80C186XL/80C188XL
Microprocessor
User’s Manual
80C186XL/80C188XL
Microprocessor
User’s Manual
1995
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© INTEL CORPORATION, 1995
CONTENTS
CHAPTER 1
INTRODUCTION
1.1
1.2
1.3
1.3.1
1.3.2
HOW TO USE THIS MANUAL....................................................................................... 1-2
RELATED DOCUMENTS.............................................................................................. 1-3
ELECTRONIC SUPPORT SYSTEMS ........................................................................... 1-4
FaxBack Service .......................................................................................................1-4
Bulletin Board System (BBS) ....................................................................................1-5
1.3.2.1
1.3.3
1.3.4
How to Find ApBUILDER Software and Hypertext Documents on the BBS ...1-6
CompuServe Forums ................................................................................................1-6
World Wide Web .......................................................................................................1-6
TECHNICAL SUPPORT ................................................................................................ 1-6
PRODUCT LITERATURE.............................................................................................. 1-7
TRAINING CLASSES.................................................................................................... 1-7
1.4
1.5
1.6
CHAPTER 2
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.1
2.1.1
ARCHITECTURAL OVERVIEW .................................................................................... 2-1
Execution Unit ...........................................................................................................2-2
Bus Interface Unit .....................................................................................................2-3
General Registers .....................................................................................................2-4
Segment Registers ...................................................................................................2-5
Instruction Pointer .....................................................................................................2-6
Flags .........................................................................................................................2-7
Memory Segmentation ..............................................................................................2-8
Logical Addresses ...................................................................................................2-10
Dynamically Relocatable Code ...............................................................................2-13
2.1.2
2.1.3
2.1.4
2.1.5
2.1.6
2.1.7
2.1.8
2.1.9
2.1.10 Stack Implementation .............................................................................................2-15
2.1.11 Reserved Memory and I/O Space ...........................................................................2-15
2.2
2.2.1
2.2.1.1
2.2.1.2
SOFTWARE OVERVIEW............................................................................................ 2-17
Instruction Set .........................................................................................................2-17
Data Transfer Instructions .............................................................................2-18
Arithmetic Instructions ...................................................................................2-19
Bit Manipulation Instructions .........................................................................2-21
String Instructions ..........................................................................................2-22
Program Transfer Instructions .......................................................................2-23
Processor Control Instructions ......................................................................2-27
Addressing Modes ..................................................................................................2-27
Register and Immediate Operand Addressing Modes ...................................2-27
Memory Addressing Modes ...........................................................................2-28
I/O Port Addressing .......................................................................................2-36
Data Types Used in the 80C186 Modular Core Family .................................2-37
2.2.1.3
2.2.1.4
2.2.1.5
2.2.1.6
2.2.2
2.2.2.1
2.2.2.2
2.2.2.3
2.2.2.4
iii
CONTENTS
2.3
2.3.1
2.3.1.1
2.3.1.2
2.3.1.3
2.3.2
2.3.3
2.3.4
2.3.5
INTERRUPTS AND EXCEPTION HANDLING............................................................ 2-39
Interrupt/Exception Processing ...............................................................................2-39
Non-Maskable Interrupts ...............................................................................2-42
Maskable Interrupts .......................................................................................2-43
Exceptions .....................................................................................................2-43
Software Interrupts ..................................................................................................2-45
Interrupt Latency .....................................................................................................2-45
Interrupt Response Time ........................................................................................2-46
Interrupt and Exception Priority ...............................................................................2-46
CHAPTER 3
BUS INTERFACE UNIT
3.1
3.2
3.2.1
3.2.2
3.3
MULTIPLEXED ADDRESS AND DATA BUS................................................................ 3-1
ADDRESS AND DATA BUS CONCEPTS..................................................................... 3-1
16-Bit Data Bus .........................................................................................................3-1
8-Bit Data Bus ...........................................................................................................3-5
MEMORY AND I/O INTERFACES................................................................................. 3-6
16-Bit Bus Memory and I/O Requirements ...............................................................3-7
8-Bit Bus Memory and I/O Requirements .................................................................3-7
BUS CYCLE OPERATION ............................................................................................ 3-7
Address/Status Phase ............................................................................................3-10
Data Phase .............................................................................................................3-13
Wait States ..............................................................................................................3-13
Idle States ...............................................................................................................3-18
BUS CYCLES .............................................................................................................. 3-20
Read Bus Cycles ....................................................................................................3-20
3.3.1
3.3.2
3.4
3.4.1
3.4.2
3.4.3
3.4.4
3.5
3.5.1
3.5.1.1
3.5.2
3.5.3
3.5.3.1
3.5.4
3.5.5
3.5.6
Refresh Bus Cycles .......................................................................................3-22
Write Bus Cycles .....................................................................................................3-22
Interrupt Acknowledge Bus Cycle ...........................................................................3-25
System Design Considerations .....................................................................3-27
HALT Bus Cycle ......................................................................................................3-28
Temporarily Exiting the HALT Bus State .................................................................3-30
Exiting HALT ...........................................................................................................3-32
3.6
SYSTEM DESIGN ALTERNATIVES ........................................................................... 3-33
Buffering the Data Bus ............................................................................................3-34
Synchronizing Software and Hardware Events .......................................................3-36
Using a Locked Bus ................................................................................................3-37
Using the Queue Status Signals .............................................................................3-38
MULTI-MASTER BUS SYSTEM DESIGNS................................................................. 3-39
Entering Bus HOLD ................................................................................................3-39
3.6.1
3.6.2
3.6.3
3.6.4
3.7
3.7.1
3.7.1.1
3.7.1.2
3.7.2
3.8
HOLD Bus Latency ........................................................................................3-40
Refresh Operation During a Bus HOLD ........................................................3-41
Exiting HOLD ..........................................................................................................3-43
BUS CYCLE PRIORITIES........................................................................................... 3-44
iv
CONTENTS
CHAPTER 4
PERIPHERAL CONTROL BLOCK
4.1
4.2
4.3
4.4
4.4.1
4.4.2
4.4.3
PERIPHERAL CONTROL REGISTERS........................................................................ 4-1
PCB RELOCATION REGISTER.................................................................................... 4-1
RESERVED LOCATIONS ............................................................................................. 4-4
ACCESSING THE PERIPHERAL CONTROL BLOCK.................................................. 4-4
Bus Cycles ...............................................................................................................4-4
READY Signals and Wait States .............................................................................4-4
F-Bus Operation .......................................................................................................4-5
4.4.3.1
4.4.3.2
4.4.3.3
4.5
4.5.1
Writing the PCB Relocation Register ...............................................................4-6
Accessing the Peripheral Control Registers ....................................................4-6
Accessing Reserved Locations .......................................................................4-6
SETTING THE PCB BASE LOCATION......................................................................... 4-6
Considerations for the 80C187 Math Coprocessor Interface ....................................4-7
CHAPTER 5
CLOCK GENERATION AND POWER MANAGEMENT
5.1
5.1.1
5.1.1.1
5.1.1.2
5.1.2
5.1.3
5.1.4
CLOCK GENERATION.................................................................................................. 5-1
Crystal Oscillator .......................................................................................................5-1
Oscillator Operation .........................................................................................5-2
Selecting Crystals ............................................................................................5-5
Using an External Oscillator ......................................................................................5-6
Output from the Clock Generator ..............................................................................5-6
Reset and Clock Synchronization .............................................................................5-6
5.2
5.2.1
5.2.1.1
5.2.1.2
POWER MANAGEMENT............................................................................................. 5-10
Power-Save Mode ..................................................................................................5-11
Entering Power-Save Mode ..........................................................................5-11
Leaving Power-Save Mode ...........................................................................5-13
Example Power-Save Initialization Code .......................................................5-13
5.2.1.3
CHAPTER 6
CHIP-SELECT UNIT
6.1
6.2
6.3
6.4
COMMON METHODS FOR GENERATING CHIP-SELECTS....................................... 6-1
CHIP-SELECT UNIT FEATURES AND BENEFITS ...................................................... 6-1
CHIP-SELECT UNIT FUNCTIONAL OVERVIEW ......................................................... 6-2
PROGRAMMING........................................................................................................... 6-6
6.4.1
6.4.2
6.4.2.1
6.4.2.2
6.4.2.3
6.4.2.4
Initialization Sequence ..............................................................................................6-6
Programming the Active Ranges ............................................................................6-12
UCS Active Range ........................................................................................6-12
LCS Active Range .........................................................................................6-13
MCS Active Range ........................................................................................6-13
PCS Active Range .........................................................................................6-15
Bus Wait State and Ready Control .........................................................................6-15
Overlapping Chip-Selects .......................................................................................6-16
6.4.3
6.4.4
v
CONTENTS
6.4.5
6.4.6
Memory or I/O Bus Cycle Decoding ........................................................................6-17
Programming Considerations ..................................................................................6-17
CHIP-SELECTS AND BUS HOLD............................................................................... 6-18
EXAMPLES ................................................................................................................. 6-18
Example 1: Typical System Configuration ..............................................................6-18
6.5
6.6
6.6.1
CHAPTER 7
REFRESH CONTROL UNIT
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.7.1
7.7.2
7.7.2.1
7.7.2.2
7.7.2.3
7.7.3
7.8
THE ROLE OF THE REFRESH CONTROL UNIT......................................................... 7-2
REFRESH CONTROL UNIT CAPABILITIES................................................................. 7-2
REFRESH CONTROL UNIT OPERATION.................................................................... 7-2
REFRESH ADDRESSES............................................................................................... 7-4
REFRESH BUS CYCLES.............................................................................................. 7-5
GUIDELINES FOR DESIGNING DRAM CONTROLLERS............................................ 7-5
PROGRAMMING THE REFRESH CONTROL UNIT..................................................... 7-7
Calculating the Refresh Interval ................................................................................7-7
Refresh Control Unit Registers .................................................................................7-7
Refresh Base Address Register ......................................................................7-8
Refresh Clock Interval Register .......................................................................7-8
Refresh Control Register .................................................................................7-9
Programming Example ...........................................................................................7-10
REFRESH OPERATION AND BUS HOLD.................................................................. 7-12
CHAPTER 8
INTERRUPT CONTROL UNIT
8.1
8.2
8.2.1
8.2.1.1
8.2.1.2
8.2.1.3
FUNCTIONAL OVERVIEW............................................................................................ 8-1
MASTER MODE ............................................................................................................ 8-2
Generic Functions in Master Mode ...........................................................................8-2
Interrupt Masking .............................................................................................8-3
Interrupt Priority ...............................................................................................8-3
Interrupt Nesting ..............................................................................................8-4
8.3
FUNCTIONAL OPERATION IN MASTER MODE ......................................................... 8-5
Typical Interrupt Sequence .......................................................................................8-5
Priority Resolution .....................................................................................................8-5
8.3.1
8.3.2
8.3.2.1
8.3.2.2
8.3.3
8.3.3.1
8.3.4
8.3.5
8.3.6
8.3.7
Priority Resolution Example ............................................................................8-6
Interrupts That Share a Single Source ............................................................8-7
Cascading with External 8259As ..............................................................................8-7
Special Fully Nested Mode ..............................................................................8-8
Interrupt Acknowledge Sequence .............................................................................8-9
Polling .......................................................................................................................8-9
Edge and Level Triggering ......................................................................................8-10
Additional Latency and Response Time .................................................................8-10
vi
CONTENTS
8.4
8.4.1
PROGRAMMING THE INTERRUPT CONTROL UNIT ............................................... 8-11
Interrupt Control Registers ......................................................................................8-12
Interrupt Request Register ......................................................................................8-16
Interrupt Mask Register ...........................................................................................8-16
Priority Mask Register .............................................................................................8-17
In-Service Register .................................................................................................8-18
Poll and Poll Status Registers .................................................................................8-19
End-of-Interrupt (EOI) Register ...............................................................................8-21
Interrupt Status Register .........................................................................................8-22
SLAVE MODE ............................................................................................................. 8-23
Slave Mode Programming ......................................................................................8-25
8.4.2
8.4.3
8.4.4
8.4.5
8.4.6
8.4.7
8.4.8
8.5
8.5.1
8.5.1.1
8.5.1.2
8.5.1.3
8.5.2
8.5.3
Interrupt Vector Register ...............................................................................8-26
End-Of-Interrupt Register ..............................................................................8-27
Other Registers .............................................................................................8-28
Interrupt Vectoring in Slave Mode ...........................................................................8-29
Initializing the Interrupt Control Unit for Master Mode .............................................8-30
CHAPTER 9
TIMER/COUNTER UNIT
9.1
9.2
9.2.1
9.2.2
9.2.3
FUNCTIONAL OVERVIEW............................................................................................ 9-1
PROGRAMMING THE TIMER/COUNTER UNIT .......................................................... 9-6
Initialization Sequence ............................................................................................9-11
Clock Sources .........................................................................................................9-12
Counting Modes ......................................................................................................9-12
9.2.3.1
Retriggering ...................................................................................................9-13
Pulsed and Variable Duty Cycle Output ..................................................................9-14
Enabling/Disabling Counters ...................................................................................9-15
Timer Interrupts .......................................................................................................9-16
Programming Considerations ..................................................................................9-16
TIMING ........................................................................................................................ 9-16
Input Setup and Hold Timings .................................................................................9-16
Synchronization and Maximum Frequency .............................................................9-17
9.2.4
9.2.5
9.2.6
9.2.7
9.3
9.3.1
9.3.2
9.3.2.1
Timer/Counter Unit Application Examples .....................................................9-17
Real-Time Clock .....................................................................................................9-17
Square-Wave Generator .........................................................................................9-17
Digital One-Shot ......................................................................................................9-17
9.3.3
9.3.4
9.3.5
CHAPTER 10
DIRECT MEMORY ACCESS UNIT
10.1 FUNCTIONAL OVERVIEW.......................................................................................... 10-1
10.1.1 The DMA Transfer ..................................................................................................10-1
10.1.1.1
10.1.1.2
DMA Transfer Directions ...............................................................................10-3
Byte and Word Transfers ..............................................................................10-3
10.1.2 Source and Destination Pointers ............................................................................10-3
vii
CONTENTS
10.1.3 DMA Requests ........................................................................................................10-3
10.1.4 External Requests ...................................................................................................10-4
10.1.4.1
10.1.4.2
Source Synchronization ................................................................................10-5
Destination Synchronization ..........................................................................10-5
10.1.5 Internal Requests ....................................................................................................10-6
10.1.5.1
10.1.5.2
Timer 2-Initiated Transfers ............................................................................10-6
Unsynchronized Transfers ............................................................................10-6
10.1.6 DMA Transfer Counts .............................................................................................10-7
10.1.7 Termination and Suspension of DMA Transfers .....................................................10-7
10.1.7.1
10.1.7.2
10.1.7.3
10.1.7.4
Termination at Terminal Count ......................................................................10-7
Software Termination ....................................................................................10-7
Suspension of DMA During NMI ...................................................................10-7
Software Suspension ....................................................................................10-7
10.1.8 DMA Unit Interrupts ................................................................................................10-8
10.1.9 DMA Cycles and the BIU ........................................................................................10-8
10.1.10 The Two-Channel DMA Unit ...................................................................................10-8
10.1.10.1
DMA Channel Arbitration ...............................................................................10-8
10.2 PROGRAMMING THE DMA UNIT ............................................................................ 10-10
10.2.1 DMA Channel Parameters ....................................................................................10-10
10.2.1.1
10.2.1.2
10.2.1.3
10.2.1.4
10.2.1.5
10.2.1.6
10.2.1.7
10.2.1.8
Programming the Source and Destination Pointers ....................................10-10
Selecting Byte or Word Size Transfers ........................................................10-14
Selecting the Source of DMA Requests ......................................................10-17
Arming the DMA Channel ............................................................................10-18
Selecting Channel Synchronization .............................................................10-18
Programming the Transfer Count Options ...................................................10-18
Generating Interrupts on Terminal Count ....................................................10-19
Setting the Relative Priority of a Channel ....................................................10-19
10.2.2 Suspension of DMA Transfers ..............................................................................10-20
10.2.3 Initializing the DMA Unit ........................................................................................10-20
10.3 HARDWARE CONSIDERATIONS AND THE DMA UNIT ......................................... 10-20
10.3.1 DRQ Pin Timing Requirements .............................................................................10-20
10.3.2 DMA Latency ........................................................................................................10-21
10.3.3 DMA Transfer Rates .............................................................................................10-21
10.3.4 Generating a DMA Acknowledge ..........................................................................10-22
10.4 DMA UNIT EXAMPLES ............................................................................................. 10-22
CHAPTER 11
MATH COPROCESSING
11.1 OVERVIEW OF MATH COPROCESSING.................................................................. 11-1
11.2 AVAILABILITY OF MATH COPROCESSING.............................................................. 11-1
11.3 THE 80C187 MATH COPROCESSOR........................................................................ 11-2
11.3.1 80C187 Instruction Set ...........................................................................................11-2
11.3.1.1
11.3.1.2
11.3.1.3
Data Transfer Instructions .............................................................................11-3
Arithmetic Instructions ...................................................................................11-3
Comparison Instructions ................................................................................11-5
viii
CONTENTS
11.3.1.4
11.3.1.5
11.3.1.6
Transcendental Instructions ..........................................................................11-5
Constant Instructions .....................................................................................11-6
Processor Control Instructions ......................................................................11-6
11.3.2 80C187 Data Types ................................................................................................11-7
11.4 MICROPROCESSOR AND COPROCESSOR OPERATION...................................... 11-7
11.4.1 Clocking the 80C187 .............................................................................................11-10
11.4.2 Processor Bus Cycles Accessing the 80C187 ......................................................11-10
11.4.3 System Design Tips ..............................................................................................11-11
11.4.4 Exception Trapping ...............................................................................................11-13
11.5 EXAMPLE MATH COPROCESSOR ROUTINES...................................................... 11-13
CHAPTER 12
ONCE MODE
12.1 ENTERING/LEAVING ONCE MODE........................................................................... 12-1
APPENDIX A
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
A.1
A.1.1
A.1.2
A.1.3
80C186 INSTRUCTION SET ADDITIONS ................................................................... A-1
Data Transfer Instructions ...................................................................................... A-1
String Instructions ................................................................................................... A-2
High-Level Instructions ........................................................................................... A-2
80C186 INSTRUCTION SET ENHANCEMENTS......................................................... A-8
Data Transfer Instructions ...................................................................................... A-8
Arithmetic Instructions ............................................................................................ A-9
Bit Manipulation Instructions ................................................................................... A-9
A.2
A.2.1
A.2.2
A.2.3
A.2.3.1
A.2.3.2
Shift Instructions ............................................................................................. A-9
Rotate Instructions ....................................................................................... A-10
APPENDIX B
INPUT SYNCHRONIZATION
B.1
B.2
WHY SYNCHRONIZERS ARE REQUIRED................................................................. B-1
ASYNCHRONOUS PINS.............................................................................................. B-2
APPENDIX C
INSTRUCTION SET DESCRIPTIONS
APPENDIX D
INSTRUCTION SET OPCODES AND CLOCK CYCLES
INDEX
ix
CONTENTS
Figure
FIGURES
Page
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
Simplified Functional Block Diagram of the 80C186 Family CPU ................................2-2
Physical Address Generation.......................................................................................2-3
General Registers ........................................................................................................2-4
Segment Registers.......................................................................................................2-6
Processor Status Word ................................................................................................2-9
Segment Locations in Physical Memory.....................................................................2-10
Currently Addressable Segments...............................................................................2-11
Logical and Physical Address ....................................................................................2-12
Dynamic Code Relocation..........................................................................................2-14
Stack Operation..........................................................................................................2-16
Flag Storage Format ..................................................................................................2-19
Memory Address Computation...................................................................................2-29
Direct Addressing.......................................................................................................2-30
Register Indirect Addressing ......................................................................................2-31
Based Addressing ......................................................................................................2-31
Accessing a Structure with Based Addressing...........................................................2-32
Indexed Addressing....................................................................................................2-33
Accessing an Array with Indexed Addressing ............................................................2-33
Based Index Addressing ............................................................................................2-34
Accessing a Stacked Array with Based Index Addressing.........................................2-35
String Operand...........................................................................................................2-36
I/O Port Addressing....................................................................................................2-36
80C186 Modular Core Family Supported Data Types................................................2-38
Interrupt Control Unit..................................................................................................2-39
Interrupt Vector Table.................................................................................................2-40
Interrupt Sequence.....................................................................................................2-42
Interrupt Response Factors........................................................................................2-46
Simultaneous NMI and Exception ..............................................................................2-47
Simultaneous NMI and Single Step Interrupts............................................................2-48
Simultaneous NMI, Single Step and Maskable Interrupt............................................2-49
Physical Data Bus Models............................................................................................3-2
16-Bit Data Bus Byte Transfers....................................................................................3-3
16-Bit Data Bus Even Word Transfers .........................................................................3-4
16-Bit Data Bus Odd Word Transfers...........................................................................3-5
8-Bit Data Bus Word Transfers.....................................................................................3-6
Typical Bus Cycle.........................................................................................................3-8
T-State Relation to CLKOUT........................................................................................3-8
BIU State Diagram .......................................................................................................3-9
T-State and Bus Phases ............................................................................................3-10
Address/Status Phase Signal Relationships ..............................................................3-11
Demultiplexing Address Information...........................................................................3-12
Data Phase Signal Relationships...............................................................................3-14
Typical Bus Cycle with Wait States ............................................................................3-15
ARDY and SRDY Pin Block Diagram.........................................................................3-15
2-9
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
2-20
2-21
2-22
2-23
2-24
2-25
2-26
2-27
2-28
2-29
2-30
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
3-10
3-11
3-12
3-13
3-14
x
CONTENTS
Page
FIGURES
Figure
3-15
3-16
3-17
3-18
3-19
3-20
3-21
3-22
3-23
3-24
3-25
3-26
3-27
3-28
3-29
3-30
3-31
3-32
3-33
3-34
3-35
3-36
3-37
4-1
5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
5-10
6-1
6-2
6-3
6-4
6-5
6-6
6-7
Generating a Normally Not-Ready Bus Signal...........................................................3-16
Generating a Normally Ready Bus Signal..................................................................3-17
Normally Not-Ready System Timing ..........................................................................3-18
Normally Ready System Timings ...............................................................................3-19
Typical Read Bus Cycle .............................................................................................3-21
Read-Only Device Interface .......................................................................................3-22
Typical Write Bus Cycle..............................................................................................3-23
16-Bit Bus Read/Write Device Interface.....................................................................3-24
Interrupt Acknowledge Bus Cycle...............................................................................3-26
Typical 82C59A Interface...........................................................................................3-27
HALT Bus Cycle.........................................................................................................3-29
Returning to HALT After a HOLD/HLDA Bus Exchange ............................................3-30
Returning to HALT After a Refresh Bus Cycle ...........................................................3-31
Returning to HALT After a DMA Bus Cycle................................................................3-32
Exiting HALT ..............................................................................................................3-33
DEN and DT/R Timing Relationships.........................................................................3-34
Buffered AD Bus System............................................................................................3-35
Qualifying DEN with Chip-Selects..............................................................................3-36
Queue Status Timing..................................................................................................3-39
Timing Sequence Entering HOLD ..............................................................................3-40
Refresh Request During HOLD..................................................................................3-42
Latching HLDA ...........................................................................................................3-43
Exiting HOLD..............................................................................................................3-44
PCB Relocation Register..............................................................................................4-2
Clock Generator ...........................................................................................................5-1
Ideal Operation of Pierce Oscillator..............................................................................5-2
Crystal Connections to Microprocessor........................................................................5-3
Equations for Crystal Calculations................................................................................5-4
Simple RC Circuit for Powerup Reset ..........................................................................5-7
Cold Reset Waveform ..................................................................................................5-8
Warm Reset Waveform ................................................................................................5-9
Clock Synchronization at Reset..................................................................................5-10
Power-Save Register .................................................................................................5-12
Power-Save Clock Transition.....................................................................................5-13
Common Chip-Select Generation Methods..................................................................6-2
Chip-Select Block Diagram...........................................................................................6-3
Chip-Select Relative Timings .......................................................................................6-4
UCS Reset Configuration.............................................................................................6-5
UMCS Register Definition.............................................................................................6-7
LMCS Register Definition.............................................................................................6-8
MMCS Register Definition............................................................................................6-9
PACS Register Definition ...........................................................................................6-10
MPCS Register Definition...........................................................................................6-11
MCS3:0 Active Ranges ..............................................................................................6-14
6-8
6-9
6-10
xi
CONTENTS
Figure
FIGURES
Page
6-11
6-12
6-13
7-1
7-2
7-3
7-4
7-5
7-6
7-7
7-8
7-9
8-1
8-2
8-3
8-4
8-5
8-6
Wait State and Ready Control Functions ...................................................................6-16
Using Chip-Selects During HOLD ..............................................................................6-18
Typical System ...........................................................................................................6-19
Refresh Control Unit Block Diagram.............................................................................7-1
Refresh Control Unit Operation Flow Chart..................................................................7-3
Refresh Address Formation..........................................................................................7-4
Suggested DRAM Control Signal Timing Relationships...............................................7-6
Formula for Calculating Refresh Interval for RFTIME Register....................................7-7
Refresh Base Address Register...................................................................................7-8
Refresh Clock Interval Register....................................................................................7-9
Refresh Control Register............................................................................................7-10
Regaining Bus Control to Run a DRAM Refresh Bus Cycle.......................................7-13
Interrupt Control Unit in Master Mode ..........................................................................8-2
Using External 8259A Modules in Cascade Mode.......................................................8-8
Interrupt Control Unit Latency and Response Time ...................................................8-11
Interrupt Control Register for Internal Sources...........................................................8-13
Interrupt Control Register for Noncascadable External Pins......................................8-14
Interrupt Control Register for Cascadable Interrupt Pins............................................8-15
Interrupt Request Register .........................................................................................8-16
Interrupt Mask Register ..............................................................................................8-17
Priority Mask Register ................................................................................................8-18
In-Service Register.....................................................................................................8-19
Poll Register ...............................................................................................................8-20
Poll Status Register....................................................................................................8-21
End-of-Interrupt Register............................................................................................8-22
Interrupt Status Register ............................................................................................8-23
Interrupt Control Unit in Slave Mode ..........................................................................8-24
Interrupt Sources in Slave Mode................................................................................8-25
Interrupt Vector Register (Slave Mode Only)..............................................................8-27
End-of-Interrupt Register in Slave Mode....................................................................8-28
Request, Mask, and In-Service Registers ..................................................................8-28
Interrupt Vectoring in Slave Mode..............................................................................8-29
Interrupt Response Time in Slave Mode....................................................................8-30
Timer/Counter Unit Block Diagram...............................................................................9-2
Counter Element Multiplexing and Timer Input Synchronization..................................9-3
Timers 0 and 1 Flow Chart ...........................................................................................9-4
Timer/Counter Unit Output Modes................................................................................9-6
Timer 0 and Timer 1 Control Registers ........................................................................9-7
Timer 2 Control Register ..............................................................................................9-9
Timer Count Registers................................................................................................9-10
Timer Maxcount Compare Registers..........................................................................9-11
TxOUT Signal Timing.................................................................................................9-15
Typical DMA Transfer.................................................................................................10-2
DMA Request Minimum Response Time ...................................................................10-4
8-7
8-8
8-9
8-10
8-11
8-12
8-13
8-14
8-15
8-16
8-17
8-18
8-19
8-20
8-21
9-1
9-2
9-3
9-4
9-5
9-6
9-7
9-8
9-9
10-1
10-2
xii
CONTENTS
Page
FIGURES
Figure
10-3
10-4
10-5
10-6
10-7
10-8
10-9
Source-Synchronized Transfers.................................................................................10-5
Destination-Synchronized Transfers ..........................................................................10-6
Two-Channel DMA Module ........................................................................................10-9
Examples of DMA Priority.........................................................................................10-10
DMA Source Pointer (High-Order Bits).....................................................................10-11
DMA Source Pointer (Low-Order Bits) .....................................................................10-12
DMA Destination Pointer (High-Order Bits)..............................................................10-13
10-10 DMA Destination Pointer (Low-Order Bits)...............................................................10-14
10-11 DMA Control Register...............................................................................................10-15
10-12 Transfer Count Register...........................................................................................10-19
11-1
11-2
11-3
11-4
12-1
A-1
A-2
A-3
A-4
A-5
80C187-Supported Data Types..................................................................................11-8
80C186 Modular Core Family/80C187 System Configuration....................................11-9
80C187 Configuration with a Partially Buffered Bus.................................................11-12
80C187 Exception Trapping via Processor Interrupt Pin..........................................11-14
Entering/Leaving ONCE Mode ...................................................................................12-2
Formal Definition of ENTER........................................................................................ A-3
Variable Access in Nested Procedures ....................................................................... A-4
Stack Frame for Main at Level 1.................................................................................. A-4
Stack Frame for Procedure A at Level 2 ..................................................................... A-5
Stack Frame for Procedure B at Level 3 Called from A............................................... A-6
Stack Frame for Procedure C at Level 3 Called from B .............................................. A-7
Input Synchronization Circuit....................................................................................... B-1
A-6
B-1
xiii
CONTENTS
Table
TABLES
Page
1-1
1-2
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
2-11
2-12
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
4-1
5-1
6-1
6-2
6-3
6-4
6-5
6-6
7-1
8-1
8-2
8-3
8-4
8-5
9-1
9-2
11-1
11-2
11-3
11-4
11-5
11-6
11-7
Comparison of 80C186 Modular Core Family Products...............................................1-2
Related Documents and Software................................................................................1-3
Implicit Use of General Registers.................................................................................2-5
Logical Address Sources............................................................................................2-13
Data Transfer Instructions..........................................................................................2-18
Arithmetic Instructions................................................................................................2-20
Arithmetic Interpretation of 8-Bit Numbers .................................................................2-21
Bit Manipulation Instructions ......................................................................................2-21
String Instructions.......................................................................................................2-22
String Instruction Register and Flag Use....................................................................2-23
Program Transfer Instructions....................................................................................2-25
Interpretation of Conditional Transfers.......................................................................2-26
Processor Control Instructions ...................................................................................2-27
Supported Data Types ...............................................................................................2-37
Bus Cycle Types ........................................................................................................3-12
Read Bus Cycle Types...............................................................................................3-20
Read Cycle Critical Timing Parameters......................................................................3-20
Write Bus Cycle Types...............................................................................................3-23
Write Cycle Critical Timing Parameters......................................................................3-25
HALT Bus Cycle Pin States........................................................................................3-29
Queue Status Signal Decoding ..................................................................................3-38
Signal Condition Entering HOLD................................................................................3-40
Peripheral Control Block...............................................................................................4-3
Suggested Values for Inductor L1 in Third Overtone Oscillator Circuit ........................5-4
Chip-Select Unit Registers ...........................................................................................6-6
UCS Block Size and Starting Address........................................................................6-12
LCS Active Range......................................................................................................6-13
MCS Active Range.....................................................................................................6-13
MCS Block Size and Start Address Restrictions ........................................................6-14
PCS Active Range......................................................................................................6-15
Identification of Refresh Bus Cycles.............................................................................7-5
Default Interrupt Priorities.............................................................................................8-3
Fixed Interrupt Types ...................................................................................................8-9
Interrupt Control Unit Registers in Master Mode ........................................................8-11
Interrupt Control Unit Register Comparison ...............................................................8-26
Slave Mode Fixed Interrupt Type Bits ........................................................................8-26
Timer 0 and 1 Clock Sources.....................................................................................9-12
Timer Retriggering......................................................................................................9-13
80C187 Data Transfer Instructions.............................................................................11-3
80C187 Arithmetic Instructions...................................................................................11-4
80C187 Comparison Instructions...............................................................................11-5
80C187 Transcendental Instructions..........................................................................11-5
80C187 Constant Instructions....................................................................................11-6
80C187 Processor Control Instructions......................................................................11-6
80C187 I/O Port Assignments..................................................................................11-10
xiv
CONTENTS
Page
TABLES
Table
C-1
C-2
C-3
C-4
D-1
D-2
D-3
D-4
D-5
Instruction Format Variables........................................................................................C-1
Instruction Operands ...................................................................................................C-2
Flag Bit Functions........................................................................................................C-3
Instruction Set .............................................................................................................C-4
Operand Variables ......................................................................................................D-1
Instruction Set Summary.............................................................................................D-2
Machine Instruction Decoding Guide...........................................................................D-9
Mnemonic Encoding Matrix (Left Half) ......................................................................D-20
Abbreviations for Mnemonic Encoding Matrix ...........................................................D-22
xv
CONTENTS
Example
EXAMPLES
Page
5-1
6-1
7-1
8-1
9-1
9-2
9-3
10-1
10-2
11-1
11-2
Initializing the Power Management Unit for Power-Save Mode .................................5-14
Initializing the Chip-Select Unit...................................................................................6-20
Initializing the Refresh Control Unit............................................................................7-11
Initializing the Interrupt Control Unit for Master Mode................................................8-31
Configuring a Real-Time Clock...................................................................................9-18
Configuring a Square-Wave Generator......................................................................9-21
Configuring a Digital One-Shot...................................................................................9-22
Initializing the DMA Unit ...........................................................................................10-23
Timed DMA Transfers ..............................................................................................10-26
Initialization Sequence for 80C187 Math Coprocessor ............................................11-15
Floating Point Math Routine Using FSINCOS..........................................................11-16
xvi
1
Introduction
CHAPTER 1
INTRODUCTION
The 8086 microprocessor was first introduced in 1978 and gained rapid support as the microcom-
puter engine of choice. There are literally millions of 8086/8088-based systems in the world to-
day. The amount of software written for the 8086/8088 is rivaled by no other architecture.
By the early 1980’s, however, it was clear that a replacement for the 8086/8088 was necessary.
An 8086/8088 system required dozens of support chips to implement even a moderately complex
design. Intel recognized the need to integrate commonly used system peripherals onto the same
silicon die as the CPU. In 1982 Intel addressed this need by introducing the 80186/80188 family
of embedded microprocessors. The original 80186/80188 integrated an enhanced 8086/8088
CPU with six commonly used system peripherals. A parallel effort within Intel also gave rise to
the 80286 microprocessor in 1982. The 80286 began the trend toward the very high performance
Intel architecture that today includes the Intel386 , Intel486 and Pentium microprocessors.
As technology advanced and turned toward small geometry CMOS processes, it became clear
that a new 80186 was needed. In 1987 Intel announced the second generation of the 80186 family:
the 80C186/C188. The 80C186 family is pin compatible with the 80186 family, while adding an
enhanced feature set. The high-performance CHMOS III process allowed the 80C186 to run at
twice the clock rate of the NMOS 80186, while consuming less than one-fourth the power.
The 80186 family took another major step in 1990 with the introduction of the 80C186EB family.
The 80C186EB heralded many changes for the 80186 family. First, the enhanced 8086/8088 CPU
was redesigned as a static, stand-alone module known as the 80C186 Modular Core. Second, the
80186 family peripherals were also redesigned as static modules with standard interfaces. The
goal behind this redesign effort was to give Intel the capability to proliferate the 80186 family
rapidly, in order to provide solutions for an even wider range of customer applications.
The 80C186EB/C188EB was the first product to use the new modular capability. The
80C186EB/C188EB includes a different peripheral set than the original 80186 family. Power
consumption was dramatically reduced as a direct result of the static design, power management
features and advanced CHMOS IV process. The 80C186EB/C188EB has found acceptance in a
wide array of portable equipment ranging from cellular phones to personal organizers.
In 1991 the 80C186 Modular Core family was again extended with the introduction of three new
products: the 80C186XL, the 80C186EA and the 80C186EC. The 80C186XL/C188XL is a high-
er performance, lower power replacement for the 80C186/C188. The 80C186EA/C188EA com-
bines the feature set of the 80C186 with new power management features for power-critical
applications. The 80C186EC/C188EC offers the highest level of integration of any of the 80C186
Modular Core family products, with 14 on-chip peripherals (see Table 1-1).
1-1
INTRODUCTION
The 80C186 Modular Core family is the direct result of ten years of Intel development. It offers
the designer the peace of mind of a well-established architecture with the benefits of state-of-the-
art technology.
Table 1-1. Comparison of 80C186 Modular Core Family Products
Feature
80C186XL
80C186EA
80C186EB
80C186EC
Enhanced 8086 Instruction Set
Low-Power Static Modular CPU
Power-Save (Clock Divide) Mode
Powerdown and Idle Modes
80C187 Interface
ONCE Mode
Interrupt Control Unit
8259
Compatible
Timer/Counter Unit
Chip-Select Unit
DMA Unit
Enhanced
Enhanced
4 Channel
2 Channel
2 Channel
Serial Communications Unit
Refresh Control Unit
Watchdog Timer Unit
I/O Ports
Enhanced
16 Total
Enhanced
22 Total
1.1 HOW TO USE THIS MANUAL
This manual uses phrases such as 80C186 Modular Core Family or 80C188 Modular Core, as
well as references to specific products such as 80C188EA. Each phrase refers to a specific set of
80C186 family products. The phrases and the products they refer to are as follows:
80C186 Modular Core Family: This phrase refers to any device that uses the modular
80C186/C188 CPU core architecture. At this time these include the 80C186EA/C188EA,
80C186EB/C188EB, 80C186EC/C188EC and 80C186XL/C188XL.
80C186 Modular Core: Without the word family, this phrase refers only to the 16-bit bus mem-
bers of the 80C186 Modular Core Family.
80C188 Modular Core: This phrase refers to the 8-bit bus products.
80C188EC: A specific product reference refers only to the named device. For example, On the
80C188EC… refers strictly to the 80C188EC and not to any other device.
1-2
INTRODUCTION
Each chapter covers a specific section of the device, beginning with the CPU core. Each periph-
eral chapter includes programming examples intended to aid in your understanding of device op-
eration. Please read the comments carefully, as not all of the examples include all the code
necessary for a specific application.
This user’s guide is a supplement to the device data sheet. Specific timing values are not dis-
cussed in this guide. When designing a system, always consult the most recent version of the de-
vice data sheet for up-to-date specifications.
1.2 RELATED DOCUMENTS
The following table lists documents and software that are useful in designing systems that incor-
porate the 80C186 Modular Core Family. These documents are available through Intel Literature.
To order a document, call the number listed for your area in “Product Literature” on page 1-7.
NOTE
If you will be transferring a design from the 80186/80188 or 80C186/80C188
to the 80C186XL/80C188XL, refer to FaxBack Document No. 2132.
Table 1-2. Related Documents and Software
Document
Document/Software Title
Order No.
Embedded Microprocessors (includes 186 family data sheets)
186 Embedded Microprocessor Line Card
272396
272079
272430
272431
80186/80188 High-Integration 16-Bit Microprocessor Data Sheet
80C186XL/C188XL-20, -12 16-Bit High-Integration Embedded Microprocessor
Data Sheet
80C186EA/80C188EA-20, -12 and 80L186EA/80L188EA-13, -8 (low power
versions) 16-Bit High-Integration Embedded Microprocessor Data Sheet
272432
272433
272434
80C186EB/80C188EB-20, -13 and 80L186EB/80L188EB-13, -8 (low power
versions) 16-Bit High-Integration Embedded Microprocessor Data Sheet
80C186EC/80C188EC-20, -13 and 80L186EC/80L188EC-13, -8 (low power
versions) 16-Bit High-Integration Embedded Microprocessor Data Sheet
80C187 80-Bit Math Coprocessor Data Sheet
270640
272324
272164
270950
270830
272047
231017
240487
Low Voltage Embedded Design
80C186/C188, 80C186XL/C188XL Microprocessor User’s Manual
80C186EA/80C188EA Microprocessor User’s Manual
80C186EB/80C188EB Microprocessor User’s Manual
80C186EC/80C188EC Microprocessor User’s Manual
8086/8088/8087/80186/80188 Programmer’s Pocket Reference Guide
8086/8088 User’s Manual Programmer’s and Hardware Reference Manual
1-3
INTRODUCTION
Table 1-2. Related Documents and Software (Continued)
Document/Software Title
Document
Order No.
ApBUILDER Software
272216
272275
80C186EA Hypertext Manual
80C186EB Hypertext Manual
80C186EC Hypertext Manual
80C186XL Hypertext Manual
ZCON - Z80 Code Converter
272296
272298
272630
Available on BBS
1.3 ELECTRONIC SUPPORT SYSTEMS
Intel’s FaxBack* service and application BBS provide up-to-date technical information. Intel
also maintains several forums on CompuServe and offers a variety of information on the World
Wide Web. These systems are available 24 hours a day, 7 days a week, providing technical infor-
mation whenever you need it.
1.3.1 FaxBack Service
FaxBack is an on-demand publishing system that sends documents to your fax machine. You can
get product announcements, change notifications, product literature, device characteristics, de-
sign recommendations, and quality and reliability information from FaxBack 24 hours a day, 7
days a week.
1-800-628-2283
916-356-3105
U.S. and Canada
U.S., Canada, Japan, APac
Europe
44(0)1793-496646
Think of the FaxBack service as a library of technical documents that you can access with your
phone. Just dial the telephone number and respond to the system prompts. After you select a doc-
ument, the system sends a copy to your fax machine.
Each document has an order number and is listed in a subject catalog. The first time you use Fax-
Back, you should order the appropriate subject catalogs to get a complete list of document order
numbers. Catalogs are updated twice monthly. In addition, daily update catalogs list the title, sta-
tus, and order number of each document that has been added, revised, or deleted during the past
eight weeks. To recieve the update for a subject catalog, enter the subject catalog number fol-
lowed by a zero. For example, for the complete microcontroller and flash catalog, request docu-
ment number 2; for the daily update to the microcontroller and flash catalog, request document
number 20.
1-4
INTRODUCTION
The following catalogs and information are available at the time of publication:
1. Solutions OEM subscription form
2. Microcontroller and flash catalog
3. Development tools catalog
4. Systems catalog
5. Multimedia catalog
6. Multibus and iRMX® software catalog and BBS file listings
7. Microprocessor, PCI, and peripheral catalog
8. Quality and reliability and change notification catalog
9. iAL (Intel Architecture Labs) technology catalog
1.3.2 Bulletin Board System (BBS)
The bulletin board system (BBS) lets you download files to your computer. The application BBS
has the latest ApBUILDER software, hypertext manuals and datasheets, software drivers, firm-
ware upgrades, application notes and utilities, and quality and reliability data.
916-356-3600
U.S., Canada, Japan, APac (up to 19.2 Kbaud)
U.S., Canada, Japan, APac (2400 baud only)
Europe
916-356-7209
44(0)1793-496340
The toll-free BBS (available in the U.S. and Canada) offers lists of documents available from
FaxBack, a master list of files available from the application BBS, and a BBS user’s guide. The
BBS file listing is also available from FaxBack (catalog number 6; see page 1-4 for phone num-
bers and a description of the FaxBack service).
1-800-897-2536
U.S. and Canada only
Any customer with a modem and computer can access the BBS. The system provides automatic
configuration support for 1200- through 19200-baud modems. Typical modem settings are 14400
baud, no parity, 8 data bits, and 1 stop bit (14400, N, 8, 1).
To access the BBS, just dial the telephone number and respond to the system prompts. During
your first session, the system asks you to register with the system operator by entering your name
and location. The system operator will set up your access account within 24 hours. At that time,
you can access the files on the BBS.
NOTE
If you encounter any difficulty accessing the high-speed modem, try the
dedicated 2400-baud modem. Use these modem settings: 2400, N, 8, 1.
1-5
INTRODUCTION
1.3.2.1
How to Find ApBUILDER Software and Hypertext Documents on the BBS
The latest ApBUILDER files and hypertext manuals and data sheets are available first from the
BBS. To access the files, complete these steps:
1. Type F from the BBS Main menu. The BBS displays the Intel Apps Files menu.
2. Type L and press <Enter>. The BBS displays the list of areas and prompts for the area
number.
3. Type 25 and press <Enter> to select ApBUILDER/Hypertext. The BBS displays several
options: one for ApBUILDER software and the others for hypertext documents for
specific product families.
4. Type 1 and press <Enter> to list the latest ApBUILDER files, or type the number of the
appropriate product family sublevel and press <Enter> for a list of available hypertext
manuals and datasheets.
5. Type the file numbers to select the files you wish to download (for example, 1,6 for files 1
and 6 or 3-7 for files 3, 4, 5, 6, and 7) and press <Enter>. The BBS displays the approx-
imate time required to download the selected files and gives you the option to download
them.
1.3.3 CompuServe Forums
The CompuServe forums provide a means for you to gather information, share discoveries, and
debate issues. Type “go intel” for access. For information about CompuServe access and service
fees, call CompuServe at 1-800-848-8199 (U.S.) or 614-529-1340 (outside the U.S.).
1.3.4 World Wide Web
Intel offers a variety of information through the World Wide Web (http://www.intel.com/). Select
“Embedded Design Products” from the Intel home page.
1.4 TECHNICAL SUPPORT
In the U.S. and Canada, technical support representatives are available to answer your questions
between 5 a.m. and 5 p.m. PST. You can also fax your questions to us. (Please include your voice
telephone number and indicate whether you prefer a response by phone or by fax). Outside the
U.S. and Canada, please contact your local distributor.
1-800-628-8686
916-356-7599
U.S. and Canada
U.S. and Canada
U.S. and Canada
916-356-6100 (fax)
1-6
INTRODUCTION
1.5 PRODUCT LITERATURE
You can order product literature from the following Intel literature centers.
1-800-468-8118, ext. 283
708-296-9333
U.S. and Canada
U.S. (from overseas)
Europe (U.K.)
Germany
44(0)1793-431155
44(0)1793-421333
44(0)1793-421777
81(0)120-47-88-32
France
Japan (fax only)
1.6 TRAINING CLASSES
In the U.S. and Canada, you can register for training classes through the Intel customer training
center. Classes are held in the U.S.
1-800-234-8806
U.S. and Canada
1-7
2
Overview of the
80C186 Family
Architecture
CHAPTER 2
OVERVIEW OF THE 80C186 FAMILY
ARCHITECTURE
The 80C186 Modular Microprocessor Core shares a common base architecture with the 8086,
8088, 80186, 80188, 80286, Intel386™ and Intel486™ processors. The 80C186 Modular Core
maintains full object-code compatibility with the 8086/8088 family of 16-bit microprocessors,
while adding hardware and software performance enhancements. Most instructions require fewer
clocks to execute on the 80C186 Modular Core because of hardware enhancements in the Bus
Interface Unit and the Execution Unit. Several additional instructions simplify programming and
reduce code size (see Appendix A, “80C186 Instruction Set Additions and Extensions”).
2.1 ARCHITECTURAL OVERVIEW
The 80C186 Modular Microprocessor Core incorporates two separate processing units: an Exe-
cution Unit (EU) and a Bus Interface Unit (BIU). The Execution Unit is functionally identical
among all family members. The Bus Interface Unit is configured for a 16-bit external data bus
for the 80C186 core and an 8-bit external data bus for the 80C188 core. The two units interface
via an instruction prefetch queue.
The Execution Unit executes instructions; the Bus Interface Unit fetches instructions, reads op-
erands and writes results. Whenever the Execution Unit requires another opcode byte, it takes the
byte out of the prefetch queue. The two units can operate independently of one another and are
able, under most circumstances, to overlap instruction fetches and execution.
The 80C186 Modular Core family has a 16-bit Arithmetic Logic Unit (ALU). The Arithmetic
Logic Unit performs 8-bit or 16-bit arithmetic and logical operations. It provides for data move-
ment between registers, memory and I/O space.
The 80C186 Modular Core family CPU allows for high-speed data transfer from one area of
memory to another using string move instructions and between an I/O port and memory using
block I/O instructions. The CPU also provides many conditional branch and control instructions.
The 80C186 Modular Core architecture features 14 basic registers grouped as general registers,
pose registers (AX, BX, CX and DX) can be used as operands for most arithmetic operations as
either 8- or 16-bit units. The four 16-bit pointer registers (SI, DI, BP and SP) can be used in arith-
metic operations and in accessing memory-based variables. Four 16-bit segment registers (CS,
DS, SS and ES) allow simple memory partitioning to aid modular programming. The status and
control registers consist of an Instruction Pointer (IP) and the Processor Status Word (PSW) reg-
ister, which contains flag bits. Figure 2-1 is a simplified CPU block diagram.
2-1
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Address Bus (20 Bits)
General
Registers
Σ
AH
AL
BL
CL
DL
Data
Bus
BH
CH
DH
(16 Bits)
CS
DS
SS
ES
IP
SP
BP
SI
DI
ALU Data Bus
(16 Bits)
Internal
Communications
Registers
Temporary
Registers
Bus
Control
Logic
External
Bus
Instruction Queue
ALU
EU
Control
1 2 3 4 5 6
System
Q Bus
(8 Bits)
Flags
Execution Unit
(EU)
Bus Interface Unit
(BIU)
A1012-0A
Figure 2-1. Simplified Functional Block Diagram of the 80C186 Family CPU
2.1.1 Execution Unit
The Execution Unit executes all instructions, provides data and addresses to the Bus Interface
Unit and manipulates the general registers and the Processor Status Word. The 16-bit ALU within
the Execution Unit maintains the CPU status and control flags and manipulates the general reg-
isters and instruction operands. All registers and data paths in the Execution Unit are 16 bits wide
for fast internal transfers.
2-2
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
The Execution Unit does not connect directly to the system bus. It obtains instructions from a
queue maintained by the Bus Interface Unit. When an instruction requires access to memory or a
peripheral device, the Execution Unit requests the Bus Interface Unit to read and write data. Ad-
dresses manipulated by the Execution Unit are 16 bits wide. The Bus Interface Unit, however,
performs an address calculation that allows the Execution Unit to access the full megabyte of
memory space.
To execute an instruction, the Execution Unit must first fetch the object code byte from the in-
struction queue and then execute the instruction. If the queue is empty when the Execution Unit
is ready to fetch an instruction byte, the Execution Unit waits for the Bus Interface Unit to fetch
the instruction byte.
2.1.2 Bus Interface Unit
The 80C186 Modular Core and 80C188 Modular Core Bus Interface Units are functionally iden-
tical. They are implemented differently to match the structure and performance characteristics of
their respective system buses. The Bus Interface Unit executes all external bus cycles. This unit
consists of the segment registers, the Instruction Pointer, the instruction code queue and several
miscellaneous registers. The Bus Interface Unit transfers data to and from the Execution Unit on
the ALU data bus.
The Bus Interface Unit generates a 20-bit physical address in a dedicated adder. The adder shifts
a 16-bit segment value left 4 bits and then adds a 16-bit offset. This offset is derived from com-
binations of the pointer registers, the Instruction Pointer and immediate values (see Figure 2-2).
Any carry from this addition is ignored.
Shift left 4 bits
1
2
0
3
2
4
0
Segment Base
Offset
15
Logical
Address
0
2
0
1
2
3
0
3
4
2
6
0
0
15
19
+
0
2
0
15
= 1
19
2
2
0
Physical Address
To Memory
A1500-0A
Figure 2-2. Physical Address Generation
2-3
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
During periods when the Execution Unit is busy executing instructions, the Bus Interface Unit
sequentially prefetches instructions from memory. As long as the prefetch queue is partially full,
the Execution Unit fetches instructions.
2.1.3 General Registers
The 80C186 Modular Core family CPU has eight 16-bit general registers (see Figure 2-3). The
general registers are subdivided into two sets of four registers. These sets are the data registers
(also called the H & L group for high and low) and the pointer and index registers (also called the
P & I group).
H
L
15
8
7
0
AX
Accumulator
Base
AH
BH
CH
DH
AL
BL
CL
DL
BX
CX
DX
Data
Group
Count
Data
SP
BP
SI
Stack Pointer
Base Pointer
Pointer
and
Index
Group
Source Index
Destination Index
DI
A1033-0A
Figure 2-3. General Registers
2-4
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
The data registers can be addressed by their upper or lower halves. Each data register can be used
interchangeably as a 16-bit register or two 8-bit registers. The pointer registers are always access-
ed as 16-bit values. The CPU can use data registers without constraint in most arithmetic and log-
ic operations. Arithmetic and logic operations can also use the pointer and index registers. Some
instructions use certain registers implicitly (see Table 2-1), allowing compact encoding.
Table 2-1. Implicit Use of General Registers
Register
Operations
AX
AL
AH
BX
CX
CL
DX
SP
SI
Word Multiply, Word Divide, Word I/O
Byte Multiply, Byte Divide, Byte I/O, Translate, Decimal Arithmetic
Byte Multiply, Byte Divide
Translate
String Operations, Loops
Variable Shift and Rotate
Word Multiply, Word Divide, Indirect I/O
Stack Operations
String Operations
DI
String Operations
The contents of the general-purpose registers are undefined following a processor reset.
2.1.4 Segment Registers
The 80C186 Modular Core family memory space is 1 Mbyte in size and divided into logical seg-
ments of up to 64 Kbytes each. The CPU has direct access to four segments at a time. The segment
registers contain the base addresses (starting locations) of these memory segments (see Figure
2-4). The CS register points to the current code segment, which contains instructions to be
fetched. The SS register points to the current stack segment, which is used for all stack operations.
The DS register points to the current data segment, which generally contains program variables.
The ES register points to the current extra segment, which is typically used for data storage. The
CS register initializes to 0FFFFH, and the SS, DS and ES registers initialize to 0000H. Programs
can access and manipulate the segment registers with several instructions.
2-5
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
15
0
CS
DS
SS
ES
Code Segment
Data Segment
Stack Segment
Extra Segment
Figure 2-4. Segment Registers
2.1.5 Instruction Pointer
The Bus Interface Unit updates the 16-bit Instruction Pointer (IP) register so it contains the offset
of the next instruction to be fetched. Programs do not have direct access to the Instruction Pointer,
but it can change, be saved or be restored as a result of program execution. For example, if the
Instruction Pointer is saved on the stack, it is first automatically adjusted to point to the next in-
struction to be executed.
Reset initializes the Instruction Pointer to 0000H. The CS and IP values comprise a starting exe-
cution address of 0FFFF0H (see “Logical Addresses” on page 2-10 for a description of address
formation).
2-6
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.1.6 Flags
The 80C186 Modular Core family has six status flags (see Figure 2-5) that the Execution Unit
posts as the result of arithmetic or logical operations. Program branch instructions allow a pro-
gram to alter its execution depending on conditions flagged by a prior operation. Different in-
structions affect the status flags differently, generally reflecting the following states:
• If the Auxiliary Flag (AF) is set, there has been a carry out from the low nibble into the high
nibble or a borrow from the high nibble into the low nibble of an 8-bit quantity (low-order
byte of a 16-bit quantity). This flag is used by decimal arithmetic instructions.
• If the Carry Flag (CF) is set, there has been a carry out of or a borrow into the high-order bit
of the instruction result (8- or 16-bit). This flag is used by instructions that add or subtract
multibyte numbers. Rotate instructions can also isolate a bit in memory or a register by
placing it in the Carry Flag.
• If the Overflow Flag (OF) is set, an arithmetic overflow has occurred. A significant digit
has been lost because the size of the result exceeded the capacity of its destination location.
An Interrupt On Overflow instruction is available that will generate an interrupt in this
situation.
• If the Sign Flag (SF) is set, the high-order bit of the result is a 1. Since negative binary
numbers are represented in standard two’s complement notation, SF indicates the sign of
the result (0 = positive, 1 = negative).
• If the Parity Flag (PF) is set, the result has even parity, an even number of 1 bits. This flag
can be used to check for data transmission errors.
• If the Zero Flag (ZF) is set, the result of the operation is zero.
Additional control flags (see Figure 2-5) can be set or cleared by programs to alter processor op-
erations:
• Setting the Direction Flag (DF) causes string operations to auto-decrement. Strings are
processed from high address to low address (or “right to left”). Clearing DF causes string
operations to auto-increment. Strings are processed from low address to high address (or
“left to right”).
• Setting the Interrupt Enable Flag (IF) allows the CPU to recognize maskable external or
internal interrupt requests. Clearing IF disables these interrupts. The Interrupt Enable Flag
has no effect on software interrupts or non-maskable interrupts.
• Setting the Trap Flag (TF) bit puts the processor into single-step mode for debugging. In
this mode, the CPU automatically generates an interrupt after each instruction. This allows
a program to be inspected instruction by instruction during execution.
The status and control flags are contained in a 16-bit Processor Status Word (see Figure 2-5). Re-
set initializes the Processor Status Word to 0F000H.
2-7
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.1.7 Memory Segmentation
user-defined segments. A segment is a logical unit of memory that can be up to 64 Kbytes long.
Each segment is composed of contiguous memory locations. Segments are independent and sep-
arately addressable. Software assigns every segment a base address (starting location) in memory
space. All segments begin on 16-byte memory boundaries. There are no other restrictions on seg-
ment locations. Segments can be adjacent, disjoint, partially overlapped or fully overlapped (see
Figure 2-6). A physical memory location can be mapped into (covered by) one or more logical
segments.
2-8
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Register Name:
Processor Status Word
PSW (FLAGS)
Register Mnemonic:
Register Function:
Posts CPU status information.
15
0
O
F
D
F
I
F
T
F
S
F
Z
F
A
F
P
F
C
F
A1035-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
OF
Overflow Flag
0
If OF is set, an arithmetic overflow has occurred.
If DF is set, string instructions are processed high
address to low address. If DF is clear, strings are
processed low address to high address.
DF
IF
Direction Flag
0
If IF is set, the CPU recognizes maskable interrupt
requests. If IF is clear, maskable interrupts are
ignored.
Interrupt
Enable Flag
0
TF
SF
ZF
Trap Flag
Sign Flag
Zero Flag
0
0
0
If TF is set, the processor enters single-step mode.
If SF is set, the high-order bit of the result of an
operation is 1, indicating it is negative.
If ZF is set, the result of an operation is zero.
If AF is set, there has been a carry from the low
nibble to the high or a borrow from the high nibble
to the low nibble of an 8-bit quantity. Used in BCD
operations.
AF
Auxiliary Flag
0
If PF is set, the result of an operation has even
parity.
PF
CF
Parity Flag
Carry Flag
0
0
If CF is set, there has been a carry out of, or a
borrow into, the high-order bit of the result of an
instruction.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products.
Figure 2-5. Processor Status Word
2-9
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Fully
Overlapped
Partly
Overlapped
Segment D
Disjoint
Logical
Segments
Contiguous
Segment C
Segment E
Segment A
Segment B
Physical
Memory
0H
10000H
20000H
30000H
A1036-0A
Figure 2-6. Segment Locations in Physical Memory
The four segment registers point to four “currently addressable” segments (see Figure 2-7). The
currently addressable segments provide a work space consisting of 64 Kbytes for code, a 64
Kbytes for stack and 128 Kbytes for data storage. Programs access code and data in another seg-
ment by updating the segment register to point to the new segment.
2.1.8 Logical Addresses
It is useful to think of every memory location as having two kinds of addresses, physical and log-
ical. A physical address is a 20-bit value that identifies a unique byte location in the memory
space. Physical addresses range from 0H to 0FFFFFH. All exchanges between the CPU and
memory use physical addresses.
out prior knowledge of where the code will be located in memory. A logical address consists of
a segment base value and an offset value. For any given memory location, the segment base value
locates the first byte of the segment. The offset value represents the distance, in bytes, of the tar-
get location from the beginning of the segment. Segment base and offset values are unsigned 16-
bit quantities. Many different logical addresses can map to the same physical location. In Figure
2-8, physical memory location 2C3H is contained in two different overlapping segments, one be-
ginning at 2B0H and the other at 2C0H.
2-10
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
FFFFFH
A
B
Data:
Code:
Stack:
Extra:
DS:
CS:
SS:
ES:
B
C
E
D
H
E
J
F
G
H
I
J
K
0H
A1037-0A
Figure 2-7. Currently Addressable Segments
in one segment type generally shares the same logical attributes (e.g., code or data). This leads to
programs that are shorter, faster and better structured.
The Bus Interface Unit must obtain the logical address before generating the physical address.
The logical address of a memory location can come from different sources, depending on the type
of reference that is being made (see Table 2-2).
Segment registers always hold the segment base addresses. The Bus Interface Unit determines
which segment register contains the base address according to the type of memory reference
made. However, the programmer can explicitly direct the Bus Interface Unit to use any currently
addressable segment (except for the destination operand of a string instruction). In assembly lan-
guage, this is done by preceding an instruction with a segment override prefix.
2-11
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Table 2-2. Logical Address Sources
Default
Segment Base
Alternate
Segment Base
Type of Memory Reference
Offset
Instruction Fetch
CS
SS
DS
DS
ES
SS
NONE
NONE
IP
Stack Operation
SP
Variable (except following)
String Source
CS, ES, SS
CS, ES, SS
NONE
Effective Address
SI
DI
String Destination
BP Used as Base Register
CS, DS, ES
Effective Address
Instructions are always fetched from the current code segment. The IP register contains the in-
rent stack segment. The Stack Pointer (SP) register contains the offset of the top of the stack from
the base of the stack. Most variables (memory operands) are assumed to reside in the current data
segment, but a program can instruct the Bus Interface Unit to override this assumption. Often, the
offset of a memory variable is not directly available and must be calculated at execution time. The
addressing mode specified in the instruction determines how this offset is calculated (see “Ad-
dressing Modes” on page 2-27). The result is called the operand’s Effective Address (EA).
Strings are addressed differently than other variables. The source operand of a string instruction
is assumed to lie in the current data segment. However, the program can use another currently
addressable segment. The operand’s offset is taken from the Source Index (SI) register. The des-
tination operand of a string instruction always resides in the current extra segment. The destina-
tion’s offset is taken from the Destination Index (DI) register. The string instructions
automatically adjust the SI and DI registers as they process the strings one byte or word at a time.
When an instruction designates the Base Pointer (BP) register as a base register, the variable is
assumed to reside in the current stack segment. The BP register provides a convenient way to ac-
cess data on the stack. The BP register can also be used to access data in any other currently ad-
dressable segment.
2.1.9 Dynamically Relocatable Code
The segmented memory structure of the 80C186 Modular Core family allows creation of dynam-
ming or multitasking system to make effective use of available memory. The processor can write
inactive programs to a disk and reallocate the space they occupied to other programs. A disk-res-
ident program can then be read back into available memory locations and restarted whenever it
is needed. If a program needs a large contiguous block of storage and the total amount is available
only in non-adjacent fragments, other program segments can be compacted to free enough con-
tinuous space. This process is illustrated in Figure 2-9.
2-13
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Before
After
Relocation
Relocation
Code
Segment
CS
SS
CS
SS
Stack
Segment
DS
ES
DS
ES
Code
Segment
Data
Segment
Stack
Segment
Data
Segment
Extra
Segment
Extra
Segment
Free Space
A1039-0A
Figure 2-9. Dynamic Code Relocation
To be dynamically relocatable, a program must not load or alter its segment registers and must
not transfer directly to a location outside the current code segment. All program offsets must be
relative to the segment registers. This allows the program to be moved anywhere in memory, pro-
vided that the segment registers are updated to point to the new base addresses.
2-14
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.1.10 Stack Implementation
Stacks in the 80C186 Modular Core family reside in memory space. They are located by the Stack
Segment register (SS) and the Stack Pointer (SP). A system can have multiple stacks, but only
one stack is directly addressable at a time. A stack can be up to 64 Kbytes long, the maximum
segment. The SS register contains the base address of the current stack. The top of the stack, not
the base address, is the origination point of the stack. The SP register contains an offset that points
to the Top of Stack (TOS).
Stacks are 16 bits wide. Instructions operating on a stack add and remove stack elements one
word at a time. An element is pushed onto the stack (see Figure 2-10) by first decrementing the
SP register by 2 and then writing the data word. An element is popped off the stack by copying
it from the top of the stack and then incrementing the SP register by 2. The stack grows down in
memory toward its base address. Stack operations never move or erase elements on the stack. The
top of the stack changes only as a result of updating the stack pointer.
2.1.11 Reserved Memory and I/O Space
Two specific areas in memory and one area in I/O space are reserved in the 80C186 Core family.
• Locations 0H through 3FFH in low memory are used for the Interrupt Vector Table.
Programs should not be loaded here.
• Locations 0FFFF0H through 0FFFFFH in high memory are used for system reset code
because the processor begins execution at 0FFFF0H.
• Locations 0F8H through 0FFH in I/O space are reserved for communication with other Intel
hardware products and must not be used. On the 80C186 core, these addresses are used as
I/O ports for the 80C187 numerics processor extension.
2-15
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
POP AX
POP BX
10
50
PUSH AX
Existing
Stack
12
34
BB AA
1062 00
11
33
55
77
99
1062 00
1060 22
105E 44
105B 66
105A 88
11
33
55
77
99
1062 00
11
33
55
77
99
1060 22
105E 44
105B 66
105A 88
1060 22
105E 44
105B 66
105A 88
TOS
TOS
1058 AA BB
1058 AA BB
1058 AA BB
TOS
1056 01
1054 45
23
67
1056 34
1054 45
12
67
1056 34
1054 45
12
67
1052 89 AB
1050 CD EF
1052 89 AB
1050 CD EF
1052 89 AB
1050 CD EF
SS
SP
SS
SP
SS
SP
10
00
50
08
10
00
50
06
10
50
00 0A
Stack operation for code sequence
PUSH AX
POP AX
POP BX
A1013-0A
Figure 2-10. Stack Operation
2-16
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.2 SOFTWARE OVERVIEW
All 80C186 Modular Core family members execute the same instructions. This includes all the
8086/8088 instructions plus several additions and enhancements (see Appendix A, “80C186 In-
struction Set Additions and Extensions”). The following sections describe the instructions by cat-
egory and provide a detailed discussion of the operand addressing modes.
Software for 80C186 core family systems need not be written in assembly language. The proces-
sor provides direct hardware support for programs written in the many high-level languages
available. The hardware addressing modes provide straightforward implementations of based
variables, arrays, arrays of structures and other high-level language data constructs. A powerful
set of memory-to-memory string operations allow efficient character data manipulation. Finally,
routines with critical performance requirements can be written in assembly language and linked
with high-level code.
2.2.1 Instruction Set
The 80C186 Modular Core family instructions treat different types of operands uniformly. Nearly
every instruction can operate on either byte or word data. Register, memory and immediate op-
erands can be specified interchangeably in most instructions. Immediate values are exceptions:
they must serve as source operands and not destination operands. Memory variables can be ma-
nipulated (added to, subtracted from, shifted, compared) without being moved into and out of reg-
isters. This saves instructions, registers and execution time in assembly language programs. In
high-level languages, where most variables are memory-based, compilers can produce faster and
shorter object programs.
The 80C186 Modular Core family instruction set can be viewed as existing on two levels. One is
the assembly level and the other is the machine level. To the assembly language programmer, the
80C186 Modular Core family appears to have about 100 instructions. One MOV (data move) in-
struction, for example, transfers a byte or a word from a register, a memory location or an imme-
diate value to either a register or a memory location. The 80C186 Modular Core family CPUs,
however, recognize 28 different machine versions of the MOV instruction.
The two levels of instruction sets address two requirements: efficiency and simplicity. Approxi-
mately 300 forms of machine-level instructions make very efficient use of storage. For example,
the machine instruction that increments a memory operand is three or four bytes long because the
address of the operand must be encoded in the instruction. Incrementing a register, however, re-
quires less information, so the instruction can be shorter. The 80C186 Core family has eight sin-
gle-byte machine-level instructions that increment different 16-bit registers.
The assembly level instructions simplify the programmer’s view of the instruction set. The pro-
grammer writes one form of an INC (increment) instruction and the assembler examines the op-
erand to determine which machine level instruction to generate. The following paragraphs
provide a functional description of the assembly-level instructions.
2-17
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.2.1.1
Data Transfer Instructions
The instruction set contains 14 data transfer instructions. These instructions move single bytes
and words between memory and registers. They also move single bytes and words between the
AL or AX register and I/O ports. Table 2-3 lists the four types of data transfer instructions and
their functions.
Table 2-3. Data Transfer Instructions
General-Purpose
MOV
Move byte or word
PUSH
POP
Push word onto stack
Pop word off stack
PUSHA
POPA
XCHG
XLAT
Push registers onto stack
Pop registers off stack
Exchange byte or word
Translate byte
Input/Output
IN
Input byte or word
Output byte or word
OUT
Address Object and Stack Frame
LEA
Load effective address
Load pointer using DS
Load pointer using ES
Build stack frame
LDS
LES
ENTER
LEAVE
Tear down stack frame
Flag Transfer
LAHF
Load AH register from flags
Store AH register in flags
Push flags from stack
Pop flags off stack
SAHF
PUSHF
POPF
Data transfer instructions are categorized as general purpose, input/output, address object and
flag transfer. The stack manipulation instructions, used for transferring flag contents and instruc-
tions used for loading segment registers are also included in this group. Figure 2-11 shows the
flag storage formats. The address object instructions manipulate the addresses of variables in-
stead of the values of the variables.
2-18
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
LAHF
SAHF
S Z U A U P U C
7 6 5 4 3 2 1 0
PUSHF
POPF
U U U U O D I T S Z U A U P U C
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
U = Undefined; Value is indeterminate
O = Overflow Flag
D = Direction Flag
I = Interrupt Enable Flag
T = Trap Flag
S = Sign Flag
Z = Zero Flag
A = Auxiliary Carry Flag
P = Parity Flag
C = Carry Flag
A1014-0A
Figure 2-11. Flag Storage Format
2.2.1.2
Arithmetic Instructions
The arithmetic instructions (see Table 2-4) operate on four types of numbers:
• Unsigned binary
• Signed binary (integers)
• Unsigned packed decimal
• Unsigned unpacked decimal
2-19
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Table 2-5 shows the interpretations of various bit patterns according to number type. Binary num-
bers can be 8 or 16 bits long. Decimal numbers are stored in bytes, two digits per byte for packed
decimal and one digit per byte for unpacked decimal. The processor assumes that the operands in
arithmetic instructions contain data that represents valid numbers for that instruction. Invalid data
may produce unpredictable results. The Execution Unit analyzes the results of arithmetic instruc-
tions and adjusts status flags accordingly.
Table 2-4. Arithmetic Instructions
Addition
ADD
ADC
INC
Add byte or word
Add byte or word with carry
Increment byte or word by 1
ASCII adjust for addition
Decimal adjust for addition
AAA
DAA
Subtraction
SUB
SBB
DEC
NEG
CMP
AAS
DAS
Subtract byte or word
Subtract byte or word with borrow
Decrement byte or word by 1
Negate byte or word
Compare byte or word
ASCII adjust for subtraction
Decimal adjust for subtraction
Multiplication
MUL
IMUL
AAM
Multiply byte or word unsigned
Integer multiply byte or word
ASCII adjust for multiplication
Division
DIV
Divide byte or word unsigned
Integer divide byte or word
ASCII adjust for division
Convert byte to word
IDIV
AAD
CBW
CWD
Convert word to double-word
2-20
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Table 2-5. Arithmetic Interpretation of 8-Bit Numbers
Unsigned
Binary
Signed
Binary
Unpacked
Decimal
Packed
Decimal
Hex
Bit Pattern
07
89
C5
0 0 0 0 0 1 1 1
1 0 0 0 1 0 0 1
1 1 0 0 0 1 0 1
7
+7
7
7
137
197
–119
–59
invalid
89
invalid
2.2.1.3
Bit Manipulation Instructions
There are three groups of instructions for manipulating bits within bytes and words. These three
groups are logical, shifts and rotates. Table 2-6 lists the bit manipulation instructions and their
functions.
Table 2-6. Bit Manipulation Instructions
Logicals
NOT
AND
OR
“Not” byte or word
“And” byte or word
“Inclusive or” byte or word
“Exclusive or” byte or word
“Test” byte or word
XOR
TEST
Shifts
SHL/SAL
SHR
Shift logical/arithmetic left byte or word
Shift logical right byte or word
SAR
Shift arithmetic right byte or word
Rotates
ROL
ROR
RCL
RCR
Rotate left byte or word
Rotate right byte or word
Rotate through carry left byte or word
Rotate through carry right byte or word
Logical instructions include the Boolean operators NOT, AND, OR and exclusive OR (XOR), as
well as a TEST instruction. The TEST instruction sets the flags as a result of a Boolean AND op-
eration but does not alter either of its operands.
Individual bits in bytes and words can be shifted either arithmetically or logically. Up to 32 shifts
can be performed, according to the value of the count operand coded in the instruction. The count
can be specified as an immediate value or as a variable in the CL register. This allows the shift
count to be a supplied at execution time. Arithmetic shifts can be used to multiply and divide bi-
nary numbers by powers of two. Logical shifts can be used to isolate bits in bytes or words.
2-21
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Individual bits in bytes and words can also be rotated. The processor does not discard the bits ro-
tated out of an operand. The bits circle back to the other end of the operand. The number of bits
to be rotated is taken from the count operand, which can specify either an immediate value or the
CL register. The carry flag can act as an extension of the operand in two of the rotate instructions.
This allows a bit to be isolated in the Carry Flag (CF) and then tested by a JC (jump if carry) or
JNC (jump if not carry) instruction.
2.2.1.4
String Instructions
Five basic string operations process strings of bytes or words, one element (byte or word) at a
time. Strings of up to 64 Kbytes can be manipulated with these instructions. Instructions are avail-
able to move, compare or scan for a value, as well as to move string elements to and from the
accumulator. Table 2-7 lists the string instructions. These basic operations can be preceded by a
one-byte prefix that causes the instruction to be repeated by the hardware, allowing long strings
to be processed much faster than is possible with a software loop. The repetitions can be termi-
nated by a variety of conditions. Repeated operations can be interrupted and resumed.
Table 2-7. String Instructions
REP
Repeat
REPE/REPZ
REPNE/REPNZ
MOVSB/MOVSW
MOVS
Repeat while equal/zero
Repeat while not equal/not zero
Move byte string/word string
Move byte or word string
Input byte or word string
Output byte or word string
Compare byte or word string
Scan byte or word string
Load byte or word string
Store byte or word string
INS
OUTS
CMPS
SCAS
LODS
STOS
String instructions operate similarly in many respects (see Table 2-8). A string instruction can
have a source operand, a destination operand, or both. The hardware assumes that a source string
resides in the current data segment. A segment prefix can override this assumption. A destination
string must be in the current extra segment. The assembler does not use the operand names to ad-
dress strings. Instead, the contents of the Source Index (SI) register are used as an offset to address
the current element of the source string. The contents of the Destination Index (DI) register are
taken as the offset of the current destination string element. These registers must be initialized to
point to the source and destination strings before executing the string instructions. The LDS, LES
and LEA instructions are useful in performing this function.
2-22
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
String instructions automatically update the SI register, the DI register, or both, before processing
the next string element. The Direction Flag (DF) determines whether the index registers are auto-
incremented (DF = 0) or auto-decremented (DF = 1). The processor adjusts the DI, SI, or both
registers by one for byte strings or by two for word strings.
If a repeat prefix is used, the count register (CX) is decremented by one after each repetition of
the string instruction. The CX register must be initialized to the number of repetitions before the
string instruction is executed. If the CX register is 0, the string instruction is not executed and
control goes to the following instruction.
Table 2-8. String Instruction Register and Flag Use
SI
Index (offset) for source string
Index (offset) for destination string
Repetition counter
DI
CX
AL/AX
Scan value
Destination for LODS
Source for STOS
DF
ZF
Direction Flag
0 = auto-increment SI, DI
1 = auto-decrement SI, DI
Scan/compare terminator
2.2.1.5
Program Transfer Instructions
The contents of the Code Segment (CS) and Instruction Pointer (IP) registers determine the in-
struction execution sequence in the 80C186 Modular Core family. The CS register contains the
base address of the current code segment. The Instruction Pointer register points to the memory
location of the next instruction to be fetched. In most operating conditions, the next instruction
will already have been fetched and will be waiting in the CPU instruction queue. Program transfer
instructions operate on the IP and CS registers. Changing the contents of these registers causes
contains the correct instruction. The Bus Interface Unit obtains the next instruction from memory
using the new IP and CS values. It then passes the instruction directly to the Execution Unit and
begins refilling the queue from the new location.
The 80C186 Modular Core family offers four groups of program transfer instructions (see Table
2-9). These are unconditional transfers, conditional transfers, iteration control instructions and in-
terrupt-related instructions.
2-23
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Unconditional transfer instructions can transfer control either to a target instruction within the
current code segment (intrasegment transfer) or to a different code segment (intersegment trans-
fer). The assembler terms an intrasegment transfer SHORT or NEAR and an intersegment trans-
fer FAR. The transfer is made unconditionally when the instruction is executed. CALL, RET and
JMP are all unconditional transfers.
CALL is used to transfer the program to a procedure. A CALL can be NEAR or FAR. A NEAR
CALL stacks only the Instruction Pointer, while a FAR CALL stacks both the Instruction Pointer
and the Code Segment register. The RET instruction uses the information pushed onto the stack
to determine where to return when the procedure finishes. Note that the RET and CALL instruc-
separately assembled programs. The JMP instruction does not push any information onto the
stack. A JMP instruction can be NEAR or FAR.
Conditional transfer instructions are jumps that may or may not transfer control, depending on
the state of the CPU flags when the instruction is executed. Each conditional transfer instruction
tests a different combination of flags for a condition (see Table 2-10). If the condition is logically
TRUE, control is transferred to the target specified in the instruction. If the condition is FALSE,
control passes to the instruction following the conditional jump. All conditional jumps are
SHORT. The target must be in the current code segment within –128 to +127 bytes of the next
instruction’s first byte. For example, JMP 00H causes a jump to the first byte of the next instruc-
tion. Jumps are made by adding the relative displacement of the target to the Instruction Pointer.
All conditional jumps are self-relative and are appropriate for position-independent routines.
2-24
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Table 2-9. Program Transfer Instructions
Conditional Transfers
Jump if above/not below nor equal
JA/JNBE
JAE/JNB
JB/JNAE
JBE/JNA
JC
Jump if above or equal/not below
Jump if below/not above nor equal
Jump if below or equal/not above
Jump if carry
JE/JZ
Jump if equal/zero
JG/JNLE
JGE/JNL
JL/JNGE
JLE/JNG
JNC
Jump if greater/not less nor equal
Jump if greater or equal/not less
Jump if less/not greater nor equal
Jump if less or equal/not greater
Jump if not carry
JNE/JNZ
JNO
Jump if not equal/not zero
Jump if not overflow
JNP/JPO
JNS
Jump if not parity/parity odd
Jump if not sign
JO
Jump if overflow
JP/JPE
JS
Jump if parity/parity even
Jump if sign
Unconditional Transfers
CALL
RET
JMP
Call procedure
Return from procedure
Jump
Iteration Control
LOOP
Loop
LOOPE/LOOPZ
LOOPNE/LOOPNZ
JCXZ
Loop if equal/zero
Loop if not equal/not zero
Jump if register CX=0
Interrupts
INT
Interrupt
INTO
BOUND
IRET
Interrupt if overflow
Interrupt if out of array bounds
Interrupt return
2-25
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Iteration control instructions can be used to regulate the repetition of software loops. These in-
structions use the CX register as a counter. Like the conditional transfers, the iteration control in-
structions are self-relative and can transfer only to targets that are within –128 to +127 bytes of
themselves. They are SHORT transfers.
The interrupt instructions allow programs and external hardware devices to activate interrupt ser-
vice routines. The effect of a software interrupt is similar to that of a hardware-initiated interrupt.
The processor cannot execute an interrupt acknowledge bus cycle if the interrupt originates in
software or with an NMI (Non-Maskable Interrupt).
Table 2-10. Interpretation of Conditional Transfers
Mnemonic
JA/JNBE
Condition Tested
(CF or ZF)=0
“Jump if…”
above/not below nor equal
above or equal/not below
below/not above nor equal
below or equal/not above
carry
JAE/JNB
JB/JNAE
JBE/JNA
JC
CF=0
CF=1
(CF or ZF)=1
CF=1
JE/JZ
ZF=1
equal/zero
JG/JNLE
JGE/JNL
JL/JNGE
JLE/JNG
JNC
((SF xor OF) or ZF)=0
(SF xor OF)=0
(SF xor OF)=1
((SF xor OF) or ZF)=1
CF=0
greater/not less nor equal
greater or equal/not less
less/not greater nor equal
less or equal/not greater
not carry
JNE/JNZ
JNO
ZF=0
not equal/not zero
not overflow
OF=0
JNP/JPO
JNS
PF=0
not parity/parity odd
not sign
SF=0
JO
OF=1
overflow
JP/JPE
JS
PF=1
parity/parity equal
sign
SF=1
NOTE: The terms above and below refer to the relationship of two unsigned values;
greater and less refer to the relationship of two signed values.
2-26
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.2.1.6
Processor Control Instructions
Processor control instructions (see Table 2-11) allow programs to control various CPU functions.
Seven of these instructions update flags, four of them are used to synchronize the microprocessor
with external events, and the remaining instruction causes the CPU to do nothing. Except for flag
operations, processor control instructions do not affect the flags.
Table 2-11. Processor Control Instructions
Flag Operations
STC
CLC
CMC
STD
CLD
STI
Set Carry flag
Clear Carry flag
Complement Carry flag
Set Direction flag
Clear Direction flag
Set Interrupt Enable flag
Clear Interrupt Enable flag
CLI
External Synchronization
HLT
Halt until interrupt or reset
Wait for TEST pin active
WAIT
ESC
Escape to external processor
Lock bus during next instruction
LOCK
No Operation
NOP
No operation
2.2.2 Addressing Modes
The 80C186 Modular Core family members access instruction operands in several ways. Oper-
ands can be contained either in registers, in the instruction itself, in memory or at I/O ports. Ad-
dresses of memory and I/O port operands can be calculated in many ways. These addressing
modes greatly extend the flexibility and convenience of the instruction set. The following para-
graphs briefly describe register and immediate modes of operand addressing. A detailed descrip-
tion of the memory and I/O addressing modes is also provided.
2.2.2.1
Register and Immediate Operand Addressing Modes
Usually, the fastest, most compact operand addressing forms specify only register operands. This
is because the register operand addresses are encoded in instructions in just a few bits and no bus
cycles are run (the operation occurs within the CPU). Registers can serve as source operands, des-
tination operands, or both.
2-27
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Immediate operands are constant data contained in an instruction. Immediate data can be either
8 or 16 bits in length. Immediate operands are available directly from the instruction queue and
can be accessed quickly. As with a register operand, no bus cycles need to be run to get an imme-
diate operand. Immediate operands can be only source operands and must have a constant value.
2.2.2.2
Memory Addressing Modes
Although the Execution Unit has direct access to register and immediate operands, memory op-
erands must be transferred to and from the CPU over the bus. When the Execution Unit needs to
read or write a memory operand, it must pass an offset value to the Bus Interface Unit. The Bus
Interface Unit adds the offset to the shifted contents of a segment register, producing a 20-bit
physical address. One or more bus cycles are then run to access the operand.
The offset that the Execution Unit calculates for memory operand is called the operand’s Effec-
tive Address (EA). This address is an unsigned 16-bit number that expresses the operand’s dis-
tance, in bytes, from the beginning of the segment in which it resides. The Execution Unit can
struction tells the Execution Unit how to calculate the effective address of each memory operand.
A compiler or assembler derives this information from the instruction written by the programmer.
Assembly language programmers have access to all addressing modes.
The Execution Unit calculates the Effective Address by summing a displacement, the contents of
a base register and the contents of an index register (see Figure 2-12). Any combination of these
can be present in a given instruction. This allows a variety of memory addressing modes.
2-28
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Single Index
Double Index
BX
or
BX
or
SI
or
BP
or
BP
DI
Encoded
in the
Instruction
+
SI
EU
or
DI
Explicit
in the
Instruction
Effective
Address
+
+
Displacement
0000
CS
or
SS 0000
or
Assumed Unless
Overridden
by Prefix
BIU
DS 0000
or
+
+
ES 0000
Physical Addr
A1015-0A
Figure 2-12. Memory Address Computation
The displacement is an 8- or 16-bit number contained in the instruction. The displacement gen-
erally is derived from the position of the operand’s name (a variable or label) in the program. The
programmer can modify this value or explicitly specify the displacement.
2-29
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
The BX or BP register can be specified as the base register for an effective address calculation.
Similarly, either the SI or the DI register can be specified as the index register. The displacement
value is a constant. The contents of the base and index registers can change during execution. This
allows one instruction to access different memory locations depending upon the current values in
the base or base and index registers. The default base register for effective address calculations
with the BP register is SS, although DS or ES can be specified.
Direct addressing is the simplest memory addressing mode (see Figure 2-13). No registers are in-
volved, and the effective address is taken directly from the displacement of the instruction. Pro-
grammers typically use direct addressing to access scalar variables.
from one of the base or index registers (see Figure 2-14). One instruction can operate on various
memory locations if the base or index register is updated accordingly. Any 16-bit general register
can be used for register indirect addressing with the JMP or CALL instructions.
In based addressing, the effective address is the sum of a displacement value and the contents of
the BX or BP register (see Figure 2-15). Specifying the BP register as a base register directs the
Bus Interface Unit to obtain the operand from the current stack segment (unless a segment over-
ride prefix is present). This makes based addressing with the BP register a convenient way to ac-
cess stack data.
Displacement
EA
Opcode
Mod R/M
A1016-0A
Figure 2-13. Direct Addressing
2-30
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Opcode
Mod R/M
BX
or
BP
or
SI
or
EA
DI
A1017-0A
Figure 2-14. Register Indirect Addressing
Mod R/M
Opcode
Displacement
BX
or
+
BP
EA
A1018-0A
Figure 2-15. Based Addressing
Based addressing provides a simple way to address data structures that may be located in different
places in memory (see Figure 2-16). A base register can be pointed at the structure. Elements of
the structure can then be addressed by their displacements. Different copies of the same structure
can be accessed by simply changing the base register.
2-31
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Displacement
High Address
Displacement
(Rate)
Age
Status
Rate
(Rate)
+
+
Vac
Sick
Div
Base
Register
Base Register
EA
Dept
Employee
EA
Age
Status
Rate
Vac
Sick
Div
Dept
Employee
Low Address
A1019-0A
Figure 2-16. Accessing a Structure with Based Addressing
With indexed addressing, the effective address is calculated by summing a displacement and the
contents of an index register (SI or DI, see Figure 2-17). Indexed addressing is often used to ac-
cess elements in an array (see Figure 2-18). The displacement locates the beginning of the array,
and the value of the index register selects one element. If the index register contains 0000H, the
processor selects the first element. Since all array elements are the same length, simple arithmetic
on the register can select any element.
2-32
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Mod R/M
Opcode
Displacement
SI
or
DI
+
EA
A1020-0A
Figure 2-17. Indexed Addressing
High Address
Array (8)
Array (7)
Array (6)
Array (5)
Array (4)
Array (3)
Array (2)
Array (1)
Array (0)
Displacement
Displacement
+
+
Index Register
14
Index Register
2
EA
EA
1 Word
Low Address
A1021-0A
Figure 2-18. Accessing an Array with Indexed Addressing
2-33
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Based index addressing generates an effective address that is the sum of a base register, an index
register and a displacement (see Figure 2-19). The two address components can be determined at
execution time, making this a very flexible addressing mode.
Opcode
Mod R/M
Displacement
BX
or
+
BP
SI
or
DI
+
EA
A1022-0A
Figure 2-19. Based Index Addressing
Based index addressing provides a convenient way for a procedure to address an array located on
a stack (see Figure 2-20). The BP register can contain the offset of a reference point on the stack.
This is typically the top of the stack after the procedure has saved registers and allocated local
displacement value. The index register can be used to access individual array elements. Arrays
contained in structures and matrices (two-dimensional arrays) can also be accessed with based
indexed addressing.
String instructions do not use normal memory addressing modes to access operands. Instead, the
index registers are used implicitly (see Figure 2-21). When a string instruction executes, the SI
register must point to the first byte or word of the source string, and the DI register must point to
the first byte or word of the destination string. In a repeated string operation, the CPU will auto-
matically adjust the SI and DI registers to obtain subsequent bytes or words. For string instruc-
tions, the DS register is the default segment register for the SI register and the ES register is the
default segment register for the DI register. This allows string instructions to operate on data lo-
cated anywhere within the 1 Mbyte address space.
2-34
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
High Address
Displacement
6
Parm 2
Parm 1
IP
Displacement
6
+
+
Old BP
Old BX
Old AX
Array (6)
Array (5)
Array (4)
Array (3)
Array (2)
Array (1)
Array (0)
Count
Base Register (BP)
Base Register
(BP)
+
+
Index Register
12
Index Register
12
EA
EA
Temp
Status
1 Word
Low Address
A1024-0A
Figure 2-20. Accessing a Stacked Array with Based Index Addressing
2-35
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Opcode
SI
DI
Source EA
Destination EA
A1025-0A
Figure 2-21. String Operand
2.2.2.3
I/O Port Addressing
Any memory operand addressing modes can be used to access an I/O port if the port is memory-
mapped. String instructions can also be used to transfer data to memory-mapped ports with an
appropriate hardware interface.
Two addressing modes can be used to access ports located in the I/O space (see Figure 2-22). For
direct I/O port addressing, the port number is an 8-bit immediate operand. This allows fixed ac-
cess to ports numbered 0 to 255. Indirect I/O port addressing is similar to register indirect address-
ing of memory operands. The DX register contains the port number, which can range from 0 to
65,535. Adjusting the contents of the DX register allows one instruction to access any port in the
I/O space. A group of adjacent ports can be accessed using a simple software loop that adjusts the
value of the DX register.
Data
Opcode
Opcode
Port Address
DX
Port Address
Direct Port
Addressing
Indirect Port
Addressing
A1026-0A
Figure 2-22. I/O Port Addressing
2-36
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.2.2.4
Data Types Used in the 80C186 Modular Core Family
The 80C186 Modular Core family supports the data types described in Table 2-12 and illustrated
in Figure 2-23. In general, individual data elements must fit within defined segment limits.
Table 2-12. Supported Data Types
Type
Integer
Description
A signed 8- or 16-bit binary numeric value (signed byte or word). All operations assume
a 2’s complement representation.
The 80C187 numerics processor extension, when added to an 80C186 Modular Core
system, directly supports signed 32- and 64-bit integers (signed double-words and
quad-words). The 80C188 Modular Core does not support the 80C187.
Ordinal
BCD
An unsigned 8- or 16-bit binary numeric value (unsigned byte or word).
A byte (unpacked) representation of a single decimal digit (0-9).
ASCII
A byte representation of alphanumeric and control characters using the ASCII
standard.
Packed BCD
String
A byte (packed) representation of two decimal digits (0-9).One digit is stored in each
nibble (4 bits) of the byte.
A contiguous sequence of bytes or words. A string can contain from 1 byte to 64
Kbytes.
Pointer
A 16- or 32-bit quantity. A 16-bit pointer consists of a 16-bit offset component; a 32-bit
pointer consists of the combination of a 16-bit base component (selector) plus a 16-bit
offset component.
Floating Point
A signed 32-, 64-, or 80-bit real number representation.
The 80C187 numerics processor extension, when added to an 80C186 Modular Core
system, directly supports floating point operands. The 80C188 Modular Core does not
support the 80C187.
2-37
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
7
0
7
0
Signed Byte
Sign Bit
Unsigned Byte
MSB
Magnitude
Magnitude
+1
0
0
+1
1514
0
15
8 7
0
8 7
Unsigned
Word
Signed Word
Sign Bit
MSB
MSB
Magnitude
0
Magnitude
24 23
+3
+2
+1
0
31
63
16 15
8 7
Signed Double
Word*
MSB
Sign Bit
Magnitude
+7
+6
+5
+4
+1
0
0
+3
+2
4847
32 31
16 15
Signed Quad
Word*
MSB
+n
Sign Bit
Magnitude
7
0
+1
7
7
0
0
0 7
0 7
0
Binary Coded
Decimal (BCD)
BCD Digit n
+n
BCD Digit 1
+1
BCD Digit 0
0
7
0
ASCII
ASCII Character n
+n
ASCII Character 1 ASCII Character 0
0
+1
0 7
7
0
7
0
Packed BCD
Least
Significant Digit
Most
Significant Digit
0
+n
7
+1
0 7
8 7
7
0
0
String
Byte Word n
+3
Byte Word 1
+1
16 15
Byte Word 0
0
+2
31
24 23
0
Pointer
Selector
+8 +7
Offset
+4
+9
+6
+5
+3
+2
+1
+0
0
79
Floating
Point*
Sign Bit
Magnitude
Exponent
NOTE: *Directly supported if the system contains an 80C187.
A1027-0B
Figure 2-23. 80C186 Modular Core Family Supported Data Types
2-38
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.3 INTERRUPTS AND EXCEPTION HANDLING
Interrupts and exceptions alter program execution in response to an external event or an error
condition. An interrupt handles asynchronous external events, for example an NMI. Exceptions
result directly from the execution of an instruction, usually an instruction fault. The user can
cause a software interrupt by executing an “INTn” instruction. The CPU processes software in-
terrupts in the same way that it handles exceptions.
The 80C186 Modular Core responds to interrupts and exceptions in the same way for all devices
within the 80C186 Modular Core family. However, devices within the family may have different
Interrupt Control Units. The Interrupt Control Unit handles all external interrupt sources and pre-
sents them to the 80C186 Modular Core via one maskable interrupt request (see Figure 2-24).
This discussion covers only those areas of interrupts and exceptions that are common to the
80C186 Modular Core family. The Interrupt Control Unit is proliferation-dependent; see Chapter
8, “Interrupt Control Unit,” for additional information.
NMI
Maskable
Interrupt
Request
Interrupt
Control
Unit
External
Interrupt
Sources
CPU
Interrupt
Acknowledge
A1028-0A
Figure 2-24. Interrupt Control Unit
2.3.1 Interrupt/Exception Processing
The 80C186 Modular Core can service up to 256 different interrupts and exceptions. A 256-entry
Interrupt Vector Table (Figure 2-25) contains the pointers to interrupt service routines. Each en-
try consists of four bytes, which contain the Code Segment (CS) and Instruction Pointer (IP) of
the first instruction in the interrupt service routine. Each interrupt or exception is given a type
number, 0 through 255, corresponding to its position in the Interrupt Vector Table. Note that in-
terrupt types 0–31 are reserved for Intel and should not be used by an application program.
2-39
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Memory
Address
Table
Entry
Vector
Definition
Memory
Address
Table
Entry
Vector
Definition
3FE
3FC
CS
IP
2E
2C
2A
CS
IP
Type 255
Type 11 - DMA1
Type 10 - DMA0
Type 9 - Reserved
Type 8 - Timer 0
Type 7 - ESC Opcode
User
Available
CS
IP
28
26
24
22
20
1E
1C
1A
18
16
14
12
10
0E
0C
0A
08
06
04
02
00
82
80
7E
7C
CS
IP
CS
IP
Type 32
Type 31
CS
IP
CS
IP
CS
IP
Reserved
52
50
4E
4C
4A
48
46
44
42
40
3E
3C
3A
38
36
34
32
30
CS
IP
CS
IP
Type 20
Type 6 - Unused
Opcode
CS
IP
CS
IP
Type 19 - Timer 2
Type 18 - Timer 1
Type 17 - Reserved
Type 16 - Numerics
Type 15 - INT3
Type 5 - Array
Bounds
CS
IP
CS
IP
Type 4 - Overflow
Type 3 - Breakpoint
Type 2 - NMI
CS
IP
CS
IP
CS
IP
CS
IP
CS
IP
CS
IP
Type 1 - Single-Step
Type 0 - Divide Error
CS
IP
CS
IP
Type 14 - INT2
CS
IP
Type 13 - INT1
2 Bytes
CS
IP
Type 12 - INT0
CS =Code Segment Value
IP = Instruction Pointer Value
2 Bytes
A1009-02
Figure 2-25. Interrupt Vector Table
When an interrupt is acknowledged, a common event sequence (Figure 2-26) allows the proces-
sor to execute the interrupt service routine.
1. The processor saves a partial machine status by pushing the Processor Status Word onto
the stack.
2-40
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2. The Trap Flag bit and Interrupt Enable bit are cleared in the Processor Status Word. This
prevents maskable interrupts or single step exceptions from interrupting the processor
during the interrupt service routine.
3. The current CS and IP are pushed onto the stack.
4. The CPU fetches the new CS and IP for the interrupt vector routine from the Interrupt
Vector Table and begins executing from that point.
The CPU is now executing the interrupt service routine. The programmer must save (usually by
pushing onto the stack) all registers used in the interrupt service routine; otherwise, their contents
will be lost. To allow nesting of maskable interrupts, the programmer must set the Interrupt En-
able bit in the Processor Status Word.
When exiting an interrupt service routine, the programmer must restore (usually by popping off
the stack) the saved registers and execute an IRET instruction, which performs the following
steps.
1. Loads the return CS and IP by popping them off the stack.
2. Pops and restores the old Processor Status Word from the stack.
The CPU now executes from the point at which the interrupt or exception occurred.
2-41
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Interrupt Enable Bit
Trap Flag
Stack
1
2
PSW
CS
0 0
Processor Status Word
IP
SP
3
Code Segment Register
Instruction Pointer
4
CS
IP
Interrupt
Vector
Table
A1029-0A
Figure 2-26. Interrupt Sequence
Non-Maskable Interrupts
2.3.1.1
The Non-Maskable Interrupt (NMI) is the highest priority interrupt. It is usually reserved for a
catastrophic event such as impending power failure. An NMI cannot be prevented (or masked)
by software. When the NMI input is asserted, the interrupt processing sequence begins after ex-
ecution of the current instruction completes (see “Interrupt Latency” on page 2-45). The CPU au-
tomatically generates a type 2 interrupt vector.
The NMI input is asynchronous. Setup and hold times are given only to guarantee recognition on
a specific clock edge. To be recognized, NMI must be asserted for at least one CLKOUT period
and meet the correct setup and hold times. NMI is edge-triggered and level-latched. Multiple
NMI requests cause multiple NMI service routines to be executed. NMI can be nested in this man-
ner an infinite number of times.
2-42
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.3.1.2
Maskable Interrupts
Maskable interrupts are the most common way to service external hardware interrupts. Software
can globally enable or disable maskable interrupts. This is done by setting or clearing the Inter-
rupt Enable bit in the Processor Status Word.
The Interrupt Control Unit processes the multiple sources of maskable interrupts and presents
them to the core via a single maskable interrupt input. The Interrupt Control Unit provides the
interrupt vector type to the 80C186 Modular Core. The Interrupt Control Unit differs among
members of the 80C186 Modular Core family; see Chapter 8, “Interrupt Control Unit,” for infor-
mation.
2.3.1.3
Exceptions
Exceptions occur when an unusual condition prevents further instruction processing until the ex-
ception is corrected. The CPU handles software interrupts and exceptions in the same way. The
interrupt type for an exception is either predefined or supplied by the instruction.
Exceptions are classified as either faults or traps, depending on when the exception is detected
and whether the instruction that caused the exception can be restarted. Faults are detected and ser-
viced before the faulting instruction can be executed. The return address pushed onto the stack
in the interrupt processing instruction points to the beginning of the faulting instruction. This al-
lows the instruction to be restarted. Traps are detected and serviced immediately after the instruc-
tion that caused the trap. The return address pushed onto the stack during the interrupt processing
points to the instruction following the trapping instruction.
Divide Error — Type 0
A Divide Error trap is invoked when the quotient of an attempted division exceeds the maximum
value of the destination. A divide-by-zero is a common example.
Single Step — Type 1
The Single Step trap occurs after the CPU executes one instruction with the Trap Flag (TF) bit set
in the Processor Status Word. This allows programs to execute one instruction at a time. Inter-
rupts are not generated after prefix instructions (e.g., REP), after instructions that modify segment
registers (e.g., POP DS) or after the WAIT instruction. Vectoring to the single-step interrupt ser-
vice routine clears the Trap Flag bit. An IRET instruction in the interrupt service routine restores
the Trap Flag bit to logic “1” and transfers control to the next instruction to be single-stepped.
2-43
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Breakpoint Interrupt — Type 3
The Breakpoint Interrupt is a single-byte version of the INT instruction. It is commonly used by
software debuggers to set breakpoints in RAM. Because the instruction is only one byte long, it
can substitute for any instruction.
Interrupt on Overflow — Type 4
The Interrupt on Overflow trap occurs if the Overflow Flag (OF) bit is set in the Processor Status
Word and the INT0 instruction is executed. Interrupt on Overflow is a common method for han-
dling arithmetic overflows conditionally.
Array Bounds Check — Type 5
An Array Bounds trap occurs when the array index is outside the array bounds during execution
of the BOUND instruction (see Appendix A, “80C186 Instruction Set Additions and Exten-
sions”).
Invalid Opcode — Type 6
Execution of an undefined opcode causes an Invalid Opcode trap.
Escape Opcode — Type 7
The Escape Opcode fault is used for floating point emulation. With 80C186 Modular Core family
members, this fault is enabled by setting the Escape Trap (ET) bit in the Relocation Register (see
Chapter 4, “Peripheral Control Block”). When a floating point instruction is executed with the
Escape Trap bit set, the Escape Opcode fault occurs, and the Escape Opcode service routine em-
ulates the floating point instruction. If the Escape Trap bit is cleared, the CPU sends the floating
point instruction to an external 80C187.
80C188 Modular Core Family members do not support the 80C187 interface and always generate
the Escape Opcode Fault. The 80C186XL will generate the Escape Opcode Fault regardless of
the state of the Escape Trap bit unless it is in Numerics Mode.
Numerics Coprocessor Fault — Type 16
The Numerics Coprocessor fault is caused by an external 80C187 numerics coprocessor. The
80C187 reports the exception by asserting the ERROR pin. The 80C186 Modular Core checks
the ERROR pin only when executing a numerics instruction. A Numerics Coprocessor Fault in-
dicates that the previous numerics instruction caused the exception. The 80C187 saves the ad-
dress of the floating point instruction that caused the exception. The return address pushed onto
the stack during the interrupt processing points to the numerics instruction that detected the ex-
ception. This way, the last numerics instruction can be restarted.
2-44
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.3.2 Software Interrupts
A Software Interrupt is caused by executing an “INTn” instruction. The n parameter corresponds
to the specific interrupt type to be executed. The interrupt type can be any number between 0 and
255. If the n parameter corresponds to an interrupt type associated with a hardware interrupt
(NMI, Timers), the vectors are fetched and the routine is executed, but the corresponding bits in
the Interrupt Status register are not altered.
The CPU processes software interrupts and exceptions in the same way. Software interrupts, ex-
ceptions and traps cannot be masked.
2.3.3 Interrupt Latency
Interrupt latency is the amount of time it takes for the CPU to recognize the existence of an inter-
rupt. The CPU generally recognizes interrupts only between instructions or on instruction bound-
aries. Therefore, the current instruction must finish executing before an interrupt can be
recognized.
The worst-case 80C186 instruction execution time is an integer divide instruction with segment
override prefix. The instruction takes 69 clocks, assuming an 80C186 Modular Core family mem-
ber and a zero wait-state external bus. The execution time for an 80C188 Modular Core family
member may be longer, depending on the queue.
This is one factor in determining interrupt latency. In addition, the following are also factors in
determining maximum latency:
1. The CPU does not recognize the Maskable Interrupt unless the Interrupt Enable bit is set.
2. The CPU does not recognize interrupts during HOLD.
3. Once communication is completely established with an 80C187, the CPU does not
recognize interrupts until the numerics instruction is finished.
The CPU can recognize interrupts only on valid instruction boundaries. A valid instruction
boundary usually occurs when the current instruction finishes. The following is a list of excep-
tions:
1. MOVs and POPs referencing a segment register delay the servicing of interrupts until
after the following instruction. The delay allows a 32-bit load to the SS and SP without an
interrupt occurring between the two loads.
2. The CPU allows interrupts between repeated string instructions. If multiple prefixes
precede a string instruction and the instruction is interrupted, only the one prefix
preceding the string primitive is restored.
3. The CPU can be interrupted during a WAIT instruction. The CPU will return to the WAIT
instruction.
2-45
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.3.4 Interrupt Response Time
Interrupt response time is the time from the CPU recognizing an interrupt until the first instruction
in the service routine is executed. Interrupt response time is less for interrupts or exceptions
which supply their own vector type. The maskable interrupt has a longer response time because
the vector type must be supplied by the Interrupt Control Unit (see Chapter 8, “Interrupt Control
Unit”).
Figure 2-27 shows the events that dictate interrupt response time for the interrupts that supply
their type. Note that an on-chip bus master, such as the DRAM Refresh Unit, can make use of
idle bus cycles. This can increase interrupt response time.
Clocks
Idle
5
4
5
Read IP
Idle
Read CS
Idle
4
4
4
3
4
4
5
Push Flags
Idle
Push CS
Push IP
Idle
First Instruction Fetch
From Interrupt Routine
Total 42
A1030-0A
Figure 2-27. Interrupt Response Factors
2.3.5 Interrupt and Exception Priority
Interrupts can be recognized only on valid instruction boundaries. If an NMI and a maskable in-
terrupt are both recognized on the same instruction boundary, NMI has precedence. The
maskable interrupt will not be recognized until the Interrupt Enable bit is set and it is the highest
priority.
2-46
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Only the single step exception can occur concurrently with another exception. At most, two ex-
ceptions can occur at the same instruction boundary and one of those exceptions must be the sin-
gle step. Single step is a special case; it is discussed on page 2-48. Ignoring single step (for now),
only one exception can occur at any given instruction boundary.
An exception has priority over both NMI and the maskable interrupt. However, a pending NMI
can interrupt the CPU at any valid instruction boundary. Therefore, NMI can interrupt an excep-
tion service routine. If an exception and NMI occur simultaneously, the exception vector is taken,
then is followed immediately by the NMI vector (see Figure 2-28). While the exception has high-
er priority at the instruction boundary, the NMI interrupt service routine is executed first.
F = 1
NMI
Divide Error
Divide
Push PSW, CS, IP
Fetch Divide Error Vector
Push PSW, CS, IP
Fetch NMI Vector
Execute NMI
Service Routine
IRET
Execute Divide
Service Routine
IRET
A1031-0A
Figure 2-28. Simultaneous NMI and Exception
2-47
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Single step priority is a special case. If an interrupt (NMI or maskable) occurs at the same instruc-
tion boundary as a single step, the interrupt vector is taken first, then is followed immediately by
the single step vector. However, the single step service routine is executed before the interrupt
service routine (see Figure 2-29). If the single step service routine re-enables single step by exe-
cuting the IRET, the interrupt service routine will also be single stepped. This can severely limit
the real-time response of the CPU to an interrupt.
To prevent the single-step routine from executing before a maskable interrupt, disable interrupts
while single stepping an instruction, then enable interrupts in the single step service routine. The
maskable interrupt is serviced from within the single step service routine and that interrupt ser-
vice routine is not single-stepped. To prevent single stepping before an NMI, the single-step ser-
vice routine must compare the return address on the stack to the NMI vector. If they are the same,
return to the NMI service routine immediately without executing the single step service routine.
NMI
Trap Flag = 1
Instruction
Push PSW, CS, IP
Fetch Divide Error Vector
Trap Flag = 0
Push PSW, CS, IP
Fetch Single Step Vector
Execute Single Step
Service Routine
IRET
Trap Flag = ???
A1032-0A
Figure 2-29. Simultaneous NMI and Single Step Interrupts
The most complicated case is when an NMI, a maskable interrupt, a single step and another ex-
ception are pending on the same instruction boundary. Figure 2-30 shows how this case is prior-
itized by the CPU. Note that if the single-step routine sets the Trap Flag (TF) bit before executing
the IRET instruction, the NMI routine will also be single stepped.
2-48
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
Interrupt Enable Bit (IE) = 1
Trap Flag (TF) = 1
NMI
Divide
Timer Interrupt
Push PSW, CS, IP
Fetch Divide Error Vector
Interrupt Enable Bit (IE) = 0
Trap Flag (TF) = 0
Interrupt Enable Bit (IE) = 0
Trap Flag (TF) = 0
Push PSW, CS, IP
Fetch NMI Vector
Push PSW, CS, IP
Fetch Single Step Vector
Interrupt Enable Bit (IE) = 0
Trap Flag (TF) = 0
Execute Single Step
Service Routine
IRET
Interrupt Enable Bit (IE) = 0
Trap Flag (TF) = ???
Interrupt Enable Bit (IE) = 1
Trap Flag (TF) = X
Push PSW, CS, IP
Fetch Single Step Vector
Interrupt Enable Bit (IE) = 1
Trap Flag (TF) = X
Execute Single Step Service Routine
IRET
A1034-0A
Figure 2-30. Simultaneous NMI, Single Step and Maskable Interrupt
2-49
3
Bus Interface Unit
CHAPTER 3
BUS INTERFACE UNIT
The Bus Interface Unit (BIU) generates bus cycles that prefetch instructions from memory, pass
data to and from the execution unit, and pass data to and from the integrated peripheral units.
The BIU drives address, data, status and control information to define a bus cycle. The start of a
bus cycle presents the address of a memory or I/O location and status information defining the
type of bus cycle. Read or write control signals follow the address and define the direction of data
flow. A read cycle requires data to flow from the selected memory or I/O device to the BIU. In a
write cycle, the data flows from the BIU to the selected memory or I/O device. Upon termination
of the bus cycle, the BIU latches read data or removes write data.
3.1 MULTIPLEXED ADDRESS AND DATA BUS
The BIU has a combined address and data bus, commonly referred to as a time-multiplexed bus.
Time multiplexing address and data information makes the most efficient use of device package
pins. A system with address latching provided within the memory and I/O devices can directly
connect to the address/data bus (or local bus). The local bus can be demultiplexed with a single
set of address latches to provide non-multiplexed address and data information to the system.
3.2 ADDRESS AND DATA BUS CONCEPTS
The programmer views the memory or I/O address space as a sequence of bytes. Memory space
consists of 1 Mbyte, while I/O space consists of 64 Kbytes. Any byte can contain an 8-bit data
element, and any two consecutive bytes can contain a 16-bit data element (identified as a word).
bus cycles are used for examples and illustration.
3.2.1 16-Bit Data Bus
The memory address space on a 16-bit data bus is physically implemented by dividing the address
space into two banks of up to 512 Kbytes each (see Figure 3-1). One bank connects to the lower
half of the data bus and contains even-addressed bytes (A0=0). The other bank connects to the
upper half of the data bus and contains odd-addressed bytes (A0=1). Address lines A19:1 select
a specific byte within each bank. A0 and Byte High Enable (BHE) determine whether one bank
or both banks participate in the data transfer.
3-1
BUS INTERFACE UNIT
Physical Implementation
of the Address Space for
16-Bit Systems
Physical Implementation
of the Address Space for
8-Bit Systems
1 MByte
512 KBytes
512 KBytes
FFFFF
FFFFE
FFFFF
FFFFD
FFFFE
FFFFC
2
1
0
5
3
1
4
2
0
A19:0
A19:1
D15:8
BHE
D7:0
A0
A1100-0A
Figure 3-1. Physical Data Bus Models
ure 3-2). A0 low enables the lower bank, while BHE high disables the upper bank. The data value
from the upper bank is ignored during a bus read cycle. BHE high prevents a write operation from
destroying data in the upper bank.
Byte transfers to odd addresses transfer information over the upper half of the data bus (see Figure
3-2). BHE low enables the upper bank, while A0 high disables the lower bank. The data value
from the lower bank is ignored during a bus read cycle. A0 high prevents a write operation from
destroying data in the lower bank.
an even address), information is transferred over both halves of the data bus (see Figure 3-3).
A19:1 select the appropriate byte within each bank. A0 and BHE drive low to enable both banks
simultaneously.
Odd-addressed word accesses require the BIU to split the transfer into two byte operations (see
Figure 3-4). The first operation transfers data over the upper half of the bus, while the second op-
eration transfers data over the lower half of the bus. The BIU automatically executes the two-byte
sequence whenever an odd-addressed word access is performed.
3-2
BUS INTERFACE UNIT
(X)
(X + 1)
A19:1
D15:8
D7:0
BHE
A0
(Low)
(Low)
A1107-0A
Figure 3-3. 16-Bit Data Bus Even Word Transfers
During a byte read operation, the BIU floats the entire 16-bit data bus, even though the transfer
occurs on only one half of the bus. This action simplifies the decoding requirements for read-only
devices (e.g., ROM, EPROM, Flash). During the byte read, an external device can drive both
halves of the bus, and the BIU automatically accesses the correct half. During the byte write op-
eration, the BIU drives both halves of the bus. Information on the half of the bus not involved in
the transfer is indeterminate. This action requires that the appropriate bank (defined by BHE or
A0 high) be disabled to prevent destroying data.
3-4
BUS INTERFACE UNIT
First Bus Cycle
Y
X
(X + 1)
A0
(High)
A19:1
D15:8
D7:0
BHE
(Low)
Second Bus Cycle
Y + 1
X + 1
(Y)
X
A0
(Low)
A19:1
D15:8
D7:0
BHE
(High)
A1108-0A
Figure 3-4. 16-Bit Data Bus Odd Word Transfers
3.2.2 8-Bit Data Bus
The memory address space on an 8-bit data bus is physically implemented as one bank of 1 Mbyte
(see Figure 3-1 on page 3-2). Address lines A19:0 select a specific byte within the bank. Unlike
transfers with a 16-bit bus, byte and word transfers (to even or odd addresses) all transfer data
over the same 8-bit bus.
Byte transfers to even or odd addresses transfer information in one bus cycle. Word transfers to
even or odd addresses transfer information in two bus cycles. The BIU automatically converts the
word access into two consecutive byte accesses, making the operation transparent to the program-
mer.
3-5
BUS INTERFACE UNIT
For word transfers, the word address defines the first byte transferred. The second byte transfer
occurs from the word address plus one. Figure 3-5 illustrates a word transfer on an 8-bit bus in-
terface.
Second Bus Cycle
(X + 1)
First Bus Cycle
(X)
A19:0
D7:0
A19:0
D7:0
A1109-0A
Figure 3-5. 8-Bit Data Bus Word Transfers
3.3 MEMORY AND I/O INTERFACES
The CPU can interface with 8- and 16-bit memory and I/O devices. Memory devices exchange
information with the CPU during memory read, memory write and instruction fetch bus cycles.
I/O (peripheral) devices exchange information with the CPU during memory read, memory write,
I/O read, I/O write and interrupt acknowledge bus cycles. Memory-mapped I/O refers to periph-
eral devices that exchange information during memory cycles. Memory-mapped I/O allows the
full power of the instruction set to be used when communicating with peripheral devices.
I/O read and I/O write bus cycles use a separate I/O address space. Only IN and OUT instructions
can access I/O address space, and information must be transferred between the peripheral device
and the AX register. The first 256 bytes (0–255) of I/O space can be accessed directly by the I/O
instructions. The entire 64 Kbyte I/O address space can be accessed only indirectly, through the
DX register. I/O instructions always force address bits A19:16 to zero.
Interrupt acknowledge, or INTA, bus cycles access an I/O device intended to increase interrupt
input capability. Valid address information is not generated as part of the INTA bus cycle, and
data is transferred only over the lower bank (16-bit device).
3-6
BUS INTERFACE UNIT
3.3.1 16-Bit Bus Memory and I/O Requirements
A 16-bit bus has certain assumptions that must be met to operate properly. Memory used to store
instruction operands (i.e., the program) and immediate data must be 16 bits wide. Instruction
prefetch bus cycles require that both banks be used. The lower bank contains the even bytes of
code and the upper bank contains the odd bytes of code.
Memory used to store interrupt vectors and stack data must be 16 bits wide. Memory address
space between 0H and 3FFH (1 Kbyte) holds the starting location of an interrupt routine. In re-
sponse to an interrupt, the BIU fetches two consecutive, even-addressed words from this 1 Kbyte
address space. Stack pushes and pops always write or read even-addressed word data.
3.3.2 8-Bit Bus Memory and I/O Requirements
An 8-bit bus interface has no restrictions on implementing the memory or I/O interfaces. All
automatically execute two consecutive byte transfers.
3.4 BUS CYCLE OPERATION
The BIU executes a bus cycle to transfer data between any of the integrated units and any external
memory or I/O devices (see Figure 3-6). A bus cycle consists of a minimum of four CPU clocks
known as “T-states.” A T-state is bounded by one falling edge of CLKOUT to the next falling
edge of CLKOUT (see Figure 3-7). Phase 1 represents the low time of the T-state and starts at the
high-to-low transition of CLKOUT. Phase 2 represents the high time of the T-state and starts at
the low-to-high transition of CLKOUT. Address, data and control signals generated by the BIU
go active and inactive at different phases within a T-state.
3-7
BUS INTERFACE UNIT
T4
T1
T2
T3
T4
CLKOUT
ALE
Valid Status
Address
S2:0
Data
AD15:0
RD / WR
A1507-0A
Figure 3-6. Typical Bus Cycle
TN
Rising
Edge
Falling
Edge
CLKOUT
Phase 1
(Low Phase)
Phase 2
(High Phase)
A1111-0A
Figure 3-7. T-State Relation to CLKOUT
Figure 3-8 shows the BIU state diagram. Typically a bus cycle consists of four consecutive T-
states labeled T1, T2, T3 and T4. A TI (idle) state occurs when no bus cycle is pending. Multiple
T3 states occur to generate wait states. The TW symbol represents a wait state.
The operation of a bus cycle can be separated into two phases:
• Address/Status Phase
• Data Phase
3-8
BUS INTERFACE UNIT
The address/status phase starts just before T1 and continues through T1. The data phase starts at
T2 and continues through T4. Figure 3-9 illustrates the T-state relationship of the two phases.
T4
Bus Ready
Request Pending
HOLD Deasserted
Halt Bus Cycle
T1
T2
T3
Bus Not
Ready
Request Pending
HOLD Deasserted
Bus Ready
No Request Pending
HOLD Deasserted
TI
RES#
Asserted
HOLD Asserted
A1533-02
Figure 3-8. BIU State Diagram
3-9
BUS INTERFACE UNIT
T4
T3
T4
T1
T2
or TI
or TW
or TI
CLKOUT
Address/
Status Phase
Data Phase
A1113-0A
Figure 3-9. T-State and Bus Phases
3.4.1 Address/Status Phase
T-state just prior to T1. Either T4 or TI precedes T1, depending on the operation of the previous
bus cycle (see Figure 3-8 on page 3-9).
ALE provides a strobe to latch physical address information. Address is presented on the multi-
plexed address/data bus during T1 (see Figure 3-10). The falling edge of ALE occurs during the
middle of T1 and provides a strobe to latch the address. Figure 3-11 presents a typical circuit for
latching addresses.
The status signals (S2:0) define the type of bus cycle (Table 3-1). S2:0 remain valid until phase
1 of T3 (or the last TW, when wait states occur). The circuit shown in Figure 3-11 can also be
used to extend S2:0 beyond the T3 (or TW) state.
3-10
BUS INTERFACE UNIT
T4
T1
T2
5
or TI
CLKOUT
ALE
4
1
2
3
6
AD15:0
A19:16
Valid
Address
Valid
S2:0
Valid
BHE
NOTES:
1. T
2. T
3. T
4. T
5. T
6. T
T
: Clock high to ALE high, S2:0 valid.
CHLH CHSV
: Clock low to address valid, BHE valid.
: Address valid to ALE low (address setup to ALE).
: Clock high to ALE low.
: Clock low to address invalid (address hold from clock low).
: ALE low to address invalid (address hold from ALE).
CLAV
AVLL
CHLL
CLAZ
LLAX
A1509-0A
Figure 3-10. Address/Status Phase Signal Relationships
3-11
BUS INTERFACE UNIT
Latched
Address Signals
Signals From CPU
4
3
I
A19:16
S2:0
4
LA19:16
O
O
I
3
LS2:0
STB
OE
8
8
AD15:8
I
8
LA15:8
O
O
STB
OE
I
AD7:0
ALE
8
LA7:0
STB
OE
A1102-0A
Figure 3-11. Demultiplexing Address Information
Table 3-1. Bus Cycle Types
Status Bit
Operation
S2
S1
S0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Interrupt Acknowledge
I/O Read
I/O Write
Halt
Instruction Prefetch
Memory Read
Memory Write
Idle (passive)
3-12
BUS INTERFACE UNIT
3.4.2 Data Phase
Figure 3-12 shows the timing relationships for the data phase of a bus cycle. The only bus cycle
type that does not have a data phase is a bus halt. During the data phase, the bus transfers infor-
mation between the internal units and the memory or peripheral device selected during the ad-
dress/status phase. Appropriate control signals become active to coordinate the transfer of data.
The data phase begins at phase 1 of T2 and continues until phase 2 of T4 or TI. The length of the
data phase varies depending on the number of wait states. Wait states occur after T3 and before
T4 or TI.
3.4.3 Wait States
Wait states extend the data phase of the bus cycle. Memory and I/O devices that cannot provide
or accept data in the minimum four CPU clocks require wait states. Figure 3-13 shows a typical
The bus ready inputs (ARDY and SRDY) and the Chip-Select Unit control bus cycle wait states.
Only the bus ready inputs are described in this chapter. (See Chapter 6, “Chip-Select Unit,” for
additional information.)
Figure 3-14 shows a simplified block diagram of the ARDY and SRDY inputs. Either ARDY or
SRDY active signals a bus ready condition; therefore, both pins must be inactive to signal a not-
ready condition. Depending on the size and characteristics of the system, ready implementation
can take one of two approaches: normally not-ready or normally ready.
3-13
BUS INTERFACE UNIT
T3
T4
T2
or TW
or TI
CLKOUT
RD/ WR
4
2
1
6
7
AD15:0
Write
Valid Write Data
5
3
Valid
Read Data
AD15:0
Read
S2:0
NOTES:
1. T
2. T
3. T
4. T
5. T
6. T
7. T
T
: Clock low to valid RD/WR active, write data valid.
CLRL/CLWL, CLOV
CLSH
DVCL
CLRH/CLWH
CLDX
WHDX
RHAV
: Clock low to status inactive.
: Data input valid to clock low.
: Clock valid to RD/WR inactive.
: Data input HOLD from clock low.
: Output data HOLD from WR high.
: Bus no longer floating from RD high.
Figure 3-12. Data Phase Signal Relationships
3-14
BUS INTERFACE UNIT
A normally not-ready system is one in which ARDY and SRDY remain low at all times except
to signal a ready condition. For any bus cycle, only the selected device drives either ready input
high to complete the bus cycle. The circuit shown in Figure 3-15 illustrates a simple circuit to
generate a normally not-ready signal. Note that if no device is selected the bus remains not-
ready indefinitely. Systems with many slow devices that cannot operate at the maximum bus
bandwidth usually implement a normally not-ready signal.
The start of a bus cycle clears the wait state module and forces ARDY low. After every rising
edge of CLKOUT, INPUT1 and INPUT2 are shifted through the module and eventually drive
ARDY high. Assuming INPUT1 and INPUT2 are valid prior to phase 2 of T2, no delay through
the module causes one wait state. Each additional clock delay through the module generates one
additional wait state. Two inputs are used to establish different wait state conditions. The same
circuit works for SRDY, but no delay through the module results in no wait states.
CS1
CS2
Wait State Module
Input 1
Input 2
CS3
CS4
Out
READY
ALE
Clear
Clock
CLKOUT
A1080-0A
Figure 3-15. Generating a Normally Not-Ready Bus Signal
A normally ready signal remains high at all times except when the selected device needs to signal
a not-ready condition. For any bus cycle, only the selected device drives the ready input (or in-
puts) low to delay the completion of the bus cycle. The circuit shown in Figure 3-16 illustrates a
simple circuit to generate a normally ready signal. Note that if no device is selected the bus re-
mains ready. Systems that have few or no devices requiring wait states usually implement a nor-
mally ready signal.
The start of a bus cycle preloads a zero shifter and forces SRDY active (high). SRDY remains
active if neither CS1 or CS2 goes low. Should either CS1 or CS2 go low, zeros are shifted out on
every rising edge of CLKOUT, causing SRDY to go inactive. At the end of the shift pattern,
SRDY is forced active again. Assuming CS1 and CS2 are active just prior to phase 2 of T2, shift-
ing one zero through the module causes one wait state. Each additional zero shifted through the
module generates one wait state. The same circuit works for ARDY, but shifting one zero through
the module generates two wait states.
3-16
BUS INTERFACE UNIT
Wait State Module
Enable
CS1
CS2
Out
READY
ALE
Load
CLKOUT
Clock
A1081-0A
Figure 3-16. Generating a Normally Ready Bus Signal
The ARDY input has two major timing concerns that can affect whether a normally ready or nor-
mally not-ready signal may be required. Two latches capture the state of the ARDY input (see
Figure 3-14 on page 3-15). The first latch captures ARDY on the phase 2 clock edge. The second
latch captures ARDY and the result of first latch on the phase 1 clock edge. The following items
define the requirements of the ARDY input to meet ready or not-ready bus conditions.
• The bus is ready if both of these two conditions are true:
— ARDY is active prior to the phase 2 clock edge, and
— ARDY remains active after the phase 1 clock edge.
• The bus is not-ready if either of these two conditions is true:
— ARDY is inactive prior to the phase 2 clock edge, or
— ARDY is inactive prior to the phase 1 clock edge.
A single latch captures the state of the SRDY input (see Figure 3-14 on page 3-15). SRDY must
be valid by the phase 1 clock edge. The following items define the requirements of the SRDY
input to meet ready or not-ready bus conditions.
• The bus is not-ready if SRDY is inactive prior to the phase 1 clock edge.
A normally not-ready system must generate a valid ARDY input at phase 2 of T2 or a valid SRDY
input at phase 1 of T3 to prevent wait states. If it cannot, then running without wait states requires
a normally ready system. Figure 3-17 illustrates the timing necessary to prevent wait states in a
normally not-ready system. Figure 3-17 also shows how to terminate a bus cycle with wait states
in a normally not-ready system.
3-17
BUS INTERFACE UNIT
T2
or T3
or TW
T3
or TW
T4
CLKOUT
2
1
3
ARDY
SRDY
In a Normally-Not-Ready system, wait states are inserted until (1 or 2) and 3 are met.
1. T
2. T
3. T
: ARDY active to clock high (assumes ARDY remains active until 3).
: SRDY active to clock low.
ARYCH
SRYCL
T
: ARDY and SRDY hold from clock low.
CLARX, CLSRY
Failure to meet SRDY setup and hold can cause a device failure
(i.e., the bus hangs or operates inappropriately).
!
A1511-0A
Figure 3-17. Normally Not-Ready System Timing
A valid not-ready input can be generated as late as phase 1 of T3 to insert wait states in a normally
ready system. A normally not-ready system must run wait states if the not-ready condition cannot
be met in time. Figure 3-18 illustrates the minimum and maximum timing necessary to insert wait
states in a normally ready system. Figure 3-18 also shows how to terminate a bus cycle with wait
states in a normally ready system.
The BIU can execute an indefinite number of wait states. However, bus cycles with large numbers
of wait states limit the performance of the CPU and the integrated peripherals. CPU performance
suffers because the instruction prefetch queue cannot be kept full. Integrated peripheral perfor-
3.4.4 Idle States
Under most operating conditions, the BIU executes consecutive (back-to-back) bus cycles. How-
ever, several conditions cause the BIU to become idle. An idle condition occurs between bus cy-
cles (see Figure 3-8 on page 3-9) and may last an indefinite period of time, depending on the
instruction sequence.
3-18
BUS INTERFACE UNIT
T2
T3
TW
T4
CLKOUT
ARDY
2
1
In a Normally-Ready system, a wait state will be inserted when 1 & 2 are met.
(Assumes SRDY is low.)
1. T
2. T
: ARDY low to clock high
ARYCH
: Clock high to ARDY high (ARDY inactive hold time)
ARYCHL
T2
T3
TW
T4
CLKOUT
2
1
ARDY
SRDY
Alternatively, in a Normally-Ready system, a wait state will be inserted
when1 & 2 are met for SRDY and ARDY.
1. T
2. T
, T
: ARDY and SRDY low to clock low
: ARDY and SRDY low from clock low
ARYCL SRYCL
, T
CHARX CLSRY
Failure to meet ARDY and SRDY setup and hold can cause a device failure
(i.e., the bus hangs or operates inappropriately).
!
A1512-0A
Figure 3-18. Normally Ready System Timings
Conditions causing the BIU to become idle include the following.
• The instruction prefetch queue is full.
• An effective address calculation is in progress.
• The bus cycle inherently requires idle states (e.g., interrupt acknowledge, locked opera-
tions).
• Instruction execution forces idle states (e.g., HLT, WAIT).
3-19
BUS INTERFACE UNIT
An idle bus state may or may not drive the bus. An idle bus state following a bus read cycle con-
tinues to float the bus. An idle bus state following a bus write cycle continues to drive the bus.
The BIU drives no control strobes active in an idle state except to indicate the start of another bus
cycle.
3.5 BUS CYCLES
There are four basic types of bus cycles: read, write, interrupt acknowledge and halt. Interrupt
acknowledge and halt bus cycles define special bus operations and require separate discussions.
Read bus cycles include memory, I/O and instruction prefetch bus operations. Write bus cycles
include memory and I/O bus operations. All read and write bus cycles have the same basic format.
The following sections present timing equations containing symbols found in the data sheet. The
timing equations provide information necessary to start a worst-case design analysis.
3.5.1 Read Bus Cycles
Figure 3-19 illustrates a typical read cycle. Table 3-2 lists the three types of read bus cycles.
Table 3-2. Read Bus Cycle Types
Status Bit
Bus Cycle Type
S2
S1
S0
0
0
1
Read I/O — Initiated by the Execution Unit for IN, OUT, INS, OUTS instructions
Access Unit”).
1
1
0
0
0
1
prefetch queue.
Read Memory — A19:0 select the desired byte or word memory location.
Figure 3-20 illustrates a typical 16-bit interface connection to a read-only device interface. The
same example applies to an 8-bit bus system, except that no devices connect to an upper bus. Four
parameters (Table 3-3) must be evaluated when determining the compatibility of a memory (or
I/O) device. TADLTCH defines the delay through the address latch.
Table 3-3. Read Cycle Critical Timing Parameters
Memory Device
Parameter
Description
Equation
TOE
TACC
TCE
Output enable (RD low) to data valid
Address valid to data valid
2T
3T
3T
– T
– T
– T
– T
DVCL
CLCL
CLCL
CLCL
RHAV
CLRL
CLAV
–T
T
ADLTCH – DVCL
Chip enable (UCS) to data valid
Output disable (RD high) to output float
– T
CLIS
CLOV2
TDF
T
3-20
BUS INTERFACE UNIT
TOE, TACC and TCE define the maximum data access requirements for the memory device. These
device parameters must be less than the value calculated in the equation column. An equal to or
greater than result indicates that wait states must be inserted into the bus cycle.
cycle begins. A TDF value greater than the equation result indicates a buffer fight. A buffer fight
means two (or more) devices are driving the bus at the same time. This can lead to short circuit
conditions, resulting in large current spikes and possible device damage.
TRHAX cannot be lengthened (other than by slowing the clock rate). To resolve a buffer fight con-
dition, choose a faster device or buffer the AD bus (see “Buffering the Data Bus” on page 3-34).
T1
T2
T3
T4
CLKOUT
S2:0
Status Valid
ALE
Address Valid
A18:16 = 0, A19=Valid Status
Valid
A19:16
A15:8
BHE
RFSH
A15:0
[AD7:0]
Data
Valid
Address
Valid
RD
DT/R
DEN
A1046-0A
Figure 3-19. Typical Read Bus Cycle
3-21
BUS INTERFACE UNIT
3.5.1.1
Refresh Bus Cycles
A refresh bus cycle operates similarly to a normal read bus cycle except for the following:
• For a 16-bit data bus, address bit A0 and BHE drive to a 1 (high) and the data value on the
bus is ignored.
• For an 8-bit data bus, address bit A0 drives to a 1 (high) and RFSH is driven active (low).
The data value on the bus is ignored. RFSH has the same bus timing as BHE.
UCS
CE
AD7:0
O
A
0-7
27C256
LA15:1
RD
0-14
OE
OE
A
0-14
0-7
27C256
AD15:8
O
CE
Note: A and BHE are not used.
0
A1105-0A
Figure 3-20. Read-Only Device Interface
3.5.2 Write Bus Cycles
Figure 3-21 illustrates a typical write bus cycle. The bus cycle starts with the transition of ALE
high and the generation of valid status bits S2:0. The bus cycle ends when WR transitions high
(inactive), although data remains valid for one additional clock. Table 3-4 lists the two types of
write bus cycles.
3-22
BUS INTERFACE UNIT
T1
T2
T3
T4
CLKOUT
S2:0
Status Valid
ALE
Address Valid
A18:16 = 0, A19=Valid Status
A19:16
BHE
[A15:8]
Valid
A15:0
[AD7:0]
Address
Valid
Data Valid
WR
DT/R
DEN
A1047-0A
Figure 3-21. Typical Write Bus Cycle
Table 3-4. Write Bus Cycle Types
Status Bits
S1
Bus Cycle Type
S2
S0
0
1
0
Write I/O — Initiated by executing IN, OUT, INS, OUTS instructions or by the
DMA Unit. A15:0 select the desired I/O port. A19:16 are driven to zero (see
1
1
0
Write Memory —Initiated by any of the Byte/ Word memory instructions or the
DMA Unit. A19:0 selects the desired byte or word memory location.
Figure 3-22 illustrates a typical 16-bit interface connection to a read/write device. Write bus cy-
cles have many parameters that must be evaluated in determining the compatibility of a memory
(or I/O) device. Table 3-5 lists some critical write bus cycle parameters.
3-23
BUS INTERFACE UNIT
Most memory and peripheral devices latch data on the rising edge of the write strobe. Address,
chip-select and data must be valid (set up) prior to the rising edge of WR. TAW, TCW and TDW de-
fine the minimum data setup requirements. The value calculated by their respective equations
must be greater than the device requirements. To increase the calculated value, insert wait states.
A0:14
OE
LA15:1
RD
AD7:0
I/O1:8
WE
CS1
LA0
WR
A0:14
OE
BHE
AD15:8
I/O1:8
WE
LCS
CS1
A1106-0A
Figure 3-22. 16-Bit Bus Read/Write Device Interface
3-24
BUS INTERFACE UNIT
The minimum device data hold time (from WR high) is defined by TDH. The calculated value
must be greater than the minimum device requirements; however, the value can be changed only
by decreasing the clock rate.
Table 3-5. Write Cycle Critical Timing Parameters
Memory Device
Parameter
Description
Equation
TWC
TAW
TCW
TWR
TDW
TDH
TWP
Write cycle time
4T
3T
3T
CLCL
Address valid to end of write strobe (WR high)
Chip enable (LCS) to end of write strobe (WR high)
Write recover time
– T
ADLTCH
CLCL
CLCL
TWHLH
2T
Data valid to write strobe (WR high)
Data hold from write strobe (WR high)
Write pulse width
CLCL
TWHDX
TWLWH
TWC and TWP define the minimum time (maximum frequency) a device can process write bus cy-
cles. TWR determines the minimum time from the end of the current write cycle to the start of the
next write cycle. All three parameters require that calculated values be greater than device re-
quirements. The calculated TWC and TWP values increase with the insertion of wait states. The cal-
culated TWR value, however, can be changed only by decreasing the clock rate.
3.5.3 Interrupt Acknowledge Bus Cycle
Interrupt expansion is accomplished by interfacing the Interrupt Control Unit with a peripheral
device such as the 82C59A Programmable Interrupt Controller. (See Chapter 8, “Interrupt Con-
trol Unit,” for more information.) The BIU controls the bus cycles required to fetch vector infor-
mation from the peripheral device, then passes the information to the CPU. These bus cycles,
collectively known as Interrupt Acknowledge bus cycles, operate similarly to read bus cycles.
However, instead of generating RD to enable the peripheral, the INTA signal is used. Figure 3-23
illustrates a typical Interrupt Acknowledge (or INTA) bus cycle.
An Interrupt Acknowledge bus cycle consists of two consecutive bus cycles. LOCK is generated
to indicate the sequential bus operation. The second bus cycle strobes vector information only
from the lower half of the bus (D7:0). In a 16-bit bus system, the upper half of the bus (D15:8)
floats.
3-25
BUS INTERFACE UNIT
T1
T2
T3
T4
TI
TI
T1
T2
T3
T4
CLKOUT
ALE
S2:0
INTA0
INTA1
Note
Note
AD15:0
[AD7:0]
Note
LOCK
DT/R
DEN
A19:16
[A15:8]
A15:8 are unknown
A19:16 are driven low
BHE
RD, WR
NOTE: Vector Type is read from AD7:0 only.
INTA occurs during T2 in slave mode.
A1048-0A
Figure 3-23. Interrupt Acknowledge Bus Cycle
3-26
BUS INTERFACE UNIT
Figure 3-24 shows a typical 82C59A interface example. Bus ready must be provided to terminate
both bus cycles in the interrupt acknowledge sequence.
NOTE
Due to an internal condition, external ready is ignored if the device is
configured in Cascade mode and the Peripheral Control Block (PCB) is
located at 0000H in I/O space. In this case, wait states cannot be added to
interrupt acknowledge bus cycles. However, you can add wait states to
interrupt acknowledge cycles if the PCB is located at any other address.
3.5.3.1
System Design Considerations
Although ALE is generated for both bus cycles, the BIU does not drive valid address information.
Actually, all address bits except A19:16 float during the time ALE becomes active (on both 8-
and 16-bit bus devices). Address-decoding circuitry must be disabled for Interrupt Acknowledge
bus cycles to prevent erroneous operation.
Processor
82C59A
INTA0
INTA
INT
IR0
IR7
INT0
RD
RD
WR
CS
A0
WR
PCS0
LA1
D7:0
AD7:0
A1065-0A
Figure 3-24. Typical 82C59A Interface
3-27
BUS INTERFACE UNIT
3.5.4 HALT Bus Cycle
Suspending the CPU reduces device power consumption and potentially reduces interrupt latency
time. The HLT instruction initiates two events:
1. Suspends the Execution Unit.
2. Instructs the BIU to execute a HALT bus cycle.
After executing a HALT bus cycle, the BIU suspends operation until one of the following events
occurs:
• A bus HOLD is generated.
• A DMA request is generated.
• A refresh request is generated.
Figure 3-25 shows the operation of a HALT bus cycle. The address/data bus either floats or drives
during T1, depending on the next bus cycle to be executed by the BIU. Under most instruction
sequences, the BIU floats the address/data bus because the next operation would most likely be
an instruction prefetch. However, if the HALT occurs just after a bus write operation, the ad-
dress/data bus drives either data or address information during T1. A19:16 continue to drive the
previous bus cycle information under most instruction sequences (otherwise, they drive the next
The Chip-Select Unit prevents a programmed chip-select from going active during a HALT bus
cycle. However, chip-selects generated by external decoder circuits must be disabled for HALT
bus cycles.
Table 3-5 lists the state of each pin after entering the HALT bus state.
3-28
BUS INTERFACE UNIT
T1
TI
TI
CLKOUT
ALE
S2:0
011
AD15:0
[AD7:0]
Note 1
Note 1
[A15:8]
A19:16
Note 2
BHE
[RFSH = 1]
NOTES:
1. The AD15:0 [AD7:0] bus can be floating, driving a previous write data value,
or driving the next instruction prefetch address value. For an 8-bit device,
A15:8 drives either the previous bus address value or the next instruction
prefetch address value.
2. The A19:16 bus drives either zero (all low) or 8H (all low except A19/S6,
which can be high if the previous bus cycle was a DMA or refresh operation).
A1513-0A
Figure 3-25. HALT Bus Cycle
Table 3-6. HALT Bus Cycle Pin States
Pin(s)
Pin State
Float
AD15:0 (AD7:0 for 8-bit)
A15:8 (8-bit)
Drive Address
Drive 8H or Zero
Drive Last Value
Drive One
A19:16
BHE (16-bit)
RD, WR, DEN, DT/R, RFSH (8-bit), S2:0
3-29
BUS INTERFACE UNIT
A DMA request, refresh request or bus hold request causes the BIU to exit the HALT bus state
temporarily. This can occur only when in the Active or Idle power management mode. The BIU
returns to the HALT bus state after it completes the desired bus operation. However, the BIU
does not execute another bus HALT cycle (i.e., ALE and bus cycle status are not regenerated).
Figures 3-26, 3-27 and 3-28 illustrate how the BIU temporarily exits and then returns to the
HALT bus state.
CLKOUT
HOLD
HLDA
AD15:0
AD7:0
A15:8
A19:16
CONTROL
A1516-0A
Figure 3-26. Returning to HALT After a HOLD/HLDA Bus Exchange
3-30
BUS INTERFACE UNIT
CLKOUT
ALE
S2:0
AD15:0
[AD7:0]
Addr
Addr
Note 1
Note 1
Note 2
[A15:8]
A19:16
Address
A19 = 1, A18:16 = 0
Note 3
BHE
RFSH
NOTES:
1. Previous bus cycle value.
2. Only occurs for BHE on the first refresh bus cycle after entering HALT.
3. BHE = 1 for 16-bit device, RFSH = 0 for 8-bit device.
A1514-0A
Figure 3-27. Returning to HALT After a Refresh Bus Cycle
3-31
BUS INTERFACE UNIT
T4 T1 T2 T3 T4 T1 T2 T3 TI TI TI TI
CLKOUT
ALE
S2:0
Valid Status
Addr
Valid Status
AD15:0
[AD7:0]
Valid Data
Addr
Note
Address
[A15:8]
A19:16
Address
8H
Addr
8H
Note
Note
Addr
BHE
[RFSH=1]
Valid
Valid
NOTE: Drives previous bus cycle value.
A1515-0A
Figure 3-28. Returning to HALT After a DMA Bus Cycle
3.5.6 Exiting HALT
An NMI or any unmasked INTn interrupt causes the BIU to exit HALT. The first bus operations
to occur after exiting HALT are read cycles to reload the CS:IP registers. Figure 3-29 shows how
the HALT bus state is exited when an NMI or INTn occurs.
3-32
BUS INTERFACE UNIT
CLKOUT
NMI/INTx
ALE
Note 1
Valid
S2:0
AD15:0
[AD7:0]
Note 2
Addr
Address
Addr
[A15:8]
A19:16
Note 2
Note 2
Note 2
BHE
RFSH
NOTES: 1. For NMI, delay = 4 1/2 clocks. For INTx, delay = 7 1/2 clocks (min).
2. Previous bus cycle value.
A1517-0A
Figure 3-29. Exiting HALT
3.6 SYSTEM DESIGN ALTERNATIVES
Most system designs require no signals other than those already provided by the BIU. However,
heavily loaded bus conditions, slow memory or peripheral device performance and off-board de-
vice interfaces may not be supported directly without modifying the BIU interface. The following
sections deal with topics to enhance or modify the operation of the BIU.
3-33
BUS INTERFACE UNIT
3.6.1 Buffering the Data Bus
The BIU generates two control signals, DEN and DT/R, to control bidirectional buffers or trans-
ceivers. The timing relationship of DEN and DT/R is shown in Figure 3-30. The following con-
ditions require transceivers:
• The capacitive load on the address/data bus gets too large.
• The current load on the address/data bus exceeds device specifications.
• Additional VOL and VOH drive is required.
• A memory or I/O device cannot float its outputs in time to prevent bus contention, even at
reset.
T1
T2
T3
T4
T1
CLKOUT
RD,WR
DT/R
DEN
Read Cycle Operation
A1094-A0
Figure 3-30. DEN and DT/R Timing Relationships
The circuit shown in Figure 3-31 illustrates how to use transceivers to buffer the address/data bus.
The connection between the processor and the transceiver is known as thelocal bus. A connection
between the transceiver and other memory or I/O devices is known as the buffered bus. A fully
buffered system has no devices attached to the local bus. A partially buffered system has devices
on both the local and buffered buses.
3-34
BUS INTERFACE UNIT
ALE
A19:16
Latch
Address Bus
Processor
AD15:0
Address
Memory
Transceiver
Data
Data Bus
CS
or
I/O
DEN
Device
DT/ R
CPU Local Bus
Buffered Bus
A1095-0A
Figure 3-31. Buffered AD Bus System
In a fully buffered system, DEN directly drives the transceiver output enable. A partially buffered
system requires that DEN be qualified with another signal to prevent the transceiver from going
active for local bus accesses. Figure 3-32 illustrates how to use chip-selects to qualify DEN.
DT/R always connects directly to the transceiver. However, an inverter may be required if the po-
larity of DT/R does not match the transceiver. DT/R goes low (0) only for memory and I/O read,
instruction prefetch and interrupt acknowledge bus cycles.
3-35
BUS INTERFACE UNIT
8
A
AD15:8
DEN
8
D15:8
B
OE
T
MCS0
Buffer
Buffered
Data
Bus
8
A
AD7:0
8
OE
T
B
D7:0
DT/R
Buffer
8
Local
Data
Bus
8
A1058-0B
Figure 3-32. Qualifying DEN with Chip-Selects
3.6.2 Synchronizing Software and Hardware Events
The execution sequence of a program and hardware events occurring within a system are often
asynchronous to each other. In some systems there may be a requirement to suspend program ex-
ecution until an event (or events) occurs, then continue program execution.
One way to synchronize software execution with hardware events requires the use of interrupts.
Executing a HALT instruction suspends program execution until an unmasked interrupt occurs.
However, there is a delay associated with servicing the interrupt before program execution can
proceed. Using the WAIT instruction removes the delay associated with servicing interrupts.
3-36
BUS INTERFACE UNIT
The WAIT instruction suspends program execution until one of two events occurs: an interrupt
is generated, or the TEST input pin is sampled low. Unlike interrupts, the TEST input pin does
not require that program execution be transferred to a new location (i.e., an interrupt routine is
not executed). In processing the WAIT instruction, program execution remains suspended as long
as TEST remains high (at least until an interrupt occurs). When TEST is sampled low, program
execution resumes.
The TEST input and WAIT instruction provide a mechanism to delay program execution until a
hardware event occurs, without having to absorb the delay associated with servicing an interrupt.
3.6.3 Using a Locked Bus
To address the problems of controlling accesses to shared resources, the BIU provides a hardware
LOCK output. The execution of a LOCK prefix instruction activates the LOCK output.
LOCK goes active in phase 1 of T1 of the first bus cycle following execution of the LOCK prefix
instruction. It remains active until phase 1 of T1 of the first bus cycle following the execution of
the instruction following the LOCK prefix. To provide bus access control in multiprocessor sys-
tems, the LOCK signal should be incorporated into the system bus arbitration logic residing in
the CPU.
During normal multiprocessor system operation, priority of the shared system bus is determined
by the arbitration circuits on a cycle by cycle basis. As each CPU requires a transfer over the sys-
tem bus, it requests access to the bus via its resident bus arbitration logic. When the CPU gains
priority (determined by the system bus arbitration scheme and any associated logic), it takes con-
trol of the bus, performs its bus cycle and either maintains bus control, voluntarily releases the
bus or is forced off the bus by the loss of priority.
The lock mechanism prevents the CPU from losing bus control (either voluntarily or by force)
and guarantees that the CPU can execute multiple bus cycles without intervention and possible
corruption of the data by another CPU. A classic use of the mechanism is the “TEST and SET
semaphore,” during which a CPU must read from a shared memory location and return data to
the location without allowing another CPU to reference the same location during the test and set
operations.
Another application of LOCK for multiprocessor systems consists of a locked block move, which
allows high speed message transfer from one CPU’s message buffer to another. During the locked
instruction (i.e., while LOCK is active), a bus hold, DMA or refresh request is recorded, but is
not acknowledged until completion of the locked instruction. However, LOCK has no effect on
interrupts. As an example, a locked HALT instruction causes bus hold, DMA or refresh bus re-
quests to be ignored, but still allows the CPU to exit the HALT state on an interrupt.
3-37
BUS INTERFACE UNIT
In general, prefix bytes (such as LOCK) are considered extensions of the instructions they pre-
cede. Interrupts, DMA requests and refresh requests that occur during execution of the prefix are
not acknowledged until the instruction following the prefix completes (except for instructions
that are servicing interrupts during their execution, such as HALT, WAIT and repeated string
primitives).Note that multiple prefix bytes can precede an instruction.
Another example is a string primitive preceded by the repetition prefix (REP), which can be in-
terrupted after each execution of the string primitive, even if the REP prefix is combined with the
LOCK prefix. This prevents interrupts from being locked out during a block move or other re-
peated string operations. However, bus hold, DMA and refresh requests remain locked out until
LOCK is removed (either when the block operation completes or after an interrupt occurs.
3.6.4 Using the Queue Status Signals
cessor. Newer devices do not require these signals because they use the 80187 math coprocessor,
which has an I/O port interface similar to that of a peripheral device.
The queue status signals, QS0 and QS1, indicate the state of the internal queue (Table 3-7). Since
the Execution Unit can remove information from the queue on any clock boundary, the queue sta-
tus pins may change state on every phase 1 clock edge (Figure 3-33). Although these signals can-
not be related to any specific T-state, the relationship between the queue status signals and BIU
operation always remains the same for a given instruction sequence.
QS0 and QS1 are alternate functions of ALE and WR, respectively. To enable QS0 and QS1, you
must connect the RD pin directly to ground. In this case, RD, WR and ALE are no longer avail-
able and must be generated by external hardware such as an 82C88 or a programmable logic de-
vice.
Table 3-7. Queue Status Signal Decoding
QS1
QS0
Queue Status
0
0
No queue operation occurred.
0
1
1
0
The first byte of a new instruction was removed from the queue.
The queue was reinitialized. All prefetch information was flushed; the BIU must begin
prefetching new queue information.
1
1
A subsequent byte of an instruction was removed from the queue. The current instruction
contains multiple opcode bytes or immediate data.
3-38
BUS INTERFACE UNIT
CLKOUT
QS0, QS1
A1059-0A
Figure 3-33. Queue Status Timing
3.7 MULTI-MASTER BUS SYSTEM DESIGNS
The BIU supports protocols for transferring control of the local bus between itself and other de-
vices capable of acting as bus masters. To support such a protocol, the BIU uses a hold request
input (HOLD) and a hold acknowledge output (HLDA) as bus transfer handshake signals. To
gain control of the bus, a device asserts the HOLD input, then waits until the HLDA output goes
3.7.1 Entering Bus HOLD
In responding to the hold request input, the BIU floats the entire address and data bus, and many
of the control signals. Figure 3-34 illustrates the timing sequence when acknowledging the hold
request. Table 3-8 lists the states of the BIU pins when HLDA is asserted. All device pins not
mentioned in Table 3-8 or shown in Figure 3-34 remain either active (e.g., CLKOUT and
T1OUT) or inactive (e.g., UCS and INTA). Refer to the data sheet for specific details of pin func-
tions during a bus hold.
3-39
BUS INTERFACE UNIT
CLKOUT
1
HOLD
4
2
HLDA
3
AD15:0
DEN
Float
A19:16
RD, WR,
DT/R,
Float
BHE, S2:0
LOCK
NOTES:
1. T
2. T
3. T
4. T
: HOLD input to clock low
: Clock high to output float
: Clock low to output float
HVCL
CHCZ
CLAZ
: Clock low to HLDA high
CLHAV
A1518-0A
Figure 3-34. Timing Sequence Entering HOLD
Table 3-8. Signal Condition Entering HOLD
Signal
HOLD Condition
A19:16, S2:0, RD, WR, DT/R, BHE (RFSH), LOCK
AD15:0 (16-bit), AD7:0 (8-bit), A15:8 (8-bit), DEN
These signals float one-half clock before HLDA
is generated (i.e., phase 2).
which HLDA is generated (i.e., phase 1).
3.7.1.1
HOLD Bus Latency
The duration between the time that the external device asserts HOLD and the time that the BIU
asserts HLDA is known as bus latency. In Figure 3-34, the two-clock delay between HOLD and
HLDA represents the shortest bus latency. Normally this occurs only if the bus is idle or halted
or if the bus hold request occurs just before the BIU begins another bus cycle.
3-40
BUS INTERFACE UNIT
The major factors that influence bus latency are listed below (in order from longest delay to short-
est delay).
1. Bus Not Ready — As long as the bus remains not ready, a bus hold request cannot be
serviced.
2. Locked Bus Cycle — As long as LOCK remains asserted, a bus hold request cannot be
serviced. Performing a locked move string operation can take several thousands of clocks.
3. Completion of Current Bus Cycle — A bus hold request cannot be serviced until the
current bus cycle completes. A bus hold request will not separate bus cycles required to
move odd-aligned word data. Also, bus cycles with long wait states will delay the
servicing of a bus hold request.
4. Interrupt Acknowledge Bus Cycle — A bus hold request is not serviced until after an
INTA bus cycle has completed. An INTA bus cycle drives LOCK active.
5. DMA and Refresh Bus Cycles — A bus hold request is not serviced until after the DMA
request or refresh bus cycle has completed. Refresh bus cycles have a higher priority than
hold bus requests. A bus hold request cannot separate the bus cycles associated with a
DMA transfer (worst case is an odd-aligned transfer, which takes four bus cycles to
complete).
3.7.1.2
Refresh Operation During a Bus HOLD
HOLD is removed. However, when a refresh bus request is generated, the HLDA output is re-
moved (driven low) to signal the need for the BIU to regain control of the local bus. The BIU does
not gain control of the bus until HOLD is removed. This procedure prevents the BIU from just
arbitrarily regaining control of the bus.
Figure 3-35 shows the timing associated with the occurrence of a refresh request while HLDA is
active. Note that HLDA can be as short as one clock in duration. This happens when a refresh
request occurs just after HLDA is granted. A refresh request has higher priority than a bus hold
request; therefore, when the two occur simultaneously, the refresh request occurs before HLDA
becomes active.
3-41
BUS INTERFACE UNIT
CLKOUT
1
3
4
HOLD
HLDA
2
5
AD15:0
DEN
A19:16
RD, WR,
BHE, S2:0
DT/R,
5
LOCK
NOTES:
1. HLDA is deasserted, signaling need to run refresh bus cycle.
2. External bus master terminates use of the bus.
3. HOLD deasserted.
4. Hold may be reasserted after one clock.
5. BIU runs refresh cycle.
A1061-0A
Figure 3-35. Refresh Request During HOLD
The device requesting a bus hold must be able to detect a HLDA pulse that is one clock in dura-
tion. A bus lockup (hang) condition can result if the requesting device fails to detect the short
HLDA pulse and continues to wait for HLDA to be asserted while the BIU waits for HOLD to be
deasserted. The circuit shown in Figure 3-36 can be used to latch HLDA.
3-42
BUS INTERFACE UNIT
+5
PRE
CLR
D
Latched HLDA
Q
HLDA
RESET
HOLD
A1535-0A
Figure 3-36. Latching HLDA
the bus and execute a refresh bus cycle. Should HOLD go active before the refresh bus cycle is
complete, the BIU will release the bus and generate HLDA.
3.7.2 Exiting HOLD
Figure 3-37 shows the timing associated with exiting the bus hold state. Normally a bus operation
(e.g., an instruction prefetch) occurs just after HOLD is released. In a 16-bit bus system, the upper
half of the bus (D15:8) floats.
3-43
BUS INTERFACE UNIT
CLKOUT
1
2
4
3
5
HOLD
HLDA
AD15:0
DEN
RD, WR, BHE,
DT/R, S2:0,
A19:16, LOCK
NOTES:
1. T
2.
: HOLD recognition setup to clock low
: HOLD internally synchronized
HVCL
3. T
4. T
5. T
: Clock low to HLDA low
: Clock high to signal active (high or low)
: Clock low to signal active (high or low)
CLHAV
CHCV
CLAV
A1063-0A
Figure 3-37. Exiting HOLD
3.8 BUS CYCLE PRIORITIES
The BIU arbitrates requests for bus cycles from the Execution Unit, the integrated peripherals
(e.g., Interrupt Control Unit) and external bus masters (i.e., bus hold requests). The list below
summarizes the priorities for all bus cycle requests (from highest to lowest).
1. Instruction execution read/write following a non-pipelined effective address calculation.
2. Refresh bus cycles.
3. Bus hold request.
4. Single step interrupt vectoring sequence.
5. Non-Maskable interrupt vectoring sequence.
3-44
BUS INTERFACE UNIT
6. Internal error (e.g., divide error, overflow) interrupt vectoring sequence.
7. Hardware (e.g., INT0, DMA) interrupt vectoring sequence.
8. 80C187 Math Coprocessor error interrupt vectoring sequence.
9. DMA bus cycles.
10. General instruction execution. This category includes read/write operations following a
pipelined effective address calculation, vectoring sequences for software interrupts and
numerics code execution. The following points apply to sequences of related execution
cycles.
— The second read/write cycle of an odd-addressed word operation is inseparable from
the first bus cycle.
— The second read/write cycle of an instruction with both load and store accesses (e.g.,
XCHG) can be separated from the first cycle by other bus cycles.
— Successive bus cycles of string instructions (e.g., MOVS) can be separated by other bus
cycles.
— When a locked instruction begins, its associated bus cycles become the highest priority
and cannot be separated (or preempted) until completed.
11. Bus cycles necessary to fill the prefetch queue.
3-45
4
Peripheral Control
Block
CHAPTER 4
PERIPHERAL CONTROL BLOCK
All integrated peripherals in the 80C186 Modular Core family are controlled by sets of registers
within an integrated Peripheral Control Block (PCB). The peripheral control registers are physi-
cally located in the peripheral devices they control, but they are addressed as a single block of
registers. The Peripheral Control Block encompasses 256 contiguous bytes and can be located on
any 256-byte boundary of memory or I/O space. The PCB Relocation Register, which is also lo-
cated within the Peripheral Control Block, controls the location of the PCB.
4.1 PERIPHERAL CONTROL REGISTERS
Each of the integrated peripherals’ control and status registers is located at a fixed offset above
the programmed base location of the Peripheral Control Block (see Table 4-1). These registers
are described in the chapters that cover the associated peripheral. “Accessing the Peripheral Con-
reading and writing them.
4.2 PCB RELOCATION REGISTER
In addition to control registers for the integrated peripherals, the Peripheral Control Block con-
tains the PCB Relocation Register (Figure 4-1). The Relocation Register is located at a fixed off-
set within the Peripheral Control Block (Table 4-1). If the Peripheral Control Block is moved, the
Relocation Register also moves.
The PCB Relocation Register allows the Peripheral Control Block to be relocated to any 256-byte
boundary within memory or I/O space. The Memory I/O bit (MEM) selects either memory space
or I/O space, and the R19:8 bits specify the starting (base) address of the PCB. The remaining bit,
Escape Trap (ET), controls access to the math coprocessor interface.
“Setting the PCB Base Location” on page 4-6 describes how to set the base location and outlines
some restrictions on the Peripheral Control Block location.
4-1
PERIPHERAL CONTROL BLOCK
Register Name:
PCB Relocation Register
RELREG
Register Mnemonic:
Register Function:
Relocates the PCB within memory or I/O space.
15
0
E
T
S
L
M
E
M
R
1
9
R
1
8
R
1
7
R
1
6
R
1
5
R
1
4
R
1
3
R
1
2
R
1
1
R
1
0
R
9
R
8
A1262-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
ET
Escape Trap
0
If ET is set, the CPU will trap when an ESC
instruction is executed.
SL
Slave/Master
Memory I/O
0
0
If SL is set, the Interrupt Control Unit operates in
slave mode. If SL is clear, it operates in master
mode.
MEM
R19:8
If MEM is set, the PCB is located in memory
space. If MEM is clear, the PCB is located in I/O
space.
PCB Base
Address
Upper Bits
0FFH
R19:8 define the upper address bits of the PCB
base address. All lower bits are zero. R19:16 are
ignored when the PCB is mapped to I/O space.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 4-1. PCB Relocation Register
4-2
PERIPHERAL CONTROL BLOCK
Table 4-1. Peripheral Control Block
PCB
Offset
PCB
Offset
PCB
Offset
PCB
Offset
Function
Function
Function
Function
00H
02H
04H
06H
08H
0AH
0CH
0EH
10H
12H
14H
16H
18H
1AH
1CH
1EH
20H
22H
24H
26H
28H
2AH
2CH
2EH
30H
32H
34H
36H
38H
3AH
3CH
3EH
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
EOI
40H
42H
44H
46H
48H
4AH
4CH
4EH
50H
52H
54H
56H
58H
5AH
5CH
5EH
60H
62H
64H
66H
68H
6AH
6CH
6EH
70H
72H
74H
76H
78H
7AH
7CH
7EH
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
T0CNT
80H
82H
84H
86H
88H
8AH
8CH
8EH
90H
92H
94H
96H
98H
9AH
9CH
9EH
A0H
A2H
A4H
A6H
A8H
AAH
ACH
AEH
B0H
B2H
B4H
B6H
B8H
BAH
BCH
BEH
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
UMCS
C0H
C2H
C4H
C6H
C8H
CAH
CCH
CEH
D0H
D2H
D4H
D6H
D8H
DAH
DCH
DEH
E0H
E2H
E4H
E6H
E8H
EAH
ECH
EEH
F0H
F2H
F4H
F6H
F8H
FAH
FCH
FEH
D0SRCL
D0SRCH
D0DSTL
D0DSTH
D0TC
D0CON
Reserved
Reserved
D1SRCL
D1SRCH
D1DSTL
D1DSTH
D1TC
T0CMPA
T0CMPB
T0CON
T1CNT
T1CMPA
T1CMPB
T1CON
D1CON
Reserved
Reserved
RFBASE
RFTIME
RFCON
T2CNT
T2CMPA
Reserved
T2CON
LMCS
POLL
PACS
POLLSTS
IMASK
MMCS
Reserved
Reserved
Reserved
Reserved
Reserved
PWRSAV
PWRCON
Reserved
STEPID
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
MPCS
PRIMSK
INSERV
REQST
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
INSTS
TCUCON
DMA0CON
DMA1CON
I0CON
Reserved
Reserved
Reserved
RELREG
I1CON
I2CON
I3CON
4-3
PERIPHERAL CONTROL BLOCK
4.3 RESERVED LOCATIONS
Many locations within the Peripheral Control Block are not assigned to any peripheral. Unused
locations are reserved. Reading from these locations yields an undefined result. If reserved reg-
isters are written (for example, during a block MOV instruction) they must be set to 0H.
NOTE
Failure to follow this guideline could result in incompatibilities with future
80C186 Modular Core family products.
4.4 ACCESSING THE PERIPHERAL CONTROL BLOCK
All communication between integrated peripherals and the Modular CPU Core occurs over a spe-
cial bus, called the F-Bus, which always carries 16-bit data. The Peripheral Control Block, like
all integrated peripherals, is always accessed 16 bits at a time.
4.4.1
Bus Cycles
The processor runs an external bus cycle for any memory or I/O cycle accessing a location within
the Peripheral Control Block. Address, data and control information is driven on the external pins
as with an ordinary bus cycle. Information returned by an external device is ignored, even if the
access does not correspond to the location of an integrated peripheral control register. This is also
true for the 80C188 Modular Core family, except that word accesses made to integrated registers
are performed in two bus cycles.
4.4.2
READY Signals and Wait States
The processor generates an internal READY signal whenever an integrated peripheral is access-
ed. External READY is ignored. READY is also generated if an access is made to a location with-
in the Peripheral Control Block that does not correspond to an integrated peripheral control
register. For accesses to timer control and counting registers, the processor inserts one wait state.
This is required to properly multiplex processor and counter element accesses to the timer control
registers. For accesses to the remaining locations in the Peripheral Control Block, the processor
does not insert wait states.
4-4
PERIPHERAL CONTROL BLOCK
4.4.3
F-Bus Operation
The F-Bus functions differently than the external data bus for byte and word accesses. All write
transfers on the F-Bus occur as words, regardless of how they are encoded. For example, the in-
struction OUT DX, AL (DX is even) will write the entire AX register to the Peripheral Control
Block register at location [DX]. If DX were an odd location, AL would be placed in [DX] and
AH would be placed at [DX–1]. A word operation to an odd address would write [DX] and [DX–
1] with AL and AH, respectively. This differs from normal external bus operation where un-
aligned word writes modify [DX] and [DX+1]. In summary, do not use odd-aligned byte or word
writes to the PCB.
Aligned word reads work normally. Unaligned word reads work differently. For example, IN AX,
DX (DX is odd) will transfer [DX] into AL and [DX–1] into AH. Byte reads from even or odd
addresses work normally, but only a byte will be read. For example, IN AL, DX will not transfer
[DX] into AX (only AL is modified).
No problems will arise if the following recommendations are adhered to.
Word reads
Byte reads
Aligned word reads of the PCB work normally. Access only even-
aligned words with IN AX, DX or MOV word register, even PCB
address.
Byte reads of the PCB work normally. Beware of reading word-wide
PCB registers that may change value between successive reads (e.g.,
timer count value).
Word writes
Always write even-aligned words to the PCB. Writing an odd-
aligned word will give unexpected results.
For the 80C186 Modular Core, use either
– OUT DX, AX or
– OUT DX, AL or
– MOV even PCB address, word register.
For the 80C188 Modular Core, using OUT DX, AX will perform an
unnecessary bus cycle and is not recommended. Use either
– OUT DX, AL or
– MOV even-aligned byte PCB address, byte register low byte.
Byte writes
Always use even-aligned byte writes to the PCB. Even-aligned byte
writes will modify the entire word PCB location. Do not perform
unaligned byte writes to the PCB.
4-5
PERIPHERAL CONTROL BLOCK
4.4.3.1
Writing the PCB Relocation Register
Whenever mapping the Peripheral Control Block to another location, the user should program the
Relocation Register with a byte write (i.e., OUT DX, AL). Internally, the Relocation Register is
written with 16 bits of the AX register, while externally the Bus Interface Unit runs a single 8-bit
bus cycle. If a word instruction (i.e., OUT DX, AX) is used with an 80C188 Modular Core family
member, the Relocation Register is written on the first bus cycle. The Bus Interface Unit then runs
an unnecessary second bus cycle. The address of the second bus cycle is no longer within the con-
trol block, since the Peripheral Control Block was moved on the first cycle. External READY
must now be generated to complete the cycle. For this reason, we recommend byte operations for
the Relocation Register.
4.4.3.2
Accessing the Peripheral Control Registers
Byte instructions should be used for the registers in the Peripheral Control Block of an 80C188
Modular Core family member. This requires half the bus cycles of word operations. Byte opera-
tions are valid only for even-addressed writes to the Peripheral Control Block. A word read (e.g.,
IN AX, DX) must be performed to read a 16-bit Peripheral Control Block register when possible.
4.4.3.3
Accessing Reserved Locations
Unused locations are reserved. If a write is made to these locations, a bus cycle occurs, but data
is not stored. If a subsequent read is made to the same location, the value written is not read back.
If reserved registers are written (for example, during a block MOV instruction) they must be
cleared to 0H.
NOTE
Failure to follow this guideline could result in incompatibilities with future
80C186 Modular Core family products.
4.5 SETTING THE PCB BASE LOCATION
Upon reset, the PCB Relocation Register (see Figure 4-1 on page 4-2) contains the value 00FFH,
which causes the Peripheral Control Block to be located at the top of I/O space (0FF00H to
0FFFFH). Writing the PCB Relocation Register allows the user to change that location.
4-6
PERIPHERAL CONTROL BLOCK
As an example, to relocate the Peripheral Control Block to the memory range 10000-100FFH, the
user would program the PCB Relocation Register with the value 1100H. Since the Relocation
Register is part of the Peripheral Control Block, it relocates to word 10000H plus its fixed offset.
NOTE
Due to an internal condition, external ready is ignored if the device is
configured in Cascade mode and the Peripheral Control Block (PCB) is
located at 0000H in I/O space. In this case, wait states cannot be added to
interrupt acknowledge bus cycles. However, you can add wait states to
interrupt acknowledge cycles if the PCB is located at any other address.
4.5.1 Considerations for the 80C187 Math Coprocessor Interface
Systems using the 80C187 math coprocessor interface must not relocate the Peripheral Control
Block to location 0000H in I/O space. The 80C187 interface uses I/O locations 0F8H through
0FFH. If the Peripheral Control Block resides in these locations, the processor communicates
with the Peripheral Control Block, not the 80C187 interface circuitry.
NOTE
If the PCB is located at 0000H in I/O space and access to the math coprocessor
interface is enabled (the Escape Trap bit is clear), a numerics (ESC) instruction
causes indeterminate system operation.
Since the 8-bit bus version of the device does not support the 80C187, it automatically traps an
ESC instruction to the Type 7 interrupt, regardless of the state of the Escape Trap (ET) bit.
For details on the math coprocessor interface, see Chapter 11, “Math Coprocessing.”
4-7
5
ClockGenerationand
Power Management
CHAPTER 5
CLOCK GENERATION AND POWER
MANAGEMENT
The clock generation and distribution circuits provide uniform clock signals for the Execution
Unit, the Bus Interface Unit and all integrated peripherals. The 80C186 Modular Core Family
processors have additional logic that controls the clock signals to provide power management
functions.
5.1 CLOCK GENERATION
The clock generation circuit (Figure 5-1) includes a crystal oscillator, a divide-by-two counter
and power-save and reset circuitry. See “Power-Save Mode” on page 5-11 for a discussion of
Power-Save mode as a power management option.
Schmitt Trigger
"Squares-up" CLKIN
Power Save
X1
1
2
Internal
Phase
Clocks
÷ 2
Clock
Clock
Divider
Phase
Drivers
To CLKOUT
X2
Reset Circuitry
Internal Reset
RES
A1519-0A
Figure 5-1. Clock Generator
5.1.1 Crystal Oscillator
The internal oscillator is a parallel resonant Pierce oscillator, a specific form of the common
phase shift oscillator.
5-1
CLOCK GENERATION AND POWER MANAGEMENT
5.1.1.1
Oscillator Operation
A phase shift oscillator operates through positive feedback, where a non-inverted, amplified ver-
sion of the input connects back to the input. A 360° phase shift around the loop will sustain the
feedback in the oscillator. The on-chip inverter provides a 180° phase shift. The combination of
the inverter’s output impedance and the first load capacitor (see Figure 5-2) provides another 90°
phase shift. At resonance, the crystal becomes primarily resistive. The combination of the crystal
and the second load capacitor provides the final 90° phase shift. Above and below resonance, the
crystal is reactive and forces the oscillator back toward the crystal’s nominal frequency.
Z = Inverter Output Z
0
90˚
90˚
180˚
NOTE:
At resonance, the crystal is essentially resistive.
Below resonance, the crystal is capacitive.
A1125-0A
Figure 5-2. Ideal Operation of Pierce Oscillator
Figure 5-3 shows the actual microprocessor crystal connections. For low frequencies, crystal ven-
dors offer fundamental mode crystals. At higher frequencies, a third overtone crystal is the only
choice. The external capacitors, CX1 at X1 and CX2 at X2, together with stray capacitance, form
the load. A third overtone crystal requires an additional inductor L1 and capacitor C1 to select the
third overtone frequency and reject the fundamental frequency. See “Selecting Crystals” on page
5-5 for a more detailed discussion of crystal vibration modes.
5-2
CLOCK GENERATION AND POWER MANAGEMENT
Choose C1 and L1 component values in the third overtone crystal circuit to satisfy the following
conditions:
• The LC components form an equivalent series resonant circuit at a frequency below the
fundamental frequency. This criterion makes the circuit inductive at the fundamental
frequency. The inductive circuit cannot make the 90° phase shift and oscillations do not
take place.
• The LC components form an equivalent parallel resonant circuit at a frequency about
halfway between the fundamental frequency and the third overtone frequency. This
criterion makes the circuit capacitive at the third overtone frequency, necessary for oscil-
lation.
• The two capacitors and inductor at X2, plus some stray capacitance, approximately equal
the 20 pF load capacitor, CX2, used alone in the fundamental mode circuit.
(a)
(b)
(c)
Fundamental
Mode Circuit
Third Overtone
Mode Circuit
Third Overtone Mode
(Equivalent Circuit)
C
X1
X1
X1
X2
C
L
X2
1
L
1
X2
C
C
C
C
1
X2
X1
X2
L
= (See text)
C
= C
X2
= 20pF
C
= 200pF
1
1
X1
A1520-0A
Figure 5-3. Crystal Connections to Microprocessor
Choosing C1 as 200 pF (at least 10 times the value of the load capacitor) simplifies the circuit
analysis. At the series resonance, the capacitance connected to L1 is 200 pF in series with 20 pF.
The equivalent capacitance is still about 20 pF and the equation in Figure 5-4(a) yields the series
resonant frequency.
5-3
CLOCK GENERATION AND POWER MANAGEMENT
To examine the parallel resonant frequency, refer to Figure 5-3(c), an equivalent circuit to Figure
5-3(b). The capacitance connected to L1 is 200 pF in parallel with 20 pF. The equivalent capaci-
tance is still about 200 pF (within 10%) and the equation in Figure 5-4(a) now yields the parallel
resonant frequency.
(a) Series or Parallel Resonant Frequency
(b) Equivalent Capacitance
ω2C1Cx2
L1 – C1 – Cx2
1
f = ------------------------
C eq = -----------------------------------------------------------
ω2C1L1 – 1
2π L1C1
Figure 5-4. Equations for Crystal Calculations
The equation in Figure 5-4(b) yields the equivalent capacitance Ceq at the operation frequency.
The desired operation frequency is the third overtone frequency marked on the crystal. Optimiz-
ing equations for the above three criteria yields Table 5-1. This table shows suggested standard
inductor values for various processor frequencies. The equivalent capacitance is about 15 pF.
Table 5-1. Suggested Values for Inductor L1 in Third Overtone Oscillator Circuit
CLKOUT
Frequency (MHz)
Third-Overtone Crystal
Frequency (MHz)
Inductor L
1
Values (µH)
10
12
16
20
20
25
32
40
10.0, 12.0, 15.0
6.8, 8.2, 10.0
3.9, 4.7, 5.6
2.2, 2.7, 3.3
5-4
CLOCK GENERATION AND POWER MANAGEMENT
5.1.1.2
Selecting Crystals
When specifying crystals, consider these parameters:
• Resonance and Load Capacitance — Crystals carry a parallel or series resonance specifi-
cation. The two types do not differ in construction, just in test conditions and expected
circuit application. Parallel resonant crystals carry a test load specification, with typical
load capacitance values of 15, 18 or 22 pF. Series resonant crystals do not carry a load
capacitance specification. You may use a series resonant crystal with the microprocessor,
even though the circuit is parallel resonant. However, it will vibrate at a frequency slightly
(on the order of 0.1%) higher than its calibration frequency.
• Vibration Mode — The vibration mode is either fundamental or third overtone. Crystal
thickness varies inversely with frequency. Vendors furnish third or higher overtone crystals
to avoid manufacturing very thin, fragile quartz crystal elements. At a given frequency, an
overtone crystal is thicker and more rugged than its fundamental mode counterpart. Below
20 MHz, most crystals are fundamental mode. In the 20 to 32 MHz range, you can purchase
both modes. You must know the vibration mode to know whether to add the LC circuit at
X2.
• Equivalent Series Resistance (ESR) — ESR is proportional to crystal thickness, inversely
proportional to frequency. A lower value gives a faster startup time, but the specification is
usually not important in microprocessor applications.
• Shunt Capacitance — A lower value reduces ESR, but typical values such as 7 pF will work
fine.
• Drive Level — Specifies the maximum power dissipation for which the manufacturer
calibrated the crystal. It is proportional to ESR, frequency, load and VCC. Disregard this
specification unless you use a third overtone crystal whose ESR and frequency will be
relatively high. Several crystal manufacturers stock a standard microprocessor crystal line.
Specifying a “microprocessor grade” crystal should ensure that the rated drive level is a
couple of milliwatts with 5-volt operation.
• Temperature Range — Specifies an operating range over which the frequency will not vary
beyond a stated limit. Specify the temperature range to match the microprocessor
temperature range.
• Tolerance — The allowable frequency deviation at a particular calibration temperature,
usually 25° C. Quartz crystals are more accurate than microprocessor applications call for;
do not pay for a tighter specification than you need. Vendors quote frequency tolerance in
percentage or parts per million (ppm). Standard microprocessor crystals typically have a
frequency tolerance of 0.01% (100 ppm). If you use these crystals, you can usually
disregard all the other specifications; these crystals are ideal for the 80C186 Modular Core
family.
5-5
CLOCK GENERATION AND POWER MANAGEMENT
An important consideration when using crystals is that the oscillator start correctly over the volt-
age and temperature ranges expected in operation. Observe oscillator startup in the laboratory.
Varying the load capacitors (within about ± 50%) can optimize startup characteristics versus sta-
bility. In your experiments, consider stray capacitance and scope loading effects.
For help in selecting external oscillator components for unusual circumstances, count on the crys-
tal manufacturer as your best resource. Using low-cost ceramic resonators in place of crystals is
possible if your application will tolerate less precise frequencies.
5.1.2 Using an External Oscillator
The microprocessor’s on-board clock oscillator allows the use of a relatively low cost crystal.
However, the designer may also use a “canned oscillator” or other external frequency source.
Connect the external frequency input (EFI) signal directly to the oscillator X1 input. Leave X2
unconnected. This oscillator input drives the internal divide-by-two counter directly, generating
the CPU clock signals. The external frequency input can have practically any duty cycle, provid-
ed it meets the minimum high and low times stated in the data sheet. Selecting an external clock
oscillator is more straightforward than selecting a crystal.
5.1.3 Output from the Clock Generator
The crystal oscillator output drives a divide-by-two circuit, generating a 50% duty cycle clock for
the processor’s integrated components. All processor timings refer to this clock, available exter-
nally at the CLKOUT pin. CLKOUT changes state on the high-to-low transition of the X1 signal,
even during reset and bus hold.
In a CMOS circuit, significant current flows only during logic level transitions. Since the micro-
processor consists mostly of clocked circuitry, the clock distribution is the basis of power man-
agement.
5.1.4 Reset and Clock Synchronization
and the clock generator synchronizes it to the CLKOUT signal.
A Schmitt trigger in the RES input ensures that the switch point for a low-to-high transition is
greater than the switch point for a high-to-low transition. The processor must remain in reset a
minimum of 4 CLKOUT cycles after VCC and CLKOUT stabilize. The hysteresis allows a simple
RC circuit to drive the RES input (see Figure 5-5). Typical applications can use about 100 milli-
seconds as an RC time constant.
5-6
CLOCK GENERATION AND POWER MANAGEMENT
Reset may be either cold (power-up) or warm. Figure 5-6 illustrates a cold reset. Assert the RES
input during power supply and oscillator startup. The processor’s pins assume their reset pin
states a maximum of 28 X1 periods after X1 and VCC stabilize. Assert RES 4 additional X1 peri-
ods after the device pins assume their reset states.
Applying RES when the device is running constitutes a warm reset (see Figure 5-7). In this case,
assert RES for at least 4 CLKOUT periods. The device pins will assume their reset states on the
second falling edge of X1 following the assertion of RES.
V
cc
-t
RC
50 k typical
V
= V 1 - e
c(t)
RESET IN
RES
1µf typical
A1521-0A
Figure 5-5. Simple RC Circuit for Powerup Reset
The processor exits reset identically in both cases. The falling RES edge generates an internal RE-
SYNC pulse (see Figure 5-8), resynchronizing the divide-by-two internal phase clock. The clock
generator samples RES on the falling X1 edge. If RES is sampled high while CLKOUT is high,
the processor forces CLKOUT low for the next two X1 cycles. The clock essentially “skips a
beat” to synchronize the internal phases. If RES is sampled high while CLKOUT is low, CLK-
OUT is already in phase.
5-7
CLOCK GENERATION AND POWER MANAGEMENT
X1
V
cc
Vcc and X1 stable to output valid 28 X1 periods (max)
CLKOUT
UCS, LCS
MCS3:0, NCS
TMR OUT0
TMR OUT1
PCS6:0
HLDA, ALE
A19:16
AD15:0, S2:0
RD, WR, DEN
DT/R, LOCK
RES
RESET
V
and X1 stable to RES high,
RES high to
first bus activity,
7 CLKOUT periods.
cc
approximately 32 X1 periods.
NOTE: CLKOUT synchronization occurs 1 1/2 X1 periods after RES is sampled low.
A1508-0B
Figure 5-6. Cold Reset Waveform
5-8
CLOCK GENERATION AND POWER MANAGEMENT
X1
CLKOUT
UCS, LCS
MCS3:0
PCS6:0,NCS
TMR OUT0
TMR OUT1
HLDA, ALE
A19/S6:
A16
AD15:0
S2:0, RD
WR, DEN
DT/R
LOCK
RES
RESET
RES high
to first bus
activity 7
CLKOUT
periods.
Minimum RES low
time 4 CLKOUT
periods.
A1522-0B
Figure 5-7. Warm Reset Waveform
At the second falling CLKOUT edge after sampling RES inactive, the processor deasserts RE-
SET. Bus activity starts 6½ CLKOUT periods after recognition of RES in the logic high state. If
an alternate bus master asserts HOLD during reset, the processor immediately asserts HLDA and
will not prefetch instructions.
5-9
CLOCK GENERATION AND POWER MANAGEMENT
X1
5
2
RES
1
RESYNC
(Internal)
1
CLKOUT
RESET
2
3
6
4
NOTES:
1. Setup of RES to falling X1.
2. RESYNC pulse generated.
3. RESYNC drives CLKOUT high, resynchronizing the clock generator.
4. RESET goes active.
5. RES allowed to go inactive after minimum 4 CLKOUT cycles.
6. RESET goes inactive 1 1/2 CLKOUT cycles after RES sampled inactive.
A1523-0A
Figure 5-8. Clock Synchronization at Reset
5.2 POWER MANAGEMENT
Many VLSI devices available today use dynamic circuitry. A dynamic circuit uses a capacitor
(usually parasitic gate or diffusion capacitance) to store information. The stored charge decays
over time due to leakage currents in the silicon. If the device does not use the stored information
before it decays, the state of the entire device may be lost. Circuits must periodically refresh dy-
namic RAMs, for example, to ensure data retention. Any microprocessor that has a minimum
clock frequency has dynamic logic. On a dynamic microprocessor, if you stop or slow the clock,
the dynamic nodes within it begin discharging. With a long enough delay, the processor is likely
to lose its present state, needing a reset to resume normal operation.
An 80C186 Modular Core microprocessor is fully static. The CPU stores its current state in
flip-flops, not capacitive nodes. The clock signal to both the CPU core and the peripherals can
stop without losing any internal information, provided the design maintains power. When the
clock restarts, the device will execute from its previous state. When the processor is inactive for
significant periods, special power management hardware takes advantage of static operation to
achieve major power savings.
5-10
CLOCK GENERATION AND POWER MANAGEMENT
5.2.1 Power-Save Mode
Power-Save mode is a means for reducing operating current. Power-Save mode enables a pro-
grammable clock divider in the clock generation circuit.
NOTE
Power-Save mode can be used to stretch bus cycles as an alternative to wait
states.
Possible clock divisor settings are 1 (undivided), 4, 8 and 16. The divided frequency feeds the
core, the integrated peripherals and CLKOUT. The processor operates at the divided clock rate
exactly as if the crystal or external oscillator frequency were lower by the same amount. Since
the processor is static, a lower limit clock frequency does not apply. It may be necessary to repro-
gram integrated peripherals such as the Timer Counter Unit and the Refresh Control Unit to com-
pensate for the overall reduced clock rate.
5.2.1.1
Entering Power-Save Mode
The Power-Save Register (Figure 5-9) controls Power-Save mode operation. The lower two bits
select the divisor. When program execution sets the PSEN bit, the processor enters Power-Save
mode. The internal clock frequency changes at the falling edge of T3 of the write to the Power-
Save Register. CLKOUT changes simultaneously and does not glitch. Figure 5-10 illustrates the
change at CLKOUT.
5-11
CLOCK GENERATION AND POWER MANAGEMENT
Register Name:
Power Save Register
PWRSAV
Register Mnemonic:
Register Function:
Enables and sets clock division factor.
15
0
P
S
E
N
F
1
F
0
A1130-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
PSEN
Power Save
Enable
0H
Setting this bit enables Power Save mode and
divides the internal operating clock by the value
defined by F1:0. Clearing this bit disables
Power-Save mode and forces the CPU to
operate at full speed. PSEN is automatically
cleared whenever an interrupt occurs.
F1:0
Clock
Division
Factor
0H
These bits control the clock division factor used
when Power Save mode is enabled. The
allowable values are listed below:
F1 F0 Divisor
0
0
1
1
0
1
0
1
By 1 (undivided)
By 4
By 8
By 16
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 5-9. Power-Save Register
5-12
CLOCK GENERATION AND POWER MANAGEMENT
T2
T3
T4
CLKOUT
WR
2
1
NOTES:
1. : Write to Power-Save Register (as viewed on the bus).
2. : Low-going edge of T3 starts new clock rate.
A1124-0A
Figure 5-10. Power-Save Clock Transition
5.2.1.2
Leaving Power-Save Mode
Power-Save mode continues until one of three events occurs: execution clears the PSEN bit in
the Power-Save Register, an unmasked interrupt occurs or an NMI occurs.
When the PSEN bit clears, the clock returns to its undivided frequency (standard divide-by-two)
at the falling T3 edge of the write to the Power-Save Register. The same result happens from re-
programming the clock divisor to a new value. The Power-Save Register can be read or written
at any time.
Unmasked interrupts include those from the Interrupt Control Unit, but not software interrupts.
If an NMI occurs, or an unmasked interrupt request has sufficient priority to pass to the core,
Power-Save mode will end. The PSEN bit clears and the clock resumes full-speed operation at
the falling edge of a bus cycle T3 state. However, the exact bus cycle of the transition is unde-
again. If you still want Power-Save mode operation, you can set the PSEN bit as part of the inter-
rupt service routine.
5.2.1.3
Example Power-Save Initialization Code
Example 5-1 illustrates programming the Power-Save Unit for a typical system. The program also
includes code to change the DRAM refresh rate to compensate for the reduced clock rate.
5-13
CLOCK GENERATION AND POWER MANAGEMENT
$mod186
name
example_PSU_code
;FUNCTION:
This function reduces CPU power consumption
by dividing the CPU operating frequency by a
divisor.
;
;
; SYNTAX:
; INPUTS:
;
extern void far power_save(int divisor);
divisor - This variable represents F0, F1 and F2
of PWRSAV.
; OUTPUTS:
None
;
;
NOTE:
Parameters are passed on the stack as required
by high-level languages
;substitute register offset
PWRSAV
RFTIME
equ
equ
xxxxH
xxxxH
;Power-Save Register
;Refresh Interval Count
;Register
;Refresh Control Register
;Power-Save enable bit
RFCON
PSEN
equ
equ
xxxxH
8000H
data
FreqTable
data
segment public 'data'
dw 1, 4, 8, 16, 32, 64, 0, 0
ends
lib_80C186
segment public 'code'
assume cs:lib_80C186, ds:data
public _power_save
proc far
_power_save
push bp
;save caller's bp
mov
bp, sp
;get current top of stack
;save registers that will
;be modified
push ax
push dx
_divisor
equ word ptr[bp+6]
;get parameter off the
;stack
mov dx, RFTIME
;get current DRAM refresh
;rate
;mask off unwanted bits
in
ax, dx
and ax, 01ffh
div FreqTable[_divisor]
;divide refresh rate
;by _divisor
out dx, ax
;set new refresh rate
;select Power-Save Register
;get divisor
mov dx, PWRSAV
mov ax, _divisor
and ax, 7
;mask off unwanted bits
;set enable bit
or
ax, PSEN
out dx, ax
pop dx
pop bx
pop ax
pop bp
ret
;divide frequency
;restore saved registers
;restore caller's bp
_power_save
lib_80C186
endp
ends
end
Example 5-1. Initializing the Power Management Unit for Power-Save Mode
5-14
6
Chip-Select Unit
CHAPTER 6
CHIP-SELECT UNIT
Every system requires some form of component-selection mechanism to enable the CPU to ac-
cess a specific memory or peripheral device. The signal that selects the memory or peripheral de-
vice is referred to as a chip-select. Besides selecting a specific device, each chip-select can be
used to control the number of wait states inserted into the bus cycle. Devices that are too slow to
keep up with the maximum bus bandwidth can use wait states to slow the bus down.
6.1 COMMON METHODS FOR GENERATING CHIP-SELECTS
One method of generating chip-selects uses latched address signals directly. An example inter-
with an active-low chip-select. Any bus cycle with an address between 10000H and 1FFFFH
(A16 = 1) enables the SRAM device. Also note that any bus cycle with an address starting at
30000H, 50000H, 70000H and so on also selects the SRAM device.
Decoding more address bits solves the problem of a chip-select being active over multiple address
ranges. In Figure 6-1(B), a one-of-eight decoder is connected to the uppermost address bits. Each
decoded output is active for one-eighth of the 1 Mbyte address space. However, each chip-select
has a fixed starting address and range. Future system memory changes could require circuit
changes to accommodate the additional memory.
6.2 CHIP-SELECT UNIT FEATURES AND BENEFITS
The Chip-Select Unit overcomes limitations of the designs shown in Figure 6-1 and has the fol-
lowing features:
• Ten chip-select outputs
• Programmable start and stop addresses
• Programmable wait-state generator
• Provision to disable a chip-select
• Provision to override bus ready
Figure 6-2 illustrates the logic blocks that generate a chip-select. Each chip-select has a duplicate
set of logic.
6-1
CHIP-SELECT UNIT
27C256
74AC138
Selects 896K to 1M
A19
A18
A17
A3
Y7
Y6
Y5
Y4
Y3
Y2
Y1
Y0
D7:0
Selects 768K to 896K
A2
A1
A0:12
A1:13
D15:8
E1
E2
ALE
RD
OE
CS
HLDA
Selects 128K to 256K
Selects 0 to 128K
A16
E3
(A)
(B)
Chip-Selects Using
Addresses Directly
Chip-Selects Using
Simple Decoder
A1168-0A
Figure 6-1. Common Chip-Select Generation Methods
6.3 CHIP-SELECT UNIT FUNCTIONAL OVERVIEW
The Chip-Select Unit (CSU) decodes bus cycle address and status information and enables the
appropriate chip-select. Figure 6-3 illustrates the timing of a chip-select during a bus cycle. Note
that the chip-select goes active in the same bus state as address goes active, eliminating any delay
through address latches and decoder circuits. The Chip-Select Unit activates a chip-select for bus
cycles initiated by the CPU, DMA Control Unit or Refresh Control Unit.
Six of the chip-selects map only into memory address space, while the remaining seven can map
into either memory or I/O address space. The chip-selects typically associate with memory and
peripheral devices as follows:
6-2
CHIP-SELECT UNIT
Internal
Address Bus
= Block Size
= Block Size
UCS
LCS
= Block Size/4
= Block Size/4
= Block Size/4
= Block Size/4
MCS3
MCS2
MCS1
MCS0
= Base
= Base
Base + 0
PCS0
PCS1
PCS2
PCS3
PCS4
Base + 128
Base + 256
Base + 384
Base + 512
Base + 640
Base + 768
Memory/
I/O Selector
MS
A
B
PCS5
PCS6
MUX
A/B
A1
A2
Internal Address Bit
EX Control Bit
A1139-0A
Figure 6-2. Chip-Select Block Diagram
6-3
CHIP-SELECT UNIT
UCS
LCS
Mapped only to the upper memory address space; selects the BOOT memory
device (EPROM or Flash memory types).
Mapped only to the lower memory address space; selects a static memory
(SRAM) device that stores the interrupt vector table, local stack, local data, and
scratch pad data.
MCS3:0
PCS6:0
Mapped only to memory address space; selects additional SRAM memory,
DRAM memory, or the system bus.
Mapped to memory or I/O address space; selects peripheral devices or generates
a DMA acknowledge strobe. Note that each PCSx is not individually config-
urable for I/O space or memory space.
T4
T1
T2
T3
T4
CLKOUT
ALE
AD15:0
A19:16
Address Valid
Status
UCS, PCS6:0
MCS3:0, LCS
S2:0
RD, WR
A1140-0A
Figure 6-3. Chip-Select Relative Timings
The UCS chip-select always ends at address location 0FFFFH; its block size (and thus its starting
address) is programmed in the UMCS register (Figure 6-5 on page 6-7). The LCS chip-select al-
256 Kbytes for both.
The MCS3:0 chip-selects access a contiguous block of memory address space. The block size can
range from 8 Kbytes to 512 Kbytes; it is programmed in the MMCS register (Figure 6-7 on page
6-9). Each chip-select goes active for one-fourth of the block. The start address is programmed
in the MPCS register (Figure 6-9 on page 6-11); it must be an integer multiple of the block size.
Because of the start address limitation, the MCS3:0 chip-selects cannot cover the entire memory
address space between the LCS and UCS chip-selects.
6-4
CHIP-SELECT UNIT
Methods such as those shown in Figure 6-1 on page 6-2 can be used to decode the remaining 256
Kbytes.
The PCS6:0 chip-selects access a contiguous, 896-byte block of memory or I/O address space.
Each chip-select goes active for one-seventh of the block (128 bytes). The start address is pro-
grammed in the PACS register (Figure 6-8 on page 6-10); it can begin on any 1 Kbyte boundary.
A chip-select goes active when it meets all of the following criteria:
1. The chip-select is enabled.
2. The bus cycle status matches the default or programmed type (memory or I/O).
3. The bus cycle address is within the default or programmed block size.
4. The bus cycle is not accessing the Peripheral Control Block.
A memory address applies to memory read, memory write and instruction prefetch bus cycles.
An I/O address applies to I/O read and I/O write bus cycles. Interrupt acknowledge and HALT
bus cycles never activate a chip-select, regardless of the address generated.
After power-on or system reset, only the UCS chip-select is initialized and active (see Figure 6-4).
Address
1
1MB
SRDY
ARDY
UCS
Data
1023K
UCS
Active For
Top 1 KByte
Memory
Map
Processor
0
NOTE:
1. 3 wait states automatically inserted. Bus READY must be provided.
A1006-0A
Figure 6-4. UCS Reset Configuration
6-5
CHIP-SELECT UNIT
6.4 PROGRAMMING
Four registers determine the operating characteristics of the chip-selects. The Peripheral Control
Block defines the location of the Chip-Select Unit registers. Table 6-1 lists the registers and their
associated programming names.
Table 6-1. Chip-Select Unit Registers
Control Register
Mnemonic
Alternate Register
Mnemonic
Chip-Select Affected
UMCS
LMCS
MMCS
PACS
None
None
UCS
LCS
MPCS
MPCS
MCS3:0
PCS6:0
The control registers (Figures 6-5 through 6-7) define the base address and bus ready and wait
state requirements for the corresponding chip-selects. The alternate control register (Figure 6-9)
defines the block size for MCS3:0. It also selects memory or I/O space for PCS6:0, selects the
function of the PCS6:5 pins, and defines the bus ready and wait state requirements for PCS6:4.
6.4.1 Initialization Sequence
Chip-selects do not have to be initialized in any specific order. However, the following guidelines
help prevent a system failure.
1. Initialize local memory chip-selects
2. Initialize local peripheral chip-selects
3. Perform local diagnostics
4. Initialize off-board memory and peripheral chip-selects
5. Complete system diagnostics
An unmasked interrupt or NMI must not occur until the interrupt vector addresses have been writ-
ten to memory. Failure to prevent an interrupt from occurring during initialization will cause a
system failure. Use external logic to generate the chip-select if interrupts cannot be masked prior
to initialization.
6-6
CHIP-SELECT UNIT
Register Name:
UCS Control Register
UMCS
Register Mnemonic:
Register Function:
Controls the operation of the UCS chip-select.
15
0
U
1
7
U
1
6
U
1
5
U
1
4
U
1
3
U
1
2
U
1
1
U
1
0
R
2
R
1
R
0
A1141-0A
Bit
Mnemonic
Reset
State
Bit Name
Start
U17:10
0FFH
Defines the starting address for the chip-select.
During memory bus cycles, U17:10 are
compared with the A17:10 address bits. An
equal to or greater than result enables the UCS
chip-select if A19:18 are both one. Table 6-2 on
page 6-12 lists the only valid programming
combinations.
Address
R2
Bus Ready
Disable
0H
3H
When R2 is clear, bus ready must be active to
complete a bus cycle. When R2 is set, R1:0
control the number of bus wait states and bus
ready is ignored.
R1:0
Wait State
Value
R1:0 define the minimum number of wait states
inserted into the bus cycle.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products. Programming
U17:10 with values other than those shown in Table 6-2 on page 6-12 results in
unreliable chip-select operation. Reading this register (before writing it) enables
the chip-select; however, none of the programmable fields will be properly initial-
ized.
Figure 6-5. UMCS Register Definition
6-7
CHIP-SELECT UNIT
Register Name:
LCS Control Register
Register Mnemonic:
Register Function:
LMCS
Controls the operation of the LCS chip-select.
15
0
U
1
7
U
1
6
U
1
5
U
1
4
U
1
3
U
1
2
U
1
1
U
1
0
R
2
R
1
R
0
A1142-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
U17:10
Ending
Address
00H
Defines the ending address for the chip-select.
During memory bus cycles, U17:10 are
compared with the A17:10 address bits. A less
than result enables the LCS chip-select if
A19:18 are both zero. Table 6.3 on page 6-13
lists the only valid programming combinations.
R2
Bus Ready
Disable
X
When R2 is clear, bus ready must be active to
complete a bus cycle. When R2 is set, R1:0
control the number of bus wait states and bus
ready is ignored.
R1:0
Wait State
Value
3H
R1:0 define the minimum number of wait states
inserted into the bus cycle.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products. Programming
U17:10 with values other than those shown in Table 6.3 on page 6-13 results in
unreliable chip-select operation. Reading this register (before writing it) enables
the chip-select; however, none of the programmable fields will be properly initial-
ized.
Figure 6-6. LMCS Register Definition
6-8
CHIP-SELECT UNIT
Register Name:
MCS Control Register
MMCS
Register Mnemonic:
Register Function:
Controls the operation of the MCS chip-selects.
15
0
U
1
9
U
1
8
U
1
7
U
1
6
U
1
5
U
1
4
U
1
3
R
2
R
1
R
0
A1143-0B
Bit
Mnemonic
Reset
State
Bit Name
Start
Function
Defines the starting address for the block of
U19:13
XXH
Address
MCS chip-selects. During memory bus cycles,
U19:13 are compared with the A19:13 address
bits. An equal to or greater than result enables
the MCS chip-select. The starting address must
be an integer multiple of the block size defined
in the MPCS register. See Table 6-5 on page
6-14 for additional information.
R2
Bus Ready
Disable
X
When R2 is clear, bus ready must be active to
complete a bus cycle. When R2 is set, R1:0
control the number of bus wait states and bus
ready is ignored.
R1:0
Wait State
Value
3H
R1:0 define the minimum number of wait states
inserted into the bus cycle.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products. A starting address
other than an integer multiple of the block size defined in the MPCS register
causes unreliable chip-select operation. (See Table 6-5 on page 6-14 for details.)
Reading this register and the MPCS register (before writing them) enables the
MCS chip-selects; however, none of the programmable fields will be properly ini-
tialized.
Figure 6-7. MMCS Register Definition
6-9
CHIP-SELECT UNIT
Register Name:
PCS Control Register
Register Mnemonic:
Register Function:
PACS
Controls the operation of the PCS chip-selects.
15
0
U
1
9
U
1
8
U
1
7
U
1
6
U
1
5
U
1
4
U
1
3
R
2
R
1
R
0
A1143-0B
Bit
Mnemonic
Reset
State
Bit Name
Start
Function
U19:13
XXH
Defines the starting address for the block of
PCS chip-selects. During memory or I/O bus
Address
cycles, U19:13 are compared with the A19:13
address bits. An equal to or greater than result
enables the PCS chip-select. U19:16 must be
programmed to zero for proper I/O bus cycle
operation.
R2
Bus Ready
Disable
X
When R2 is clear, bus ready must be active to
complete a bus cycle. When R2 is set, R1:0
control the number of bus wait states and bus
ready is ignored.
R1:0
Wait State
Value
3H
R1:0 define the minimum number of wait states
inserted into the bus cycle.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products. U19:16 must be
programmed to zero for proper I/O bus cycle operation. Reading this register and
the MPCS register (before writing them) enables the PCS chip-selects; however,
none of the programmable fields will be properly initialized.
Figure 6-8. PACS Register Definition
6-10
CHIP-SELECT UNIT
Register Name:
MCS and PCS Alternate Control Register
Register Mnemonic:
Register Function:
MPCS
Controls operation of the MCS and PCS chip-
selects.
15
0
M
6
M
5
M
4
M
3
M
2
M
1
M
0
E
X
M
S
R
2
R
1
R
0
A1144-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
M6:0
Block Size
XXH
XH
Defines the block size for the MCS chip-selects.
Table 6-5 on page 6-14 lists allowable values.
EX
MS
R2
Pin Selector
Setting EX configures PCS6:5 as chip-selects.
Clearing EX configures the pins as latched
address bits A2:A1.
Bus Cycle
Selector
XH
X
Clearing MS activates PCS6:0 for I/O bus
cycles. Setting MS activates PCS6:0 for
memory bus cycles.
Bus Ready
Disable for
PCS6:4
Applies only to PCS6:4. When R2 is clear, bus
ready must be active to complete a bus cycle.
When R2 is set, R1:0 control the number of bus
wait states and bus ready is ignored.
R1:0
Wait State
Value for
PCS6:4
3H
Apply only to PCS6:4. R1:0 define the minimum
number of wait states inserted into the bus
cycle.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products. A starting address
other than an integer multiple of the block size defined in this register causes
unreliable chip-select operation. Reading this register and the MMCS or PACS
register (before writing them) enables the associated chip-selects; however, none
of the programmable fields will be properly initialized.
Figure 6-9. MPCS Register Definition
6-11
CHIP-SELECT UNIT
The UMCS and LMCS registers can be programmed in any sequence. To program the MCS and
PCS chip-selects, follow this sequence:
1. Program the MPCS register
2. Program the MMCS register to enable the MCS chip-selects.
3. Program the PACS register to enable the PCS chip-selects.
6.4.2 Programming the Active Ranges
The active ranges of the chip-selects are determined by a combination of their starting or ending
select.
6.4.2.1
UCS Active Range
The UCS starting address is 100000H (1 Mbyte) minus the block size; its ending address is fixed
at 0FFFFFH.Table 6-2 defines the acceptable values for the field (U17:10) in the UMCS register
that determines the UCS block size and starting address.
Table 6-2. UCS Block Size and Starting Address
UMCS Field
U17:10
Block Size
(Kbytes)
Starting Address
00H
80H
C0H
E0H
F0H
F8H
FCH
FEH
FFH
256
128
64
32
16
8
0C0000H
0E0000H
0F0000H
0F8000H
0FC000H
0FE000H
0FF000H
0FF800H
0FFC00H
4
2
1
6-12
CHIP-SELECT UNIT
6.4.2.2
LCS Active Range
The LCS starting address is fixed at zero in memory address space; its ending address is the pro-
grammed block size minus one. Table 6.3 defines the acceptable values for the field (U17:10) in
the LMCS register that determines the LCS block size and ending address.
Table 6.3 LCS Active Range
LMCS Field
U17:10
Block Size
(Kbytes)
Ending Address
00H
01H
03H
07H
0FH
1FH
3FH
7FH
1
2
003FFH
007FFH
00FFFH
01FFFH
03FFFH
07FFFH
0FFFFH
1FFFFH
3FFFFH
4
8
16
32
64
128
256
6.4.2.3
MCS Active Range
The starting and ending addresses of the individual MCS chip-selects are determined by the base
address programmed in the MMCS register and the block size programmed in the MPCS register
(see Table 6-4 and Figure 6-10). The base address must be an integer multiple of the block size.
Table 6-5 lists the allowable block sizes and base address limitations.
Table 6-4. MCS Active Range
Active Range
Chip-
Select
Start Address
Base
Ending Address
MCS0
MCS1
MCS2
MCS3
Base + (1/4 block size –1)
Base + 1/4 block size Base + (1/2 block size –1)
Base + 1/2 block size Base + (3/4 block size –1)
Base + 3/4 block size Base + (block size – 1)
6-13
CHIP-SELECT UNIT
Table 6-5. MCS Block Size and Start Address Restrictions
MPCS Block Size Bits
Block Size
(Kbytes)
MMCS Start Address
Restrictions
M6
M5
M4
M3
M2
M1
M0
0
0
0
0
0
0
1
0
0
0
0
0
1
X
0
0
0
0
1
X
X
0
0
0
0
0
1
1
X
X
X
X
X
X
8
None
16
U13 must be zero.
0
1
X
X
X
X
X
32
U14:13 must be zero.
U15:13 must be zero.
U16:13 must be zero.
U17:13 must be zero.
1
X
X
X
X
64
X
X
X
128
256
512
U18:13 must be zero.
NOTE: If U19 is one, will overlap UCS.
X= don’t care, but should be 0 for future compatibility.
Starting Address
Ending Address
Block Size is
defined by M6:0
Base + (Block Size-1)
MCS3 Active Range
Base + 3/4 Block Size
Base + 1/2 Block Size
Base + 1/4 Block Size
Base + (3/4 Block Size-1)
Base + (1/2 Block Size-1)
Base + (1/4 Block Size-1)
MCS2 Active Range
MCS1 Active Range
MCS0 Active Range
MCS Base
(Defined by U19:13)
Memory Map
A1136-0C
Figure 6-10. MCS3:0 Active Ranges
6-14
CHIP-SELECT UNIT
6.4.2.4
PCS Active Range
Each PCS chip-select starts at an offset above the base address programmed in the PACS register
and is active for 128 bytes. The base address can start on any 1 Kbyte memory or I/O address
location. Table 6-6 lists the active range for each PCS chip-select.
Table 6-6. PCS Active Range
Active Range
Chip-
Select
Start Address
Base
Ending Address
PCS0
PCS1
PCS2
PCS3
PCS4
PCS5
PCS6
Base + 127 (7FH)
Base + 255 (0FFH)
Base + 383 (17FH)
Base + 511 (1FFH)
Base + 639 (27FH)
Base + 767 (2FFH)
Base + 895 (37FH)
Base + 128 (080H)
Base + 256 (100H)
Base + 384 (180H)
Base + 512 (200H)
Base + 640 (280H)
Base + 768 (300H)
6.4.3 Bus Wait State and Ready Control
Normally, the bus ready input must be inactive at the appropriate time to insert wait states into
the bus cycle. The Chip-Select Unit can ignore the state of the bus ready input to extend and com-
three or fewer wait states. However, accessing such devices as a dual-port memory, an expansion
bus interface, a system bus interface or remote peripheral devices can require more than three
wait states to complete a bus cycle.
A three-bit field (R2:0) in the control registers defines the number of wait states and the ready
requirements for the chip-selects. Figure 6-11 shows a simplified logic diagram of the wait state
and ready control functions.
6-15
CHIP-SELECT UNIT
BUS READY
R2 Control Bit
READY
Wait
State
Counter
Wait
State
Ready
Wait State Value (R1:0)
A1137-0A
Figure 6-11. Wait State and Ready Control Functions
The R2 control bit determines whether the bus cycle completes normally (requires bus ready) or
unconditionally (ignores bus ready). The R1:0 bits define the number of wait states to insert into
the bus cycle. For devices requiring three or fewer wait states, you can set R2 (ignore bus ready)
and program R1:0 with the number of required wait states. For devices that may require more than
three wait states, you must clear R2 (require bus ready).
A bus cycle with wait states automatically inserted cannot be shortened. A bus cycle that ignores
bus ready cannot be lengthened.
6.4.4 Overlapping Chip-Selects
The Chip-Select Unit activates all enabled chip-selects programmed to cover the same physical
address space. This is true if any portion of the chip-selects’ address ranges overlap (i.e., chip-
selects’ ranges do not need to overlap completely to all go active). There are various reasons for
overlapping chip-selects. For example, a system might have a need for overlapping a portion of
read-only memory with read/write memory or copying data to two devices simultaneously.
If overlapping chip-selects do not have identical wait state and bus ready programming, the Chip-
Select Unit uses the following priority scheme:
1. If any MCS chip-select is active, it uses the R2:0 bits in the MPCS register.
2. If the PCS chip-selects overlap LCS, it uses the R2:0 bits in the LMCS register.
3. If the PCS chip-selects overlap UCS, it uses the R2:0 bits in the UMCS register.
6-16
CHIP-SELECT UNIT
For example, assume MCS3 overlaps UCS. MCS3 is programmed for two wait states and re-
quires bus ready, while UCS is programmed for no wait states and ignores bus ready. An access
to the overlapped region has two wait states and requires bus ready (the values programmed in
the R2:0 bits in the MPCS register).
Be cautious when overlapping chip-selects with different wait state or bus ready programming.
The following two conditions require special attention to ensure proper system operation:
1. When all overlapping chip-selects ignore bus ready but have different wait states, verify
that each chip-select still works properly using the highest wait state value. A system
failure may result when too few or too many wait states occur in the bus cycle.
2. If one or more of the overlapping chip-selects requires bus ready, verify that all chip-
selects that ignore bus ready still work properly using both the smallest wait state value
and the longest possible bus cycle. A system failure may result when too few or too many
wait states occur in the bus cycle.
6.4.5 Memory or I/O Bus Cycle Decoding
The UCS, LCS and MCS chip-selects activate only for memory bus cycles. The PCS chip-selects
activate for either memory or I/O bus cycles, depending on the state of the MS bit in the MPCS
register (Figure 6-9 on page 6-11). Memory bus cycles consist of memory read, memory write
and instruction prefetch cycles. I/O bus cycles consist of I/O read and I/O write cycles.
Chip-selects go active for bus cycles initiated by the CPU, DMA Control Unit and Refresh Con-
trol Unit.
6.4.6 Programming Considerations
When programming the PCS chip-selects active for I/O bus cycles, remember that eight bytes of
I/O are reserved by Intel. These eight bytes (locations 00F8H through 00FFH) control the inter-
face to an 80C187 math coprocessor. A chip-select can overlap this reserved space provided there
is no intention of using the 80C187. However, to avoid possible future compatibility issues, Intel
recommends that the PCS chip-selects not start at I/O address location 0H.
Reading or writing the chip-select registers enables the corresponding chip-select. Reading a reg-
ister before writing to it enables the chip-select without initializing the programmable fields,
which causes indeterminate operation. For example, reading the LMCS register enables the LCS
chip-select, but it does not ensure that LCS is programmed correctly. Once you enable a chip-
select, you cannot disable it, but you can change its operation by writing to the appropriate reg-
ister.
6-17
CHIP-SELECT UNIT
The Chip-Select Unit decodes only internally generated address and bus state information. An ex-
ternal bus master cannot make use of the Chip-Select Unit. During HLDA, all chip-selects remain
inactive.
The circuit shown in Figure 6-12 allows an external bus master to access a device during bus
HOLD.
CSU Chip Select
Device select
External Master Chip Select
A1167-0A
Figure 6-12. Using Chip-Selects During HOLD
6.6 EXAMPLES
The following sections provide examples of programming the Chip-Select Unit to meet the needs
of a particular application. The examples do not go into hardware analysis or design issues.
6.6.1 Example 1: Typical System Configuration
Figure 6-13 illustrates a block diagram of a typical system design with a 128 Kbyte EPROM and
a 32 Kbyte SRAM. The peripherals are mapped to I/O address space. Example 6.1 shows a pro-
gram template for initializing the Chip-Select Unit.
6-18
CHIP-SELECT UNIT
$
$
TITLE
MOD186XREF
NAME
(Chip-Select Unit Initialization)
CSU_EXAMPLE_1
; External reference from this module
include(PCBMAP.INC
$
;File declares register
;locations and names.
; Module equates
; Configuration equates
INTRDY EQU
EXTRDY EQU
0004H
0000H
0080H
0040H
;Internal bus ready modifier
;External bus ready modifier
;PCS Memory/IO select modifier
;PCS/Latched address modifier
IO
EQU
ALLPCS EQU
;Below is a list of the default system memory and I/O environment. These
;defaults configure the Chip-Select Unit for proper system operation.
;EPROM memory is located from 0E0000 to 0FFFFF (128 Kbytes).
;Wait states are calculated assuming 16MHz operation.
;UCS# controls the accesses to EPROM memory space.
EPROM_SIZE EQU 128
EPROM_BASE EQU 1024 - EPROM_SIZE;Start address in Kbytes
EPROM_WAIT EQU 1 ;Wait states
;Size in Kbytes
;The UMCS register values are calculated using the above system contraints
;and the equations below.
UMCS_VAL EQU
(EPROM_BASE SHL 6)OR (0C038H) OR
(EPROM_RDY) OR (EPROM_WAIT)
&
;SRAM memory starts at 0H and continues to 7FFFH (32 Kbytes).
;Wait states are calculated assuming 16MHz operation.
;LCS# controls the accesses to SRAM memory space.
SRAM_SIZE EQU 32
SRAM_BASE EQU 0
SRAM_WAIT EQU 0
SRAM_RDY EQU INTRDY
;Size in Kbytes
;Start address in Kbytes
;Wait states
;Ignore bus ready
;The LMCS register value is calculated using the above system constraints
;and the equations below
LMCS_VAL EQU ((SRAM_SIZE - 1)SHL6) OR (0038H) OR
&
(SRAM_RDY)
OR (SRAM_WAIT)
;A DRAM interface is selected by the MCS3:0 chip-selects. The BASE value
;defines the starting address of the DRAM window. The SIZE value (along with
;the BASE ;value) defines the ending address. Zero wait state performance
;is assumed. The Refresh Control Unit uses DRAM_BASE to properly configure
;refresh operation.
Example 6-1. Initializing the Chip-Select Unit
6-20
CHIP-SELECT UNIT
DRAM_BASE EQU
DRAM_SIZE EQU
DRAM_WAIT EQU
256
256
0
;window start address in Kbytes
;window size in Kbytes
;wait states
DRAM_RDY
EQU
INTRDY
;ignore bus ready
;The MPCS register is used to program both the MCS and PCS chip-selects.
;Below are the equates for the I/O peripherals (also used to program the PACS
;register.
IO_WAIT
IO_RDY
EQU
EQU
4
;IO wait states
INTRDY
IO
;ignore bus ready
PCS_SPACE EQU
;put PCS# chip-selects in I/O space
;generate PCS5# and PCS6#
PCS_FUNC
EQU
ALLPCS
;The MMCS and MPCS register values are calculated using the above system
;constraints and the equations below:
MMCS_VAL
MPCS_VAL
&
EQU
EQU
(DRAM_BASE SHL 6) OR (001F8H) OR (DRAM_RDY) OR (DRAM_WAIT)
(DRAM_SIZE SHL 5) OR (08038H) OR (PCS_SPACE) OR (PCS_FUNC) OR
(IO_RDY) OR (IO_WAIT)
;I/O is selected using the PCS0# chip-select. Wait states assume operation at
;16 MHz. For this example, the Floppy Disk Controller is connected to PCS2# and
;PCS1# provides the DACK signal.
IO_BASE
EQU
1
;I/O start address in Kbytes
;The PACS register value is calculated using the above system constraints and
;the equation below.
PACS_VAL
EQU
(IO_BASE SHL 6) OR (0038H) OR (IO_RDY) OR (IO_WAIT)
;The following statements define the default assumptions for segment locations.
ASSUME CS:CODE
ASSUME DS:DATA
ASSUME SS:DATA
ASSUME ES:DATA
CODE
;
SEGMENT PUBLIC 'CODE'
;Entry point on power-up
;
FW_START
LABEL FAR
CLI
;forces far jump
;disable interrupts
;Place register initialization code here
;
;Set up chip-selects.
;UCS - EPROM Select
;LCS - SRAM Select
;PCS - I/O Select
;MCS - DRAM Select
(initialized during POWER_ON code)
(set to SRAM size)
(PCS1:0 to support floppy)
(set to DRAM size)
mov
mov
out
dx, LMCS_REG
ax, LMCS_VAL
dx, al
;set up LMCS register
;remember that byte writes are OK
Example 6-1. Initializing the Chip-Select Unit (Continued)
6-21
CHIP-SELECT UNIT
mov
mov
out
dx, MPCS_REG
ax, MPCS_VAL
dx, al
;ready for PCS lines 4-6
;as well as MCS programming
mov
mov
out
dx, MMCS_REG
ax, MMCS_VAL
dx, al
;set up DRAM chip-selects
;set up I/O chip-select
mov
mov
out
dx, PACS_REG
ax, PACS_VAL
dx, al
CODE
;
ENDS
;Power-on reset code to get started
;
ASSUME CS:POWER_ON
POWER_ON SEGMENT AT 0FFFFH
mov
mov
out
jmp
ENDS
dx, UMCS_REG
ax, UMCS_VAL
dx, al
;point to UMCS register
;reprogram UMCS to match system
;requirements
FW_START
;jump to initialization code
POWER_ON
;
;Data segment
;
DATA
SEGMENTPUBLIC 'DATA'
DD 256 DUP (?)
;Place memory variables here
DW 500 DUP (?)
STACK_TOP LABEL WORD
DATA ENDS
;Program ends
;reserved for interrupt vectors
;stack allocation
END
Example 6-1. Initializing the Chip-Select Unit (Continued)
6-22
7
Refresh Control Unit
CHAPTER 7
REFRESH CONTROL UNIT
The Refresh Control Unit (RCU) simplifies dynamic memory controller design with its integrat-
ed address and clock counters. Figure 7-1 shows the relationship between the Bus Interface Unit
and the Refresh Control Unit. Integrating the Refresh Control Unit into the processor allows an
external DRAM controller to use chip-selects, wait state logic and status lines.
Refresh Clock
Interval Register
CPU
Clock
9-Bit Down
Refresh Request
Counter
BIU
Interface
Refresh Acknowledge
CLR
REQ
F-Bus
Refresh Control
Register
9-Bit Address Counter
Refresh Address
Register
Refresh Base
Address Register
7
13
20-Bit
Refresh Address
A1539-01
Figure 7-1. Refresh Control Unit Block Diagram
7-1
REFRESH CONTROL UNIT
7.1 THE ROLE OF THE REFRESH CONTROL UNIT
Like a DMA controller, the Refresh Control Unit runs bus cycles independent of CPU execution.
Unlike a DMA controller, however, the Refresh Control Unit does not run bus cycle bursts nor
does it transfer data. The DRAM refresh process freshens individual DRAM rows in “dummy
read” cycles, while cycling through all necessary addresses.
The microprocessor interface to DRAMs is more complicated than other memory interfaces. A
complete DRAM controller requires circuitry beyond that provided by the processor even in the
simplest configurations. This circuitry must respond correctly to reads, writes and DRAM refresh
cycles. The external DRAM controller generates the Row Address Strobe (RAS), Column Ad-
dress Strobe (CAS) and other DRAM control signals.
Pseudo-static RAMs use dynamic memory cells but generate address strobes and refresh address-
es internally. The address counters still need external timing pulses. These pulses are easy to de-
rive from the processor’s bus control signals. Pseudo-static RAMs do not need a full DRAM
controller.
7.2 REFRESH CONTROL UNIT CAPABILITIES
with up to 9 rows of memory cells (9 refresh address bits). This includes all practical DRAM sizes
for the processor’s 1 Mbyte address space.
7.3 REFRESH CONTROL UNIT OPERATION
Figure 7-2 illustrates Refresh Control Unit counting, address generation and BIU bus cycle gen-
eration in flowchart form.
The nine-bit down-counter loads from the Refresh Interval Register on the falling edge of CLK-
OUT. Once loaded, it decrements every falling CLKOUT edge until it reaches one. Then the
down-counter reloads and starts counting again, simultaneously triggering a refresh request.
Once enabled, the DRAM refresh process continues indefinitely until the user reprograms the Re-
fresh Control Unit, a reset occurs, or the processor enters Powerdown mode. Power-Save mode
divides the Refresh Control Unit clocks, so reprogramming the Refresh Interval Register be-
comes necessary.
The refresh request remains active until the bus becomes available. When the bus is free, the BIU
will run its “dummy read” cycle. Refresh bus requests have higher priority than most CPU bus
cycles, all DMA bus cycles and all interrupt vectoring sequences. Refresh bus cycles also have a
higher priority than the HOLD/HLDA bus arbitration protocol (see “Refresh Operation and Bus
HOLD” on page 7-12).
7-2
REFRESH CONTROL UNIT
Refresh Control
Unit Operation
BIU Refresh
Bus Operation
Refresh Request
Acknowledged
Set "E" Bit
Execute
Memory Read
Load Counter
From Refresh Clock
Interval Register
Increment
Address
Counter = ?
Remove
Request
Executed
Every
Clock
Continue
Decrement
Counter
Generated BIU
Request
A1265-0A
Figure 7-2. Refresh Control Unit Operation Flow Chart
The nine-bit refresh clock counter does not wait until the BIU services the refresh request to con-
tinue counting. This operation ensures that refresh requests occur at the correct interval. Other-
wise, the time between refresh requests would be a function of varying bus activity. When the
BIU services the refresh request, it clears the request and increments the refresh address.
7-3
REFRESH CONTROL UNIT
The BIU does not queue DRAM refresh requests. If the Refresh Control Unit generates another
request before the BIU handles the present request, the BIU loses the present request. However,
the address associated with the request is not lost. The refresh address changes only after the BIU
that the processor will successfully refresh the corresponding row of cells in the DRAM, retaining
the data.
7.4 REFRESH ADDRESSES
Figure 7-3 shows the physical address generated during a refresh bus cycle. This figure applies
to both the 8-bit and 16-bit data bus microprocessor versions. Refresh address bits RA19:13 come
from the Refresh Base Address Register. (See “Refresh Base Address Register” on page 7-8.)
From Refresh Base
Address Register
Fixed
0
From Refresh Address Counter Fixed
0 RA RA RA RA RA RA RA RA RA 1
RA RA RA RA RA RA RA 0
19 18 17 16 15 14 13
9
8
7
6
5
4
3
2
1
19
0
20-Bit Refresh Address
A1502-0A
Figure 7-3. Refresh Address Formation
Refresh address bits RA12:10 are always zero. A linear-feedback shift counter generates address
bits RA9:1 and RA0 is always one. The counter does not count linearly from 0 through 1FFH.
However, the counting algorithm cycles uniquely through all possible 9-bit values. It matters only
that each row of DRAM memory cells is refreshed at a specific interval. The order of the rows is
unimportant.
Address bit A0 is fixed at one during all refresh operations. In applications based on a 16-bit data
bus processor, A0 typically selects memory devices placed on the low (even) half of the bus. Ap-
plications based on an 8-bit data bus processor typically use A0 as a true address bit. The DRAM
controller must not route A0 to row address pins on the DRAMs.
7-4
REFRESH CONTROL UNIT
7.5 REFRESH BUS CYCLES
Refresh bus cycles look exactly like ordinary memory read bus cycles except for the control sig-
nals listed in Table 7-1. These signals can be ANDed in a DRAM controller to detect a refresh
bus cycle. The 16-bit bus processor drives both the BHE and A0 pins high during refresh cycles.
The 8-bit bus version replaces the BHE pin with RFSH, which has the same timings. The 8-bit
bus processor drives RFSH low and A0 high during refresh cycles.
Table 7-1. Identification of Refresh Bus Cycles
Data Bus Width
BHE/RFSH
A0
16-Bit Device
8-Bit Device
1
0
1
1
7.6 GUIDELINES FOR DESIGNING DRAM CONTROLLERS
The basic DRAM access method consists of four phases:
1. The DRAM controller supplies a row address to the DRAMs.
2. The DRAM controller asserts a Row Address Strobe (RAS), which latches the row
address inside the DRAMs.
3. The DRAM controller supplies a column address to the DRAMs.
4. The DRAM controller asserts a Column Address Strobe (CAS), which latches the column
address inside the DRAMs.
Most 80C186 Modular Core family DRAM interfaces use only this method. Others are not dis-
cussed here.
The DRAM controller’s purpose is to use the processor’s address, status and control lines to gen-
erate the multiplexed addresses and strobes. These signals must be appropriate for three bus cycle
types: read, write and refresh. They must also meet specific pulse width, setup and hold timing
requirements. DRAM interface designs need special attention to transmission line effects, since
DRAMs represent significant loads on the bus.
DRAM controllers may be either clocked or unclocked. An unclocked DRAM controller requires
a tapped digital delay line to derive the proper timings.
Clocked DRAM controllers may use either discrete or programmable logic devices. A state ma-
chine design is appropriate, especially if the circuit must provide wait state control (beyond that
possible with the processor’s Chip-Select Unit). Because of the microprocessor’s four-clock bus,
clocking some logic elements on each CLKOUT phase is advantageous (see Figure 7-4).
7-5
REFRESH CONTROL UNIT
T4
T1
T2
T3/TW
T4
CLKOUT
Muxed
Address
Column
Row
S2:0
CS
RAS
CAS
WE
1
2
NOTES:
1. CAS is unnecessary for refresh cycles only.
2. WE is necessary for write cycles only.
A1267-0A
Figure 7-4. Suggested DRAM Control Signal Timing Relationships
The cycle begins with presentation of the row address. RAS should go active on the falling edge
of T2. At the rising edge of T2, the address lines should switch to a column address. CAS goes
active on the falling edge of T3. Refresh cycles do not require CAS. When CAS is present, the
“dummy read” cycle becomes a true read cycle (the DRAM drives the bus), and the DRAM row
still gets refreshed.
Both RAS and CAS stay active during any wait states. They go inactive on the falling edge of T4.
At the rising edge of T4, the address multiplexer shifts to its original selection (row addressing),
preparing for the next DRAM access.
7-6
REFRESH CONTROL UNIT
7.7 PROGRAMMING THE REFRESH CONTROL UNIT
Given a specific processor operating frequency and information about the DRAMs in the system,
the user can program the Refresh Control Unit registers.
7.7.1 Calculating the Refresh Interval
DRAM data sheets show DRAM refresh requirements as a number of refresh cycles necessary
and the maximum period to run the cycles. (The number of refresh cycles is the same as the num-
ber of rows.) You must compensate for bus latency — the time it takes for the Refresh Control
Unit to gain control of the bus. This is typically 1–5%, but if an external bus master will be ex-
tremely slow to release the bus, increase the overhead percentage. At standard operating frequen-
cies, DRAM refresh bus overhead totals 2–3% of the total bus bandwidth.
Given this information and the CPU operating frequency, use the formula in Figure 7-5 to deter-
mine the correct value for the RFTIME Register value.
RPERIOD × FCPU
------------------------------------------------------------------------------- = RFTIME RegisterValue
Rows + (Rows × Overhead%)
RPERIOD
FCPU
=
=
=
Maximum refresh period specified by DRAM manufacturer (in µs).
Operating frequency (in MHz).
Rows
Total number of rows to be refreshed.
Overhead % = Derating factor to compensate for missed refresh requests (typically 1 – 5 %).
Figure 7-5. Formula for Calculating Refresh Interval for RFTIME Register
If the processor enters Power-Save mode, the refresh rate must increase to offset the reduced CPU
clock rate to preserve memory. At lower frequencies, the refresh bus overhead increases. At fre-
quencies less than about 1.5 MHz, the Bus Interface Unit will spend almost all its time running
refresh cycles. There may not be enough bandwidth left for the processor to perform other activ-
ities, especially if the processor must share the bus with an external master.
7.7.2 Refresh Control Unit Registers
Three contiguous Peripheral Control Block registers operate the Refresh Control Unit: the Re-
fresh Base Address Register, Refresh Clock Interval Register and the Refresh Control Register.
7-7
REFRESH CONTROL UNIT
7.7.2.1
Refresh Base Address Register
The Refresh Base Address Register (Figure 7-6) programs the base (upper seven bits) of the re-
fresh address. Seven-bit mapping places the refresh address at any 4 Kbyte boundary within the
1 Mbyte address space. When the partial refresh address from the 9-bit address counter (see Fig-
ure 7-1 and “Refresh Control Unit Capabilities” on page 7-2) passes 1FFH, the Refresh Control
Unit does not increment the refresh base address. Setting the base address ensures that the address
driven during a refresh bus cycle activates the DRAM chip select.
Register Name:
Refresh Base Address Register
RFBASE
Register Mnemonic:
Register Function:
Determines upper 7 bits of refresh address.
15
0
R
A
1
R
A
1
R
A
1
R
A
1
R
A
1
R
A
1
R
A
1
9
8
7
6
5
4
3
A1503-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
RA19:13
Refresh
Base
00H
Uppermost address bits for DRAM refresh
cycles.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 7-6. Refresh Base Address Register
Refresh Clock Interval Register
7.7.2.2
The Refresh Clock Interval Register (Figure 7-7) defines the time between refresh requests. The
higher the value, the longer the time between requests. The down-counter decrements every fall-
ing CLKOUT edge, regardless of core activity. When the counter reaches one, the Refresh Con-
trol Unit generates a refresh request, and the counter reloads the value from the register. Since
Power-Save mode divides the clock to the Refresh Control Unit, this register will require repro-
gramming if Power-Save mode is used.
7-8
REFRESH CONTROL UNIT
Register Name:
Refresh Clock Interval Register
Register Mnemonic:
Register Function:
RFTIME
Sets refresh rate.
15
0
R
C
8
R
C
7
R
C
6
R
C
5
R
C
4
R
C
3
R
C
2
R
C
1
R
C
0
A1288-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
RC8:0
Refresh Counter
Reload Value
000H
Sets the desired clock count between refresh
cycles.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products.
Figure 7-7. Refresh Clock Interval Register
7.7.2.3
Refresh Control Register
Figure 7-8 shows the Refresh Control Register. The user may read or write the REN bit at any
time to turn the Refresh Control Unit on or off. The lower nine bits contain the current nine-bit
down-counter value. The user cannot program these bits. Disabling the Refresh Control Unit
clears both the counter and the corresponding counter bits in the control register.
7-9
REFRESH CONTROL UNIT
Register Name:
Refresh Control Register
RFCON
Register Mnemonic:
Register Function:
Controls Refresh Unit operation.
15
0
R
E
N
R
C
8
R
C
7
R
C
6
R
C
5
R
C
4
R
C
3
R
C
2
R
C
1
R
C
0
A1311-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
REN
Refresh
Control Unit
Enable
0
Setting REN enables the Refresh Unit. Clearing
REN disables the Refresh Unit.
RC8:0
Refresh
Counter
000H
These bits contain the present value of the
down-counter that triggers refresh requests.
The user cannot program these bits.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 7-8. Refresh Control Register
7.7.3 Programming Example
Example 7-1 contains sample code to initialize the Refresh Control Unit. Example 5-2 on page
5-14 shows the additional code to reprogram the Refresh Control Unit upon entering Power-Save
mode.
7-10
REFRESH CONTROL UNIT
$mod186
name
example_80C186_RCU_code
; FUNCTION: This function initializes the DRAM Refresh
; Control Unit to refresh the DRAM starting at dram_addr
; at clock_time intervals.
; SYNTAX:
; extern void far config_rcu(int dram_addr, int clock_time);
; INPUTS:
;
dram_addr - Base address of DRAM to refresh
clock_time - DRAM refresh rate
; OUTPUTS:
None
;
;
NOTE: Parameters are passed on the stack as
required by high-level languages.
RFBASE
RFTIME
RFCON
equ xxxxh
equ xxxxh
equ xxxxh
equ 8000h
;substitute register offset
Enable
;enable bit
lib_80186
segment public 'code'
assume cs:lib_80186
public _config_rcu
proc far
_config_rcu
push bp
;save caller's bp
mov bp, sp
;get current top of stack
_clock_time
_dram_addr
equ word ptr[bp+6]
equ word ptr[bp+8]
;get parameters off
;the stack
push ax
push cx
push dx
push di
;save registers that
;will be modified
Example 7-1. Initializing the Refresh Control Unit
7-11
REFRESH CONTROL UNIT
mov dx, RFBASE
mov ax, _dram_addr
out dx, al
;set upper 7 address bits
;set clock pre_scaler
;Enable RCU
mov dx, RFTIME
mov ax, _clock_time
out dx, al
mov dx, RFCON
mov ax, Enable
out dx, al
mov cx, 8
;8 dummy cycles are
;required by DRAMs
;before actual use
xor di, di
_exercise_ram:
mov word ptr [di], 0
loop _exercise_ram
pop di
pop dx
pop cx
pop ax
pop bp
;restore saved registers
;restore caller’s bp
ret
_config_rcu
lib_80186
endp
ends
end
Example 7-1. Initializing the Refresh Control Unit (Continued)
7.8 REFRESH OPERATION AND BUS HOLD
When another bus master controls the bus, the processor keeps HLDA active as long as the
HOLD input remains active. If the Refresh Control Unit generates a refresh request during bus
hold, the processor drives the HLDA signal inactive, indicating to the current bus master that it
wishes to regain bus control (see Figure 7-9). The BIU begins a refresh bus cycle only after the
alternate master removes HOLD. The user must design the system so that the processor can re-
gain bus control. If the alternate master asserts HOLD after the processor starts the refresh cycle,
the CPU will relinquish control by asserting HLDA when the refresh cycle is complete.
7-12
REFRESH CONTROL UNIT
T1
T1
T1
T1
T1
T4
T1
CLKOUT
4
3
1
HOLD
HLDA
2
6
AD15:0
DEN
RD, WR,
BHE, S2:0
DT / R,
5
A19:16
NOTES:
1. HLDA is deasserted; signaling need to run DRAM refresh cycles less than T
2. External bus master terminates use of the bus.
.
CLHAV
3. HOLD deasserted; greater than T
.
HVCL
4. Hold may be reasserted after one clock.
5. Lines come out of float in order to run DRAM refresh cycle.
A1534-0A
Figure 7-9. Regaining Bus Control to Run a DRAM Refresh Bus Cycle
7-13
8
Interrupt Control
Unit
CHAPTER 8
INTERRUPT CONTROL UNIT
The 80C186 Modular Core has a single maskable interrupt input. (See “Interrupts and Exception
Handling” on page 2-39.) The Interrupt Control Unit (ICU) expands the interrupt capabilities be-
modes: Master or Slave.
In Master mode, the ICU controls the maskable interrupt input to the CPU. Interrupts can origi-
nate from the on-chip peripherals and from four external interrupt pins. The ICU synchronizes
and prioritizes all interrupt sources and presents the correct interrupt type vector to the CPU. (See
Figure 8-1.) Most systems use master mode.
In Slave mode, an external 8259A module controls the maskable interrupt input to the CPU and
acts as the master interrupt controller. The ICU processes only those interrupts from the on-chip
peripherals and acts as an interrupt input to the 8259A. (See Figure 8-15 on page 8-24.) This mode
can be useful in larger system designs.
The Interrupt Control Unit has the following features:
• Programmable priority of each interrupt source
• Individual masking of each interrupt source
• Nesting of interrupt sources
• Support for polled operation
• Support for cascading external 8259A modules to expand external interrupt sources
8.1 FUNCTIONAL OVERVIEW
All microcomputer systems must communicate in some way with the external world. A typical
system might have a keyboard, a disk drive and a communications port, all requiring CPU atten-
tion at different times. There are two distinct ways to process peripheral I/O requests: polling and
interrupts.
Polling requires that the CPU check each peripheral device in the system periodically to see-
whether it requires servicing. It would not be unusual to poll a low-speed peripheral (a serial port,
for instance) thousands of times before it required servicing. In most cases, the use of polling has
a detrimental effect on system throughput. Any time used to check the peripherals is time spent
away from the main processing tasks.
8-1
INTERRUPT CONTROL UNIT
Interrupts eliminate the need for polling by signalling the CPU that a peripheral device requires
servicing. The CPU then stops executing the main task, saves its state and transfers execution to
CPU’s original state is restored and execution continues at the point of interruption in the main
task.
8.2 MASTER MODE
Figure 8-1 shows a block diagram of the Interrupt Control Unit in Master mode. In this mode, the
ICU processes all interrupt requests, both external and internal. The three timer interrupt requests
share a single input, while the others are supported directly.
DMA
0
DMA
1
Timer 0 Timer 1 Timer 2
INT0 INT1 INT2 INT3
Interrupt
Priority
Resolver
Vector
Generation
Logic
To CPU Interrupt Request
F - Bus
A1506-A0
Figure 8-1. Interrupt Control Unit in Master Mode
8.2.1 Generic Functions in Master Mode
Several functions of the Interrupt Control Unit are common among most interrupt controllers.
This section describes how those generic functions are implemented in the Interrupt Control Unit.
8-2
INTERRUPT CONTROL UNIT
8.2.1.1
Interrupt Masking
There are circumstances in which a programmer may need to disable an interrupt source tempo-
rarily (for example, while executing a time-critical section of code or servicing a high-priority
task). This temporary disabling is called interrupt masking. All interrupts from the Interrupt Con-
trol Unit can be masked either globally or individually.
The Interrupt Enable bit in the Processor Status Word globally enables or disables the maskable
interrupt request from the Interrupt Control Unit. The programmer controls the Interrupt Enable
bit with the STI (set interrupt) and CLI (clear interrupt) instructions.
Besides being globally enabled or disabled by the Interrupt Enable bit, each interrupt source can
be individually enabled or disabled. The Interrupt Mask register has a single bit for each interrupt
source. The programming can selectively mask (disable) or unmask (enable) each interrupt
source by setting or clearing the corresponding bit in the Interrupt Mask register.
8.2.1.2
Interrupt Priority
One critical function of the Interrupt Control Unit is to prioritize interrupt requests. When multi-
systems, an interrupt handler may itself be interrupted by another interrupt source. This is known
as interrupt nesting. With interrupt nesting, priority determines whether an interrupt source can
preempt an interrupt handler that is currently executing.
Each interrupt source is assigned a priority between zero (highest) and seven (lowest). After reset,
the interrupts default to the priorities shown in Table 8-1. Because the timers share an interrupt
source, they also share a priority. Within the assigned priority, each has a relative priority (Timer
0 has the highest relative priority and Timer 2 has the lowest).
Table 8-1. Default Interrupt Priorities
Interrupt Name
Relative Priority
Timer 0
Timer 1
Timer 2
DMA0
DMA1
INT0
0 (a)
0 (b)
0 (c)
1
2
3
4
5
6
INT1
INT2
INT3
8-3
INTERRUPT CONTROL UNIT
The priority of each source is programmable. The Interrupt Control register enables the
programmer to assign each source a priority that differs from the default. The priority must still
be between zero (highest) and seven (lowest). Interrupt sources can be programmed to share a
priority. The Interrupt Control Unit uses the default priorities (see Table 8-1) within the shared
priority level to determine which interrupt to service first. For example, assume that INT0 and
INT1 are both programmed to priority seven. Because INT0 has the higher default priority, it is
serviced first.
Interrupt sources can be masked on the basis of their priority. The Priority Mask register masks
all interrupts with priorities lower than its programmed value. After reset, the Priority Mask reg-
ister contains priority seven, which effectively enables all interrupts. The programmer can then
program the register with any valid priority level.
8.2.1.3
Interrupt Nesting
When entering an interrupt handler, the CPU pushes the Processor Status Word onto the stack
and clears the Interrupt Enable bit. The processor enters all interrupt handlers with maskable in-
terrupts disabled. Maskable interrupts remain disabled until either the IRET instruction restores
the Interrupt Enable bit or the programmer explicitly enables interrupts. Enabling maskable in-
terrupts within an interrupt handler allows interrupts to be nested. Otherwise, interrupts are pro-
cessed sequentially; one interrupt handler must finish before another executes.
The simplest way to use the Interrupt Control Unit is without nesting. The operation and servicing
of all sources of maskable interrupts is straightforward. However, the application tradeoff is that
an interrupt handler will finish executing even if a higher priority interrupt occurs. This can add
considerable latency to the higher priority interrupt.
In the simplest terms, the Interrupt Control Unit asserts the maskable interrupt request to the CPU,
waits for the interrupt acknowledge, then presents the interrupt type of the highest priority un-
masked interrupt to the CPU. The CPU then executes the interrupt handler for that interrupt. Be-
cause the interrupt handler never sets the Interrupt Enable bit, it can never be interrupted.
The function of the Interrupt Control Unit is more complicated with interrupt nesting. In this case,
another. Two rules apply for interrupt nesting:
• An interrupt source cannot preempt interrupts of higher priority.
• An interrupt source cannot preempt itself. The interrupt handler must finish executing
before the interrupt is serviced again. (Special Fully Nested Mode is an exception. See
“Special Fully Nested Mode” on page 8-8.)
8-4
INTERRUPT CONTROL UNIT
8.3 FUNCTIONAL OPERATION IN MASTER MODE
This section covers the process in which the Interrupt Control Unit receives interrupts and asserts
the maskable interrupt request to the CPU.
8.3.1 Typical Interrupt Sequence
When the Interrupt Control Unit first detects an interrupt, it sets the corresponding bit in the In-
terrupt Request register to indicate that the interrupt is pending. The Interrupt Control Unit checks
all pending interrupt sources. If the interrupt is unmasked and meets the priority criteria (see “Pri-
ority Resolution” on page 8-5), the Interrupt Control Unit asserts the maskable interrupt request
to the CPU, then waits for the interrupt acknowledge.
When the Interrupt Control Unit receives the interrupt acknowledge, it passes the interrupt type
to the CPU. At that point, the CPU begin the interrupt processing sequence.(See “Interrupt/Ex-
ception Processing” on page 2-39 for details.) The Interrupt Control Unit always passes the vector
that has the highest priority at the time the acknowledge is received. If a higher priority interrupt
occurs before the interrupt acknowledge, the higher priority interrupt has precedence.
When it receives the interrupt acknowledge, the Interrupt Control Unit clears the corresponding
bit in the Interrupt Request register and sets the corresponding bit in the In-Service register. The
In-Service register keeps track of which interrupt handlers are being processed. At the end of an
interrupt handler, the programmer must issue an End-of-Interrupt (EOI) command to explicitly
clear the In-Service register bit. If the bit remains set, the Interrupt Control Unit cannot process
any additional interrupts from that source.
8.3.2 Priority Resolution
The decision to assert the maskable interrupt request to the CPU is somewhat complicated. The
complexity is needed to support interrupt nesting. First, an interrupt occurs and the corre-
sponding Interrupt Request register bit is set. The Interrupt Control Unit then asserts the
maskable interrupt request to the CPU, if the pending interrupt satisfies these requirements:
1. its Interrupt Mask bit is cleared (it is unmasked)
2. its priority is higher than the value in the Priority Mask register
3. its In-Service bit is cleared
4. its priority is equal to or greater than that of any interrupt whose In-Service bit is set
The In-Service register keeps track of interrupt handler execution. The Interrupt Control Unit
uses this information to decide whether another interrupt source has sufficient priority to preempt
an interrupt handler that is executing.
8-5
INTERRUPT CONTROL UNIT
8.3.2.1
Priority Resolution Example
This example illustrates priority resolution. Assume these initial conditions:
• the Interrupt Control Unit has been initialized
• no interrupts are pending
• no In-Service bits are set
• the Interrupt Enable bit is set
• all interrupts are unmasked
• the default priority scheme is being used
• the Priority Mask register is set to the lowest priority (seven)
The example uses two external interrupt sources, INT0 and INT3, to describe the process.
1. A low-to-high transition on INT0 sets its Interrupt Request bit. The interrupt is now
pending.
2. Because INT0 is the only pending interrupt, it meets all the priority criteria. The Interrupt
Control Unit asserts the interrupt request to the CPU and waits for an acknowledge.
3. The CPU acknowledges the interrupt.
4. The Interrupt Control Unit passes the interrupt type (in this case, type 12) to the CPU.
5. The Interrupt Control Unit clears the INT0 bit in the Interrupt Request register and sets the
INT0 bit in the In-Service register.
6. The CPU executes the interrupt processing sequence and begins executing the interrupt
handler for INT0.
7. During execution of the INT0 interrupt handler, a low-to-high transition on INT3 sets its
Interrupt Request bit.
8. The Interrupt Control Unit determines that INT3 has a lower priority than INT0, which is
currently executing (INT0’s In-Service bit is set). INT3 does not meet the priority criteria,
so no interrupt request is sent to the CPU. (If INT3were programmed with a higher
priority than INT0, the request would be sent.) INT3 remains pending in the Interrupt
Request register.
9. The INT0 interrupt handler completes and sends an EOI command to clear the INT0 bit in
the In-Service register.
10. INT3 is still pending and now meets all the priority criteria. The Interrupt Control Unit
asserts the interrupt request to the CPU and the process begins again.
8-6
INTERRUPT CONTROL UNIT
8.3.2.2
Interrupts That Share a Single Source
Multiple interrupt requests can share a single interrupt input to the Interrupt Control Unit. (For
example, the three timers share a single input.) Although these interrupts share an input, each has
its own interrupt vector. (For example, when a Timer 0 interrupt occurs, the Timer 0 interrupt
handler is executed.) This section uses the three timers as an example to describe how these in-
terrupts are prioritized and serviced.
The Interrupt Status register acts as a second-level request register to process the timer interrupts.
It contains a bit for each timer interrupt. When a timer interrupt occurs, both the individual Inter-
rupt Status register bit and the shared Interrupt Request register bit are set. From this point, the
interrupt is processed like any other interrupt source.
When the shared interrupt is acknowledged, the timer interrupt with the highest priority (see Ta-
ble 8-1 on page 8-3) at that time is serviced first and that timer’s Interrupt Status bit is cleared.
If no other timer Interrupt Status bits are set, the shared Interrupt Request bit is also cleared. If
other timer interrupts are pending, the Interrupt Request bit remains set.
When the timer interrupt is acknowledged, the shared In-Service bit is set. No other timer inter-
rupts can occur when the In-Service bit is set. If a second timer interrupt occurs while another
timer interrupt is being serviced, the second interrupt remains pending until the interrupt handler
for the first interrupt finishes and clears the In-Service bit. (This is true even if the second interrupt
has a higher priority than the first.)
8.3.3 Cascading with External 8259As
rupt Control Unit, external 8259A modules can be used to increase the number of external inter-
rupt pins. The cascade mode of the Interrupt Control Unit supports the external 8259As. The
INT2/INTA0 and INT3/INTA1 pins can serve either of two functions. Outside cascade mode,
they serve as external interrupt inputs. In cascade mode, they serve as interrupt acknowledge out-
puts. INTA0 is the acknowledge for INT0, and INTA1 is the acknowledge for INT1. (See Figure
8-2.)
The INT2/INTA0 and INT3/INTA1 pins are inputs after reset until the pins are confiugred as out-
puts. The pullup resistors ensure that the INTA pins never float (which would cause a spurious
interrupt acknowledge to the 8259A). The value of the resistors must be high enough to prevent
excessive loading on the pins.
8-7
INTERRUPT CONTROL UNIT
INT
INT0
V
8259A
or
CC
82C59A
INTA
INT
INTA0
INT1
Interrupt
Control
Unit
V
8259A
or
CC
82C59A
INTA
INTA1
A1211-A0
Figure 8-2. Using External 8259A Modules in Cascade Mode
Special Fully Nested Mode
8.3.3.1
Special fully nested mode is an optional feature normally used with cascade mode. It is applicable
only to INT0 and INT1. In special fully nested mode, an interrupt request is serviced even if its
In-Service bit is set.
In cascade mode, an 8259A controls up to eight external interrupts that share a single interrupt
input pin. Special fully nested mode allows the 8259A’s priority structure to be maintained. For
example, assume that the CPU is servicing a low-priority interrupt from the 8259A. While the
interrupt handler is executing, the 8259A receives a higher priority interrupt from one of its sourc-
es. The 8259A applies its own priority criteria to that interrupt and asserts its interrupt to the In-
terrupt Control Unit. Special fully nested mode allows the higher priority interrupt to be serviced
even though the In-Service bit for that source is already set. A higher priority interrupt has pre-
empted a lower priority interrupt, and interrupt nesting is fully maintained.
Special fully nested mode can also be used without cascade mode. In this case, it allows a single
external interrupt pin (either INT0 or INT1) to preempt itself.
8-8
INTERRUPT CONTROL UNIT
8.3.4 Interrupt Acknowledge Sequence
to the CPU. The CPU then multiplies the interrupt type by four to derive the interrupt vector ad-
dress in the interrupt vector table. (“Interrupt/Exception Processing” on page 2-39 describes the
interrupt acknowledge sequence and Figure 2-25 on page 2-40 illustrates the interrupt vector ta-
ble.)
The interrupt types for all sources are fixed and unalterable (see Table 8-2). The Interrupt Control
Unit passes these types to the CPU internally. The first external indication of the interrupt ac-
knowledge sequence is the CPU fetch from the interrupt vector table.
In cascade mode, the external 8259A supplies the interrupt type. In this case, the CPU runs an
external interrupt acknowledge cycle to fetch the interrupt type from the 8259A (see “Interrupt
Acknowledge Bus Cycle” on page 3-25).
Table 8-2. Fixed Interrupt Types
Interrupt Name
Interrupt Type
Timer 0
Timer 1
Timer 2
DMA0
DMA1
INT0
8
18
19
10
11
12
13
14
15
INT1
INT2
INT3
8.3.5 Polling
In some applications, it is desirable to poll the Interrupt Control Unit. The CPU polls the Interrupt
Control Unit for any pending interrupts, and software can service interrupts whenever it is con-
venient. The Poll and Poll Status registers support polling.
Software reads the Poll register to get the type of the highest priority pending interrupt, then calls
the corresponding interrupt handler. Reading the Poll register also acknowledges the interrupt.
This clears the Interrupt Request bit and sets the In-Service bit for the interrupt. The Poll Status
register has the same format as the Poll register, but reading the Poll Status register does not ac-
knowledge the interrupt.
8-9
INTERRUPT CONTROL UNIT
8.3.6 Edge and Level Triggering
The external interrupts (INT3:0) can be programmed for either edge or level triggering (see “In-
terrupt Control Registers” on page 8-12). Both types of triggering are active high. An edge-trig-
gered interrupt is generated by a zero-to-one transition on an external interrupt pin. The pin must
remain high until after the CPU acknowledges the interrupt, then must go low to reset the edge-
detection circuitry. (See the current data sheet for timing requirements.) The edge-detection cir-
cuitry must be reset to enable further interrupts to occur.
A level-triggered interrupt is generated by a valid logic one on the external interrupt pin. The pin
must remain high until after the CPU acknowledges the interrupt. Unlike edge-triggered inter-
rupts, level-triggered interrupts will continue to occur if the pin remains high. A level-triggered
external interrupt pin must go low before the EOI command to prevent another interrupt.
NOTE
When external 8259As are cascaded into the Interrupt Control Unit, INT0 and
8.3.7 Additional Latency and Response Time
The Interrupt Control Unit adds 5 clocks to the interrupt latency of the CPU. Cascade mode adds
13 clocks to the interrupt response time because the CPU must run the interrupt acknowledge bus
cycles. (See Figure 8-3 on page 8-11 and Figure 2-27 on page 2-46.)
8-10
INTERRUPT CONTROL UNIT
Clocks
Interrupt presented to control unit
Interrupt presented to CPU
5
INTA
IDLE
INTA
IDLE
READ IP
IDLE
4
2
4
5
4
Cascade Mode Only
3 (5 if not cascade mode)
READ CS
IDLE
PUSH FLAGS
IDLE
PUSH CS
PUSH IP
IDLE
4
4
4
3
4
4
5
First instruction fetch
from interrupt routine
Total 55
A1212-A0
Figure 8-3. Interrupt Control Unit Latency and Response Time
8.4 PROGRAMMING THE INTERRUPT CONTROL UNIT
Table 8-3 lists the Interrupt Control Unit registers in master mode with their Peripheral Control
Block offset addresses. The remainder of this section describes the functions of the registers.
Table 8-3. Interrupt Control Unit Registers in Master Mode
Register Name
INT3 Control
Offset Address
3EH
3CH
3AH
38H
34H
36H
32H
30H
2EH
INT2 Control
INT1 Control
INT0 Control
DMA0 Control
DMA1 Control
Timer Control
Interrupt Status
Interrupt Request
8-11
INTERRUPT CONTROL UNIT
Table 8-3. Interrupt Control Unit Registers in Master Mode (Continued)
Register Name
In-Service
Offset Address
2CH
2AH
28H
26H
24H
22H
Priority Mask
Interrupt Mask
Poll Status
Poll
EOI
8.4.1 Interrupt Control Registers
Each interrupt source has its own Interrupt Control register. The Interrupt Control register allows
you to define the behavior of each interrupt source. Figure 8-4 shows the registers for the timers
and DMA channels, Figure 8-5 shows the registers for INT3:2, and Figure 8-6 shows the registers
for INT0 and INT1.
All Interrupt Control registers have a three-bit field (PM2:0) that defines the priority level for the
is the same as the one in the Interrupt Mask register. Modifying a bit in either register also mod-
ifies that same bit in the other register.
The Interrupt Control registers for the external interrupt pins also have a bit (LVL) that selects
level-triggered or edge-triggered mode for that interrupt. (See “Edge and Level Triggering” on
page 8-10.)
The Interrupt Control registers for the cascadable external interrupt pins (INT0 and INT1) have
two additional bits to support the external 8259As. The CAS bit enables cascade mode, and the
SFNM bit enables special fully nested mode.
8-12
INTERRUPT CONTROL UNIT
Register Name:
Interrupt Control Register (internal sources)
Register Mnemonic:
Register Function:
TCUCON, DMA0CON, DMA1CON
Control register for the internal interrupt sources
15
0
M
S
K
P
M
2
P
M
1
P
M
0
A1213-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
MSK
Interrupt
Mask
1
Clear to enable interrupts from this source.
Defines the priority level for this source.
PM2:0
Priority
Level
111
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-4. Interrupt Control Register for Internal Sources
8-13
INTERRUPT CONTROL UNIT
.
Register Name:
Interrupt Control Register (non-cascadable pins)
I2CON, I3CON
Register Mnemonic:
Register Function:
Control register for the non-cascadable external
internal interrupt pins
15
0
L
V
L
M
S
K
P
M
2
P
M
1
P
M
0
A1214-A0
Bit
Reset
State
Bit Name
Mnemonic
Function
LVL
Level-trigger
0
Selects the interrupt triggering mode:
0 = edge triggering
1 = level triggering.
MSK
Interrupt
Mask
1
Clear to enable interrupts from this source.
Defines the priority level for this source.
PM2:0
Priority
Level
111
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-5. Interrupt Control Register for Noncascadable External Pins
8-14
INTERRUPT CONTROL UNIT
Register Name:
Interrupt Control Register (cascadable pins)
Register Mnemonic:
Register Function:
I0CON, I1CON
Control register for the cascadable external
interrupt pins
15
0
S
F
N
M
C
A
S
L
V
L
M
S
K
P
M
2
P
M
1
P
M
0
A1215-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
SFNM
Special
Fully
0
Set to enable special fully nested mode.
Nested
Mode
CAS
LVL
Cascade
Mode
0
0
Set to enable cascade mode.
Level-trigger
Selects the interrupt triggering mode:
0 = edge triggering
1 = level triggering.
The LVL bit must be set when external 8259As
are cascaded into the Interrupt Control Unit.
MSK
Interrupt
Mask
1
Clear to enable interrupts from this source.
Defines the priority level for this source.
PM2:0
Priority
Level
111
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-6. Interrupt Control Register for Cascadable Interrupt Pins
8-15
INTERRUPT CONTROL UNIT
8.4.2 Interrupt Request Register
The Interrupt Request register (Figure 8-7) has one bit for each interrupt source. When a source
requests an interrupt, its Interrupt Request bit is set (without regard to whether the interrupt is
masked). The Interrupt Request bit is cleared when the interrupt is acknowledged. An external
interrupt pin must remain asserted until its interrupt is acknowledged. Otherwise, the Interrupt
Request bit will be cleared, but the interrupt will not be serviced.
Register Name:
Interrupt Request Register
REQST
Register Mnemonic:
Register Function:
Stores pending interrupt requests
15
0
I
I
I
I
D
M
A
1
D
M
A
0
T
M
R
N
T
3
N
T
2
N
T
1
N
T
0
A1201-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
INT3:0
External
Interrupts
0000 0
A bit is set to indicate a pending interrupt from
the corresponding external interrupt pin.
DMA1:0
TMR
DMA
Interrupt
0
0
A bit is set to indicate a pending interrupt from
the corresponding DMA channel.
Timer
Interrupt
This bit is set to indicate a pending interrupt
from one of the timers.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-7. Interrupt Request Register
8.4.3 Interrupt Mask Register
The Interrupt Mask register (Figure 8-8) contains a mask bit for each interrupt source. This reg-
ister allows you to mask (disable) individual interrupts. Set a mask bit to disable interrupts from
the corresponding source. The mask bit is the same as the one in the Interrupt Control register.
Modifying a bit in either register also modifies that same bit in the other register.
8-16
INTERRUPT CONTROL UNIT
Register Name:
Interrupt Mask Register
IMASK
Register Mnemonic:
Register Function:
Masks individual interrupt sources
15
0
I
I
I
I
D
M
A
1
D
M
A
0
T
M
R
N
T
3
N
T
2
N
T
1
N
T
0
A1202-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
Set a bit to mask (disable) interrupt requests
INT3:0
External
Interrupt
Mask
0000 0
from the corresponding external interrupt pin.
DMA1:0
TMR
DMA
Interrupt
Mask
0
0
Set to mask (disable) interrupt requests from
the corresponding DMA channel .
Timer
Interrupt
Mask
Set to mask (disable) interrupt requests from
the timers.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-8. Interrupt Mask Register
8.4.4 Priority Mask Register
The Priority Mask register (Figure 8-9) contains a three-level field that holds a priority value.
This register allows you to mask interrupts based on their priority levels. Write a priority value
to the PM2:0 field to specify the lowest priority interrupt to be serviced. This disables (masks)
any interrupt source whose priority is lower than the PM2:0 value. After reset, the Priority Mask
register is set to the lowest priority (seven), which enables all interrupts of any priority.
8-17
INTERRUPT CONTROL UNIT
Register Name:
Priority Mask Register
PRIMSK
Register Mnemonic:
Register Function:
Masks lower-priority interrupt sources
15
0
P
M
2
P
M
1
P
M
0
A1216-A0
Bit
Reset
State
Bit Name
Mnemonic
Function
Defines a priority-based interrupt mask.
Interrupts whose priority is lower than this value
are masked.
PM2:0
Priority
Mask
111
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-9. Priority Mask Register
8.4.5 In-Service Register
The In-Service register has a bit for each interrupt source. The bits indicate which source’s inter-
rupt handlers are currently executing. The In-Service bit is set when an interrupt is acknowl-
edged; the interrupt handler must clear it with an End-of-Interrupt (EOI) command. The Interrupt
Control Unit uses the In-Service register to support interrupt nesting.
8-18
INTERRUPT CONTROL UNIT
Register Name:
In-Service Register
INSERV
Register Mnemonic:
Register Function:
Indicates which interrupt handlers are in process
15
0
I
I
I
I
D
M
A
1
D
M
A
0
T
M
R
N
T
3
N
T
2
N
T
1
N
T
0
A1192-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
INT3:0
External
Interrupt In-
Service
0000 0
A bit is set to indicate that the corresponding
external interrupt is being serviced.
DMA1:0
TMR
DMA
Interrupt In-
Service
0
0
This bit is set to indicate that the corresponding
DMA channel interrupt is being serviced.
Timer
Interrupt In-
Service
This bit is set to indicate that a timer interrupt is
being serviced.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-10. In-Service Register
8.4.6 Poll and Poll Status Registers
The Poll and Poll Status registers allow you to poll the Interrupt Control Unit and service inter-
rupts through software. You can read these registers to determine whether an interrupt is pending
and, if so, the interrupt type. The registers contain identical information, but reading them pro-
duces different results.
8-19
INTERRUPT CONTROL UNIT
Reading the Poll register (Figure 8-11) acknowledges the pending interrupt, just as if the CPU
quest, In-Service, Poll, and Poll Status registers, as it does in the normal interrupt acknowledge
sequence. However, the processor does not run an interrupt acknowledge sequence or fetch the
vector from the vector table. Instead, software must read the interrupt type and execute the proper
routine to service the pending interrupt.
Reading the Poll Status register (Figure 8-12) will merely transmit the status of the polling bits
without modifying any of the other Interrupt Controller registers.
Register Name:
Poll Register
POLL
Register Mnemonic:
Register Function:
Read to check for and acknowledge pending
interrupts when polling
15
0
I
V
T
4
V
T
3
V
T
2
V
T
1
V
T
0
R
E
Q
A1208-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
IREQ
Interrupt
Request
0
This bit is set to indicate a pending interrupt.
VT4:0
Vector Type
0
Contains the interrupt type of the highest
priority pending interrupt.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-11. Poll Register
8-20
INTERRUPT CONTROL UNIT
Register Name:
Poll Status Register
POLLSTS
Register Mnemonic:
Register Function:
Read to check for pending interrupts when polling
15
0
I
V
T
4
V
T
3
V
T
2
V
T
1
V
T
0
R
E
Q
A1209-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
IREQ
Interrupt
Request
0
This bit is set to indicate a pending interrupt.
VT4:0
Vector Type
0
Contains the interrupt type of the highest
priority pending interrupt.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-12. Poll Status Register
8.4.7 End-of-Interrupt (EOI) Register
The End-of-Interrupt register (Figure 8-13) issues an End-of-Interrupt (EOI) command to the In-
terrupt Control Unit, which clears the In-Service bit for the associated interrupt. An interrupt han-
dler typically ends with an EOI command. There are two types of EOI commands: nonspecific
and specific. A nonspecific EOI simply clears the In-Service bit of the highest priority interrupt.
To issue a nonspecific EOI command, set the NSPEC bit. (Write 8000H to the EOI register.)
A specific EOI clears a particular In-Service bit. To issue a specific EOI command, clear the
NSPEC bit and write the VT4:0 bits with the interrupt type of the interrupt whose In-Service bit
you wish to clear. For example, to clear the In-Service bit for INT2, write 000EH to the EOI reg-
ister. The timer interrupts share an In-Service bit. To clear the In-Service bit for any timer inter-
rupt with a specific EOI, write 0008H (interrupt type 8) to the EOI register.
8-21
INTERRUPT CONTROL UNIT
Register Name:
End-of-Interrupt Register
EOI
Register Mnemonic:
Register Function:
Used to issue an EOI command
15
0
N
S
P
E
C
V
T
4
V
T
3
V
T
2
V
T
1
V
T
0
A1210-A0
Bit
Reset
State
Bit Name
Mnemonic
Function
NSPEC
VT4:0
Nonspecific
EOI
0
Set to issue a nonspecific EOI.
Interrupt
Type
0 0000
Write with the interrupt type of the interrupt
whose In-Service bit is to be cleared.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-13. End-of-Interrupt Register
8.4.8 Interrupt Status Register
The Interrupt Status register (Figure 8-14) contains the DMA Halt bit and one bit for each timer
interrupt. The CPU sets the DMA Halt bit to suspend DMA transfers while an NMI is processed.
Software can also read and write this bit. See “Suspension of DMA Transfers” on page 10-20 for
details. A timer bit is set to indicate a pending interrupt and is cleared when the interrupt request
is acknowledged. Any number of bits can be set at any one time.
8-22
INTERRUPT CONTROL UNIT
Register Name:
Interrupt Status Register
INTSTS
Register Mnemonic:
Register Function:
Indicates pending shared-source interrupts and
DMA suspension
15
0
D
H
L
T
M
R
2
T
M
R
1
T
M
R
0
T
A1193-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
This bit is set to suspend DMA activity.
DHLT
DMA Halt
0
TMR2:0
Timer
Interrupt
Pending
000
A bit is set to indicate a pending interrupt from
the corresponding timer.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-14. Interrupt Status Register
NOTE
Do not write to the DHLT bit while Timer/Counter Unit interrupts are enabled.
A conflict with the internal use of the register may cause incorrect processing
of timer interrupts.
The DHLT bit does not function when the interrupt controller is in slave mode.
8.5 SLAVE MODE
Although Master mode is the most common, Slave mode is useful in larger system designs. In
Slave mode, an external 8259A module controls the interrupt input to the CPU and acts as the
master interrupt controller. The Interrupt Control Unit processes only the internal interrupt re-
quests and acts as an interrupt input to the external 8259A. In simplest terms, the Interrupt Control
Unit behaves like a cascaded 8259A to the master 8259A. (See Figures 8-15 and 8-16.)
8-23
INTERRUPT CONTROL UNIT
DMA
0
DMA
1
Timer 0 Timer 1 Timer 2
Interrupt
Priority
Resolver
Vector
Generation
Logic
To External 8259A
Interrupt Request
F - Bus
A1195-A0
Figure 8-16. Interrupt Sources in Slave Mode
8.5.1 Slave Mode Programming
Some registers differ between Slave mode and Master mode. Slave mode adds the Interrupt Vec-
tor Register; it does not support the Poll, Poll Status Registers, INT3 and INT2 Control registers;
and it replaces the Timer, INT1 and INT0 Control registers with individual Timer 0, Timer 1, and
Timer 2 Control registers. The remaining registers retain the same functions as in Master mode;
however, some bit positions change to accommodate the addition of the individual timer inter-
rupts and the deletion of the external interrupts. Table 8-4 compares the Master and Slave mode
registers and lists their PCB offset addresses.
8-25
INTERRUPT CONTROL UNIT
8.5.1.1
Interrupt Vector Register
The Interrupt Vector Register is used only in Slave mode. In Master mode, the interrupt vector
types are fixed; in Slave mode they are programmable. The Interrupt Vector Register is used to
specify the five most-significant bits of the interrupt vector type. The three least-significant bits
are fixed (Table 8-5).
Table 8-4. Interrupt Control Unit Register Comparison
Master Mode
Register Name
Slave Mode
Register Name
PCB Offset
Address
INT3 Control
INT2 Control
INT1 Control
INT0 Control
DMA1 Control
DMA0 Control
Timer Control
Interrupt Status
Interrupt Request
In-Service
(not used)
3EH
3CH
3AH
38H
36H
34H
32H
30H
2EH
2CH
2AH
28H
26H
24H
22H
20H
(not used)
Timer 2 Control
Timer 1 Control
DMA1 Control
DMA0 Control
Timer 0 Control
Interrupt Status
Interrupt Request
In-Service
Priority Mask
Interrupt Mask
Poll Status
Priority Mask
Interrupt Mask
(not used)
Poll
(not used)
EOI
EOI
(not used)
Interrupt Vector
Table 8-5. Slave Mode Fixed Interrupt Type Bits
Type Bits
Interrupt Source
2
1
0
Timer 0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
(reserved)
DMA 0
DMA 1
Timer 1
Timer 2
(reserved)
(reserved)
8-26
INTERRUPT CONTROL UNIT
Register Name:
Interrupt Vector Register (Slave Mode only)
Register Mnemonic:
Register Function:
INTVEC
Specifies the five most-significant bit of the interrupt
vector types for the internal interrupt sources
15
0
T
4
T
3
T
2
T
1
T
0
A1196-A0
Bit
Mnemonic
Reset
State
Bit Name
Function
Specifies the five most-significant bits of the
interrupt vector types for the internal interrupt
sources. The three least-significant bits are
fixed (see Table 8-5).
T4:0
Interrupt
Vector Type
Field
00000
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-17. Interrupt Vector Register (Slave Mode Only)
End-Of-Interrupt Register
8.5.1.2
The End-of-Interrupt (EOI) register has the same function in Slave mode as in Master mode.
However, non-specific EOI commands are not supported, so the NSPEC bit is omitted from the
register. Only specific EOI commands can be issued. To clear an In-Service bit in Slave mode,
write the three least-significant bits of the interrupt type (from Table 8-5) to the VT2:0 bits.
8-27
INTERRUPT CONTROL UNIT
Register Name:
End-of-Interrupt Register (in Slave Mode)
EOI
Register Mnemonic:
Register Function:
Used to issue the EOI command
15
0
V
T
2
V
T
1
V
T
0
A1197-A0
Bit
Reset
State
Bit Name
Mnemonic
Function
VT2:0
Interrupt
Type
0
Write the three LSBs of the interrupt type (see
Table 8-5) to these bits to issue an EOI
command in slave mode.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 8-18. End-of-Interrupt Register in Slave Mode
Other Registers
8.5.1.3
The Priority Mask register is identical in Slave mode and Master mode. The Interrupt Request,
Interrupt Mask, and In-Service registers retain the same function, but individual bits differ to ac-
commodate the addition of the individual timer interrupts and the deletion of the external inter-
rupts. Figure 8-19 shows the bit positions for Slave mode.
15
0
T
M
R
2
T
M
R
1
D
M
A
0
D
M
A
1
T
M
R
0
Figure 8-19. Request, Mask, and In-Service Registers
8-28
INTERRUPT CONTROL UNIT
In Slave mode, the external 8259A module acts as the master interrupt controller. Therefore, in-
terrupt acknowledge cycles are required for every interrupt, including those from integrated pe-
ripherals. During the first interrupt acknowledge cycle, the external 8259A determines which
slave interrupt controller has the highest priority interrupt request. It then drives that slave’s ad-
dress onto its CAS2:0 pins (Figure 8-20). External logic must decode the correct slave address
from the CAS2:0 pins to drive the SELECT pin.
T1
T2
T3
T4
TI
TI
T1
T2
T3
T4
CLKOUT
S2:0
INTA
INTA
INTA0
SELECT
LOCK
Slave Cascade Address From 8259A
CAS2:0
NOTES:
1. INT1/SELECT has the SELECT function in slave mode.
2. INT2/INTA0 has the INTA0 function in slave mode.
3. Cascade address is driven by the external 8259A.
4. SELECT must be driven before phase 2 of T2 of the second INTA.
5. SELECT read by processor.
6. ALE is generated for each INTA.
7. RD is inactive.
A1199-A0
Figure 8-20. Interrupt Vectoring in Slave Mode
The SELECT pin is the slave-select input to the Interrupt Control Unit. During the second inter-
rupt acknowledge cycle, the highest-priority slave interrupt controller transfers the interrupt type
of its highest priority interrupt to the CPU. If the Interrupt Control Unit is the highest-priority
slave, it passes the interrupt type to the CPU internally; however, the interrupt acknowledge cycle
still must occur for the benefit of the external 8259A module.
8-29
INTERRUPT CONTROL UNIT
External interrupt acknowledge cycles must be run for every maskable interrupt. Therefore, the
interrupt response time for every interrupt will be 55 clocks, as shown in Figure 8-21.
Clocks
Interrupt presented to Interrupt Control Unit
5
Interrupt presented to external 82C59A
INTA
IDLE
INTA
IDLE
READ IP
IDLE
READ CS
IDLE
PUSH FLAGS
IDLE
PUSH CS
PUSH IP
IDLE
4
2
4
5
4
3
4
4
4
3
4
4
5
First instruction fetch
from interrupt routine
Total 55
A1200-A0
Figure 8-21. Interrupt Response Time in Slave Mode
8.5.3 Initializing the Interrupt Control Unit for Master Mode
Follow these steps to initialize the Interrupt Control Unit for Master mode.
1. Determine which interrupt sources you want to use.
2. Determine whether to use the default priority scheme or devise your own.
3. Program the Interrupt Control register for each interrupt source.
— For external interrupt pins, select edge or level triggering.
— For INT0 or INT1, enable cascade mode, special fully nested mode, or both, if you
wish to use them.
— If you are using a custom priority scheme, program the priority level for each interrupt
source.
4. Program the Priority Mask with a priority mask level, if you wish to mask interrupts based
on priority. (The default is level seven, which enables all interrupt levels.)
8-30
INTERRUPT CONTROL UNIT
5. Set the mask bit in the Interrupt Mask register for any interrupts that you wish to disable.
Example 8-1 shows sample code to initialize the Interrupt Control Unit.
$mod186
name
;
example_80C186_ICU_initialization
;This routine configures the interrupt controller to provide two cascaded
;interrupt inputs (through an external 8259A connected to INT0 and INTA0#)
;and two direct interrupt inputs connected to INT1 and INT3. The default
;priorities are used.
;
;The example assumes that the register addresses have been properly defined.
;
code
segment
assume cs:code
proc near
push dx
set_int_
push ax
mov ax,0110111B
mov dx,I0CON
out dx,ax
mov ax,01001101B
mov dx,IMASK
out dx,ax
pop ax
;cascade mode, priority seven
;INT0 control register
;unmask INT1 and INT3
pop dx
ret
set_int_
code
endp
ends
end
Example 8-1. Initializing the Interrupt Control Unit for Master Mode
8-31
9
Timer/Counter Unit
CHAPTER 9
TIMER/COUNTER UNIT
The Timer/Counter Unit can be used in many applications. Some of these applications include a
real-time clock, a square-wave generator and a digital one-shot. All of these can be implemented
in a system design. A real-time clock can be used to update time-dependent memory variables. A
square-wave generator can be used to provide a system clock tick for peripheral devices. (See
“Timer/Counter Unit Application Examples” on page 9-17 for code examples that configure the
Timer/Counter Unit for these applications.)
9.1 FUNCTIONAL OVERVIEW
The Timer/Counter Unit is composed of three independent 16-bit timers (see Figure 9-1). The op-
eled as a single counter element, time-multiplexed to three register banks. The register banks are
dual-ported between the counter element and the CPU. During a given bus cycle, the counter el-
ement and CPU can both access the register banks; these accesses are synchronized.
The Timer/Counter Unit is serviced over four clock periods, one timer during each clock, with an
idle clock at the end (see Figure 9-2). No connection exists between the counter element’s se-
quencing through timer register banks and the Bus Interface Unit’s sequencing through T-states.
Timer operation and bus interface operation are asynchronous. This time-multiplexed scheme re-
sults in a delay of 2½ to 6½ CLKOUT periods from timer input to timer output.
Each timer keeps its own running count and has a user-defined maximum count value. Timers 0
and 1 can use one maximum count value (single maximum count mode) or two alternating max-
ue. The control register for each timer determines the counting mode to be used. When a timer is
serviced, its present count value is incremented and compared to the maximum count for that tim-
er. If these two values match, the count value resets to zero. The timers can be configured either
to stop after a single cycle or to run continuously.
Timers 0 and 1 are functionally identical. Figure 9-3 illustrates their operation. Each has a
latched, synchronized input pin and a single output pin. Each timer can be clocked internally or
externally. Internally, the timer can either increment at ¼ CLKOUT frequency or be prescaled by
Timer 2. A timer that is prescaled by Timer 2 increments when Timer 2 reaches its maximum
count value.
9-1
TIMER/COUNTER UNIT
Timer 0 Timer 1 Timer 2
Timer 0 Timer 1 Timer 2
Timer 0
ServicedServicedServiced Dead ServicedServicedServiced Dead Serviced
3
4
1
T0IN
2
5
T1IN
T0OUT
T1OUT
NOTES:
1. T0IN resolution time (setup time met).
2. T1IN resolution time (setup time not met).
3. Modified count value written into Timer 0 count register.
4. T1IN resolution time, count value written into Timer 1 count register.
5. T1IN resolution time.
A1293-0A
Figure 9-2. Counter Element Multiplexing and Timer Input Synchronization
9-3
TIMER/COUNTER UNIT
Timer
Enabled
(EN = 1)
?
No
Done
Start
Yes
External
Clocking
(EXT = 1)
?
No
Yes
No
Yes
Retrigger
(RTG = 1)
?
Lo to Hi
transition on input
pin since last
service
Lo to Hi
Yes
Yes
No
Timer Input
at High Level
?
transition on input
pin since last
service
?
?
Yes
No
No
Clear Count
Register
No
Prescaler On
(P = 1)
?
Yes
Done
Did Timer 2
Reach Maxcount
Last Service
State
Yes
Increment
Counter
?
No
Continued
"A"
Done
A1294-0A
Figure 9-3. Timers 0 and 1 Flow Chart
9-4
TIMER/COUNTER UNIT
Continued From
"A"
Alternating
Maxcount Regs
(ALT = 1)
?
Yes
No
No
(Use"B")
Yes
(Use"A")
Using
Counter =
Compare "A"
?
Yes
Maxcount A
(RIU = 0)
?
No
No
No
Counter =
Compare "A"
?
Counter =
Compare "B"
?
Done
Yes
Yes
Pulse TOUT Pin
Low For 1 Clock
Set RIU Bit
TOUT Pin Driven Low
Clear RIU Bit
TOUT Pin Driven High
Yes
Yes
Continuous Mode
Continuous Mode
(CONT=1)
?
(CONT=1)
?
No
No
No
Interrupt Bit Set
?
Clear Enable Bit
(Stop Counting)
Clear Enable Bit
(Stop Counting)
Yes
Request Interrupt
Clear Counter
Done
A1295-0A
Figure 9-3. Timers 0 and 1 Flow Chart (Continued)
9-5
TIMER/COUNTER UNIT
When configured for internal clocking, the Timer/Counter Unit uses the input pins either to en-
able timer counting or to retrigger the associated timer. Externally, a timer increments on low-to-
high transitions on its input pin (up to ¼ CLKOUT frequency).
the end of a timing cycle, or a variable duty cycle wave. These two output options correspond to
single maximum count mode and dual maximum count mode, respectively (Figure 9-4). Inter-
rupts can be generated at the end of every timing cycle.
Timer 2 has no input or output pins and can be operated only in single maximum count mode
(Figure 9-4). It can be used as a free-running clock and as a prescaler to Timers 0 and 1. Timer 2
can be clocked only internally, at ¼ CLKOUT frequency. Timer 2 can also generate interrupts at
the end of every timing cycle.
Maxcount A
Maxcount B
Dual Maximum
Count Mode
One CPU
Clock
Maxcount A
Single Maximum
Count Mode
Figure 9-4. Timer/Counter Unit Output Modes
9.2 PROGRAMMING THE TIMER/COUNTER UNIT
Each timer has three registers: a Timer Control register (Figure 9-5 and Figure 9-6), a Timer
Count register (Figure 9-7) and a Timer Maxcount Compare register (Figure 9-8). Timers 0 and
1 also have access to an additional Maxcount Compare register. The Timer Control register con-
trols timer operation. The Timer Count register holds the current timer count value, and the Max-
count Compare register holds the maximum timer count value.
9-6
TIMER/COUNTER UNIT
Register Name:
Timer 0 and 1 Control Registers
T0CON, T1CON
Register Mnemonic:
Register Function:
Defines Timer 0 and 1 operation.
15
0
E
N
I
N
H
I
N
T
R
I
U
M
C
R
T
G
P
E
X
T
A
L
T
C
O
N
T
A1297-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
EN
Enable
0
Set to enable the timer. This bit can be written only
when the INH bit is set.
INH
INT
RIU
MC
Inhibit
X
X
X
X
Set to enable writes to the EN bit. Clear to ignore
writes to the EN bit. The INH bit is not stored; it
always reads as zero.
Interrupt
Set to generate an interrupt request when the Count
register equals a Maximum Count register. Clear to
disable interrupt requests.
Register In
Use
Indicates which compare register is in use. When set,
the current compare register is Maxcount Compare B;
when clear, it is Maxcount Compare A.
Maximum
Count
This bit is set when the counter reaches a maximum
count. The MC bit must be cleared by writing to the
Timer Control register. This is not done automati-
cally. If MC is clear, the counter has not reached a
maximum count.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products.
Figure 9-5. Timer 0 and Timer 1 Control Registers
9-7
TIMER/COUNTER UNIT
Register Name:
Timer 0 and 1 Control Registers
T0CON, T1CON
Register Mnemonic:
Register Function:
Defines Timer 0 and 1 operation.
15
0
E
N
I
N
H
I
N
T
R
I
U
M
C
R
T
G
P
E
X
T
A
L
T
C
O
N
T
A1297-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
RTG
Retrigger
X
This bit specifies the action caused by a low-to-high
transition on the TMR INx input. Set RTG to reset the
count; clear RTG to enable counting. This bit is
ignored with external clocking (EXT=1).
P
Prescaler
X
Set to increment the timer when Timer 2 reaches its
maximum count. Clear to increment the timer at ¼
CLKOUT. This bit is ignored with external clocking
(EXT=1).
EXT
ALT
External
Clock
X
X
Set to use external clock; clear to use internal clock.
The RTG and P bits are ignored with external clocking
(EXT set).
Alternate
Compare
Register
This bit controls whether the timer runs in single or
dual maximum count mode (see Figure 9-4 on page
9-6). Set to specify dual maximum count mode; clear
to specify single maximum count mode.
CONT
Continuous
Mode
X
Set to cause the timer to run continuously. Clear to
disable the counter (clear the EN bit) after each
counting sequence.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products.
Figure 9-5. Timer 0 and Timer 1 Control Registers (Continued)
9-8
TIMER/COUNTER UNIT
Register Name:
Timer 2 Control Register
T2CON
Register Mnemonic:
Register Function:
Defines Timer 2 operation.
15
0
E
N
I
N
H
I
N
T
M
C
C
O
N
T
A1298-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
Set to enable the timer. This bit can be written
EN
Enable
0
only when the INH bit is set.
INH
INT
MC
Inhibit
X
X
X
Set to enable writes to the EN bit. Clear to
ignore writes to the EN bit. The INH bit is not
stored; it always reads as zero.
Interrupt
Set to generate an interrupt request when the
Count register equals a Maximum Count
register. Clear to disable interrupt requests.
Maximum
Count
This bit is set when the counter reaches a
maximum count. The MC bit must be cleared
by writing to the Timer Control register. This
is not done automatically. If MC is clear, the
counter has not reached a maximum count.
CONT
Continuous
Mode
X
Set to cause the timer to run continuously.
Clear to disable the counter (clear the EN bit)
after each counting sequence.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 9-6. Timer 2 Control Register
9-9
TIMER/COUNTER UNIT
Register Name:
Timer Count Register
Register Mnemonic:
Register Function:
T0CNT, T1CNT, T2CNT
Contains the current timer count.
15
0
T
C
1
T
C
1
T
C
1
T
C
1
T
C
1
T
C
1
T
C
9
T
C
8
T
C
7
T
C
6
T
C
5
T
C
4
T
C
3
T
C
2
T
C
1
T
C
0
5
4
3
2
1
0
A1299-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
TC15:0
Timer Count
Value
XXXXH
Contains the current count of the associated
timer.
Figure 9-7. Timer Count Registers
9-10
TIMER/COUNTER UNIT
Register Name:
Timer Maxcount Compare Register
Register Mnemonic:
Register Function:
T0CMPA, T0CMPB, T1CMPA, T1CMPB, T2CMPA
Contains timer maximum count value.
15
0
T
C
1
T
C
1
T
C
1
T
C
1
T
C
1
T
C
1
T
C
9
T
C
8
T
C
7
T
C
6
T
C
5
T
C
4
T
C
3
T
C
2
T
C
1
T
C
0
5
4
3
2
1
0
A1300-0A
Bit
Mnemonic
Reset
State
Bit Name
Timer
Compare
Value
Function
TC15:0
XXXXH
Contains the maximum value a timer will count
to before resetting its Count register to zero.
Figure 9-8. Timer Maxcount Compare Registers
9.2.1 Initialization Sequence
When initializing the Timer/Counter Unit, the following sequence is suggested:
1. If timer interrupts will be used, program interrupt vectors into the Interrupt Vector Table.
2. Clear the Timer Count register. This must be done before the timer is enabled because
the count register is undefined at reset. Clearing the count register ensures that counting
begins at zero.
3. Write the desired maximum count value to the Timer Maxcount Compare register. For
dual maximum count mode, write a value to both Maxcount Compare A and B.
4. Program the Timer Control register to enable the timer. When using Timer 2 to prescale
another timer, enable Timer 2 last. If Timer 2 is enabled first, it will be at an unknown
point in its timing cycle when the timer to be prescaled is enabled. This results in an
unpredictable duration of the first timing cycle for the prescaled timer.
9-11
TIMER/COUNTER UNIT
9.2.2 Clock Sources
The 16-bit Timer Count register increments once for each timer event. A timer event can be a
low-to-high transition on a timer input pin (Timers 0 and 1), a pulse generated every fourth CPU
16
clock (all timers) or a timeout of Timer 2 (Timers 0 and 1). Up to 65536 (2 ) events can be count-
ed.
Timers 0 and 1 can be programmed to count low-to-high transitions on their input pins as timer
events by setting the External (EXT) bit in their control registers. Transitions on the external pin
are synchronized to the CPU clock before being presented to the timer circuitry. The timer counts
transitions on this pin. The input signal must go low, then high, to cause the timer to increment.
The maximum count-rate for the timers is ¼ the CPU clock rate (measured at CLKOUT) because
the timers are serviced only once every four clocks.
increments every fourth CPU clock due to the counter element’s time-multiplexed servicing
scheme. Timer 2 can use only the internal clock as a timer event.
Timers 0 and 1 can also use Timer 2 reaching its maximum count as a timer event. In this config-
uration, Timer 0 or Timer 1 increments each time Timer 2 reaches its maximum count. See Table
9-1 for a summary of clock sources for Timers 0 and 1. Timer 2 must be initialized and running
in order to increment values in other timer/counters.
Table 9-1. Timer 0 and 1 Clock Sources
EXT
P
Clock Source
0
0
1
0
1
Timer clocked internally at ¼ CLKOUT frequency.
Timer clocked internally, prescaled by Timer 2.
Timer clocked externally at up to ¼ CLKOUT frequency.
X
9.2.3 Counting Modes
All timers have a Timer Count register and a Maxcount Compare A register. Timers 0 and 1 also
have access to a second Maxcount Compare B register. Whenever the contents of the Timer
Count register equal the contents of the Maxcount Compare register, the count register resets to
zero. The maximum count value will never be stored in the count register. This is because the
counter element increments, compares and resets a timer in one clock cycle. Therefore, the max-
imum value is never written back to the count register. The Maxcount Compare register can be
written at any time during timer operation.
9-12
TIMER/COUNTER UNIT
The timer counting from its initial count (usually zero) to its maximum count (either Maxcount
Compare A or B) and resetting to zero defines one timing cycle. A Maxcount Compare value of
0 implies a maximum count of 65536, a Maxcount Compare value of 1 implies a maximum count
of 1, etc.
Only equivalence between the Timer Count and Maxcount Compare registers is checked. The
count does not reset to zero if its value is greater than the maximum count. If the count value ex-
ceeds the Maxcount Compare value, the timer counts to 0FFFFH, increments to zero, then counts
to the value in the Maxcount Compare register. Upon reaching a maximum count value, the Max-
imum Count (MC) bit in the Timer Control register sets. The MC bit must be cleared by writing
to the Timer Control register. This is not done automatically.
The Timer/Counter Unit can be configured to execute different counting sequences. The timers
can operate in single maximum count mode (all timers) or dual maximum count mode (Timers 0
and 1 only). They can also be programmed to run continuously in either of these modes. The Al-
ternate (ALT) bit in the Timer Control register determines the counting modes used by Timers 0
and 1.
All timers can use single maximum count mode, where only Maxcount Compare A is used. The
timer will count to the value contained in Maxcount Compare A and reset to zero. Timer 2 can
operate only in this mode.
Timers 0 and 1 can also use dual maximum count mode. In this mode, Maxcount Compare A and
Maxcount Compare B are both used. The timer counts to the value contained in Maxcount Com-
pare A, resets to zero, counts to the value contained in Maxcount Compare B, and resets to zero
again. The Register In Use (RIU) bit in the Timer Control register indicates which Maxcount
Compare register is currently in use.
count modes. The Continuous (CONT) bit in the Timer Control register determines whether a
timer is disabled after a single counting sequence.
9.2.3.1
Retriggering
The timer input pins affect timer counting in three ways (see Table 9-2). The programming of the
External (EXT) and Retrigger (RTG) bits in the Timer Control register determines how the input
signals are used. When the timers are clocked internally, the RTG bit determines whether the in-
put pin enables timer counting or retriggers the current timing cycle.
When the EXT and RTG bits are clear, the timer counts internal timer events. In this mode, the
input is level-sensitive, not edge-sensitive. A low-to-high transition on the timer input is not re-
quired for operation. The input pin acts as an external enable. If the input is high, the timer will
count through its sequence, provided the timer remains enabled.
9-13
TIMER/COUNTER UNIT
Table 9-2. Timer Retriggering
EXT
RTG
Timer Operation
0
0
0
1
Timer counts internal events, if input pin remains high.
Timer counts internal events; count resets to zero on every low-to-high transition on
the input pin.
1
X
Timer input acts as clock source.
When the EXT bit is clear and the RTG bit is set, every low-to-high transition on the timer input
pin causes the Count register to reset to zero. After the timer is enabled, counting begins only after
the first low-to-high transition on the input pin. If another low-to-high transition occurs before
the end of the timer cycle, the timer count resets to zero and the timer cycle begins again. In dual
maximum count mode, the Register In Use (RIU) bit does not clear when a low-to-high transition
occurs. For example, if the timer retriggers while Maxcount Compare B is in use, the timer resets
to zero and counts to maximum count B before the RIU bit clears. In dual maximum count
mode, the timer retriggering extends the use of the current Maxcount Compare register.
Timers 0 and 1 each have an output pin that can perform two functions. First, the output can be a
single pulse, indicating the end of a timing cycle (single maximum count mode). Second, the out-
put can be a level, indicating the Maxcount Compare register currently in use (dual maximum
count mode). The output occurs one clock after the counter element services the timer when the
maximum count is reached (see Figure 9-9).
With external clocking, the time between a transition on a timer input and the corresponding tran-
sition of the timer output varies from 2½ to 6½ clocks. This delay occurs due to the time-multi-
plexed servicing scheme of the Timer/Counter Unit. The exact timing depends on when the input
occurs relative to the counter element’s servicing of the timer. Figure 9-2 on page 9-3 shows the
occurs immediately before the timer is serviced. Timer 1 demonstrates the worst possible case,
where the input is latched, but the setup time is not met and the input is not recognized until the
counter element services the timer again.
In single maximum count mode, the timer output pin goes low for one CPU clock period (see Fig-
ure 9-4 on page 9-6). This occurs when the count value equals the Maxcount Compare A value.
If programmed to run continuously, the timer generates periodic pulses.
9-14
TIMER/COUNTER UNIT
Timer 0
Serviced
1
Internal Count Value
TxOUT Pin
Maxcount - 1
0
NOTE: 1. T
CLOV1
A1301-0A
Figure 9-9. TxOUT Signal Timing
In dual maximum count mode, the timer output pin indicates which Maxcount Compare register
is currently in use. A low output indicates Maxcount Compare B, and a high output indicates
Maxcount Compare A (see Figure 9-4 on page 9-6). If programmed to run continuously, a repet-
itive waveform can be generated. For example, if Maxcount Compare A contains 10, Maxcount
Compare B contains 20, and CLKOUT is 12.5 MHz, the timer generates a 33 percent duty cycle
waveform at 104 KHz. The output pin always goes high at the end of the counting sequence (even
if the timer is not programmed to run continuously).
9.2.5 Enabling/Disabling Counters
Each timer has an Enable (EN) bit in its Control register to allow or prevent timer counting. The
Inhibit (INH) bit controls write accesses to the EN bit. Timers 0 and 1 can be programmed to use
their input pins as enable functions also. If a timer is disabled, the count register does not incre-
ment when the counter element services the timer.
The Enable bit can be altered by programming or the timers can be programmed to disable them-
selves at the end of a counting sequence with the Continuous (CONT) bit. If the timer is not pro-
grammed for continuous operation, the Enable bit automatically clears at the end of a counting
sequence. In single maximum count mode, this occurs after Maxcount Compare A is reached. In
dual maximum count mode, this occurs after Maxcount Compare B is reached (Timers 0 and 1
only).
9-15
TIMER/COUNTER UNIT
The input pins for Timers 0 and 1 provide an alternate method for enabling and disabling timer
counting. When using internal clocking, the input pin can be programmed either to enable the tim-
er or to reset the timer count, depending on the state of the Retrigger (RTG) bit in the control reg-
ister. When used as an enable function, the input pin either allows (input high) or prevents (input
low) timer counting. To ensure recognition of an input level, it must be valid for four CPU clocks.
This is due to the counter element’s time-multiplexed servicing scheme for the timers.
9.2.6 Timer Interrupts
All timers can generate internal interrupt requests. Although all three timers share a single inter-
rupt request to the CPU, each has its own vector location and internal priority. Timer 0 has the
highest interrupt priority and Timer 2 has the lowest.
Timer Interrupts are enabled or disabled by the Interrupt (INT) bit in the Timer Control register.
If enabled, an interrupt is generated every time a maximum count value is reached. In dual max-
imum count mode, an interrupt is generated each time the value in Maxcount Compare A or Max-
count Compare B is reached. If the interrupt is disabled after a request has been generated, but
before a pending interrupt is serviced, the interrupt request remains active (the Interrupt Control-
ler latches the request). If a timer generates a second interrupt request before the CPU services
the first interrupt request, the first request is lost.
9.2.7 Programming Considerations
Timer registers can be read or written whether the timer is operating or not. Since processor ac-
cesses to timer registers are synchronized with counter element accesses, a half-modified count
register will never be read.
When Timer 0 and Timer 1 use an internal clock source, the input pin must be high to enable
counting.
9.3 TIMING
Certain timing considerations need to be made with the Timer/Counter Unit. These include input
setup and hold times, synchronization and operating frequency.
9.3.1 Input Setup and Hold Timings
To ensure recognition, setup and hold times must be met with respect to CPU clock edges. The
timer input signal must be valid TCHIS before the rising edge of CLKOUT and must remain valid
TCHIH after the same rising edge. If these timing requirements are not met, the input will not be
recognized until the next clock edge.
9-16
TIMER/COUNTER UNIT
9.3.2 Synchronization and Maximum Frequency
All timer inputs are latched and synchronized with the CPU clock. Because of the internal logic
required to synchronize the external signals, and the multiplexing of the counter element, the
Timer/Counter Unit can operate only up to ¼ of the CLKOUT frequency. Clocking at greater fre-
quencies will result in missed clocks.
9.3.2.1
Timer/Counter Unit Application Examples
The following examples are possible applications of the Timer/Counter Unit. They include a real-
time clock, a square wave generator and a digital one-shot.
9.3.3 Real-Time Clock
Example 9-1 contains sample code to configure Timer 2 to generate an interrupt request every 10
milliseconds. The CPU then increments memory-based clock variables.
9.3.4 Square-Wave Generator
A square-wave generator can be useful to act as a system clock tick. Example 9-2 illustrates how
to configure Timer 1 to operate this way.
9.3.5 Digital One-Shot
Example 9-3 configures Timer 1 to act as a digital one-shot.
9-17
TIMER/COUNTER UNIT
$mod186
name example_80186_family_timer_code
;FUNCTION: This function sets up the timer and interrupt controller
;
to cause the timer to generate an interrupt every
10 milliseconds and to service interrupts to
implement a real time clock.
;
;
;
;
Timer 2 is used in this example because no input or
output signals are required.
;
;
;SYNTAX:
extern void far set_time(hour, minute, second, T2Compare)
;
;INPUTS:
hour - hour to set time to.
;
minute - minute to set time to.
second - second to set time to.
T2Compare - T2CMPA value (see note below)
;
;
;
;OUTPUTS:
None
;NOTE:
Parameters are passed on the stack as required by
high-level languages
;
;
;
;
;
;
;
;
;
;
;
For a CLKOUT of 16Mhz,
f(timer2)
= 16Mhz/4
= 4Mhz
= 0.25us for T2CMPA = 1
T2CMPA(10ms)
= 10ms/0.25us
= 10e-3/0.25e-6
= 40000
;substitute register offsets
T2CON equ xxxxh
T2CMPA equ xxxxh
T2CNT equ xxxxh
TCUCON equ xxxxh
;Timer 2 Control register
;Timer 2 Compare register
;Timer 2 Counter register
;Int. Control register
;End Of Interrupt register
;Interrupt Status register
;timer 2:vector type 19
EOI
equ xxxxh
INTSTS equ xxxxh
timer_2_int equ 19
data segment public ’data’
public _hour, _minute, _second, _msec
_hour
db ?
db ?
db ?
db ?
_minute
_second
_msec
data ends
Example 9-1. Configuring a Real-Time Clock
9-18
TIMER/COUNTER UNIT
lib_80186 segment public ’code’
assume cs:lib_80186, ds:data
public _set_time
_set_time proc far
push
mov
bp
bp, sp
;save caller’s bp
;get current top of stack
hour
equ word ptr[bp+6]
;get parameters off stack
minute equ word ptr[bp+8]
second equ word ptr[bp+10]
T2Compare equ word ptr[bp+12]
push
push
push
ax
dx
si
;save registers used
;set interrupt vector
push
xor
mov
mov
mov
ds
ax, ax
ds, ax
si, 4*timer_2_int
word ptr ds:[si], offset
timer_2_interrupt_routine
inc
inc
mov
pop
si
si
ds:[si], cs
ds
mov
mov
mov
mov
mov
mov
mov
ax, hour
;set time
_hour, al
ax, minute
_minute, al
ax, second
_second, al
_msec, 0
mov
xor
out
dx, T2CNT
ax, ax
dx, al
;clear Count register
mov
mov
out
mov
mov
out
dx, T2CMPA
ax, T2Compare
dx, al
dx, T2CON
ax, 0E001H
dx, al
;set maximum count value
;see note in header above
;set up the control word:
;enable counting,
;generate interrupt on MC,
;continuous counting
mov
xor
out
dx, TCUCON
ax, ax
dx, al
;set up interrupt controller
;unmask highest priority interrupt
Example 9-1. Configuring a Real-Time Clock (Continued)
9-19
TIMER/COUNTER UNIT
sti
;enable interrupts
pop
pop
pop
pop
ret
si
dx
ax
bp
;restore saved registers
;restore caller’s bp
_set_time endp
timer_2_interrupt_routine proc far
push
push
cmp
ax
;save registers used
dx
_msec, 99
bump_second
_msec
;has 1 sec passed?
;if above or equal...
jae
inc
jmp
short reset_int_ctl
bump_second:
mov
cmp
jae
inc
jmp
_msec, 0
;reset millisecond
;has 1 minute passed?
_minute, 59
bump_minute
_second
short reset_int_ctl
bump_minute:
mov
cmp
jae
inc
jmp
_second, 0
;reset second
;has 1 hour passed?
_minute, 59
bump_hour
_minute
short reset_int_ctl
bump_hour:
mov
_minute, 0
_hour, 12
reset_hour
_hour
;reset minute
;have 12 hours passed?
cmp
jae
inc
jmp
reset_int_ctl
reset_hour:
mov
_hour, 1
;reset hour
reset_int_ctl:
mov
mov
out
pop
pop
iret
dx, EOI
ax, 8000h
dx, al
dx
;non-specific end of interrupt
ax
timer_2_interrupt_routine endp
lib_80186
ends
end
Example 9-1. Configuring a Real-Time Clock (Continued)
9-20
TIMER/COUNTER UNIT
$mod186
name
example_timer1_square_wave_code
;FUNCTION: This function generates a square wave of given
;
;
frequency and duty cycle on Timer 1 output pin.
; SYNTAX: extern void far clock(int mark, int space)
;
; INPUTS: mark - This is the mark (1) time.
;
;
;
;
;
;
;
space - This is the space (0) time.
The register compare value for a given time can be
easily calculated from the formula below.
CompareValue = (req_pulse_width*f)/4
; OUTPUTS: None
;
;
;
NOTE: Parameters are passed on the stack as required by
high-level Languages
T1CMPA equ xxxxH
;substitute register offsets
T1CMPB equ xxxxH
T1CNT equ xxxxH
T1CON equ xxxxH
lib_80186
segment public ’code’
assume cs:lib_80186
public
_clock
_clock
proc far
push
mov
bp
;save caller’s bp
;get current top of stack
;get parameters off the stack
bp, sp
_space equ word ptr[bp+6]
_mark equ word ptr[bp+8]
push
push
push
ax
bx
dx
;save registers that will be
;modified
mov
mov
out
dx, T1CMPA
ax, _mark
dx, al
;set mark time
mov
mov
out
dx, T1CMPB
ax, _space
dx, al
;set space time
;Clear Timer 1 Counter
;start Timer 1
mov
xor
out
dx, T1CNT
ax, ax
dx, al
mov
mov
out
dx, T1CON
ax, C003H
dx, al
Example 9-2. Configuring a Square-Wave Generator
9-21
TIMER/COUNTER UNIT
pop
pop
pop
dx
bx
ax
;restore saved registers
;restore caller’s bp
pop
ret
bp
_clock
endp
ends
lib_80186
end
Example 9-2. Configuring a Square-Wave Generator (Continued)
$mod186
name example_timer1_1_shot_code
; FUNCTION: This function generates an active-low one-shot pulse
;
on Timer 1 output pin.
;
; SYNTAX:
extern void far one_shot(int CMPB);
;
; INPUTS:
CMPB - This is the T1CMPB value required to generate a
pulse of a given pulse width. This value is calculated
from the formula below.
;
;
;
;
;
CMPB = (req_pulse_width*f)/4
; OUTPUTS: None
;
; NOTE:
;
Parameters are passed on the stack as required by
high-level languages
T1CNT equ xxxxH
;substitute register offsets
T1CMPA equ xxxxH
T1CMPB equ xxxxH
T1CON equ xxxxH
MaxCount equ 0020H
lib_80186
segment public ’code’
assume cs:lib_80186
public
_one_shot
_one_shot
proc far
push
mov
bp
bp, sp
;save caller’s bp
;get current top of stack
Example 9-3. Configuring a Digital One-Shot
9-22
TIMER/COUNTER UNIT
_CMPB equ word ptr[bp+6]
;get parameter off the stack
push
push
mov
xor
out
mov
mov
out
mov
mov
out
mov
mov
out
ax
;save registers that will be
;modified
dx
dx, T1CNT
ax, ax
dx, al
dx, T1CMPA
ax, 1
dx, al
dx, T1CMPB
ax, _CMPB
dx, al
dx, T1CON
ax, C002H
dx, al
;Clear Timer 1 Counter
;set time before t_shot to 0
;set pulse time
;start Timer 1
CountDown: in ax, dx
;read in T1CON
test
jz
ax, MaxCount
CountDown
;max count occurred?
;no: then wait
and
out
ax, not MaxCount
dx, al
;clear max count bit
;update T1CON
pop
pop
dx
ax
bp
;restore saved registers
;restore caller’s bp
pop
ret
_one_shot
lib_80186
end
endp
ends
Example 9-3. Configuring a Digital One-Shot (Continued)
9-23
10
Direct Memory
Access Unit
CHAPTER 10
DIRECT MEMORY ACCESS UNIT
In many applications, large blocks of data must be transferred between memory and I/O space. A
disk drive, for example, usually reads and writes data in blocks that may be thousands of bytes
long. If the CPU were required to handle each byte of the transfer, the main tasks would suffer a
severe performance penalty. Even if the data transfers were interrupt driven, the overhead for
transferring control to the interrupt handler would still decrease system throughput.
Direct Memory Access, or DMA, allows data to be transferred between memory and peripherals
without the intervention of the CPU. Systems that use DMA have a special device, known as
the DMA controller, that takes control of the system bus and performs the transfer between mem-
ory and the peripheral device. When the DMA controller receives a request for a transfer from a
peripheral, it signals the CPU that it needs control of the system bus. The CPU then releases con-
trol of the bus and the DMA controller performs the transfer. In many cases, the CPU releases the
bus and continues to execute instructions from the prefetch queue. If the DMA transfers are rel-
atively infrequent, there is no degradation of software performance; the DMA transfer is trans-
parent to the CPU.
The DMA Unit has two channels. Each channel can accept DMA requests from one of three
sources: an external request pin, the Timer/Counter Unit or direct programming. Data can be
transferred between any combination of memory and I/O space. The DMA Unit can access the
entire memory and I/O space in either byte or word increments.
10.1 FUNCTIONAL OVERVIEW
The DMA Unit consists of two channels that are functionally identical. The following discussion
is hierarchical, beginning with an overview of a single channel and ending with a description of
the two-channel unit.
10.1.1 The DMA Transfer
A DMA transfer begins with a request. The requesting device may either have data to transmit (a
source request) or it may require data (a destination request). Alternatively, transfers may be ini-
tiated by the system software without an external request.
10-1
DIRECT MEMORY ACCESS UNIT
When the DMA request is granted, the Bus Interface Unit provides the bus signals for the DMA
transfer, while the DMA channel provides the address information for the source and destination
controller chips (an acknowledge can be synthesized, however). The DMA channel continues
transferring data as long as the request is active and it has not exceeded its programmed transfer
limit.
Every DMA transfer consists of two distinct bus cycles: a fetch and a deposit (see Figure 10-1 on
page 10-2). During the fetch cycle, the byte or word is read from the data source and placed in an
internal temporary storage register. The data in the temporary storage register is written to the
destination during the deposit cycle. The two bus cycles are indivisible; they cannot be separated
by a bus hold request, a refresh request or another DMA request.
Deposit
Fetch
TI
T1
T2
T3
T4
T1
T2
T3
T4
CLKOUT
ALE
AD15:0
Source
Address
Source
Data
Destination Destination
Address Data
RD
WR
A1186-0A
Figure 10-1. Typical DMA Transfer
10-2
DIRECT MEMORY ACCESS UNIT
10.1.1.1
DMA Transfer Directions
The source and destination addresses for a DMA transfer are programmable and can be in either
memory or I/O space. DMA transfers can be programmed for any of the following four direc-
tions:
• from memory space to I/O space
• from I/O space to memory space
• from memory space to memory space
• from I/O space to I/O space
DMA transfers can access the Peripheral Control Block.
10.1.1.2
Byte and Word Transfers
DMA transfers can be programmed to handle either byte or word transfers. The handling of byte
and word data is the same as that for normal bus cycles and is dependent upon the processor bus
width. For example, odd-aligned word DMA transfers on a processor with a 16-bit bus requires
two fetches and two deposits (all back-to-back). BIU bus cycles are covered in Chapter 3, “Bus
Interface Unit.” Word transfers are illegal on the 8-bit bus device.
10.1.2 Source and Destination Pointers
Each DMA channel maintains a twenty-bit pointer for the source of data and a twenty-bit pointer
for the destination of data. The twenty-bit pointers allow access to the full 1 Mbyte of memory
space. The DMA Unit views memory as a linear (unsegmented) array.
With a twenty-bit pointer, it is possible to create an I/O address that is above the CPU limit of 64
Kbytes. The DMA Unit will run I/O DMA cycles above 64K, even though these addresses are
not accessible through CPU instructions (e.g., IN and OUT). Some applications may wish to
make use of this by swapping pages of data from I/O space above 64K to standard CPU memory.
The source and destination pointers can be individually programmed to increment, decrement or
remain constant after each transfer. The programmed data width (byte or word) determines the
amount that a pointer is incremented or decremented. Word transfers change the pointer by two;
byte transfers change the pointer by one.
10.1.3 DMA Requests
There are three distinct sources of DMA requests: the external DRQ pin, the internal DMA re-
quest line and the system software. In all three cases, the system software must arm a DMA chan-
nel before it recognizes DMA requests. (See “Arming the DMA Channel” on page 10-18.)
10-3
DIRECT MEMORY ACCESS UNIT
10.1.4 External Requests
External DMA requests are asserted on the DRQ pins. The DRQ pins are sampled on the falling
edge of CLKOUT. It takes a minimum of four clocks before the DMA cycle is initiated by the
BIU (see Figure 10-2). The DMA request is cleared four clocks before the end of the DMA cycle
(effectively re-arming the DRQ input).
T1
T1 or
TW or
TI
T2 or
TW or
TI
T3 or
TW or
TI
of
DMA
Cycle
T4 or
TI
4
DRQ
1
2
3
NOTES:
1. T
: DMA request to clock low.
INVCL
2. Synchronizer resolution time.
3. DMA unit priority arbitration and overhead.
4. Bus interface unit latches DMA request and decides to run DMA cycle.
A1528-0A
Figure 10-2. DMA Request Minimum Response Time
External requests (and the resulting DMA transfer) are classified as either source-synchronized
sending data. For example, a disk controller in the process of reading data from a disk would use
a source-synchronized request (data would be moving from the disk to memory). A destination-
synchronized request originates from the peripheral that is receiving data. If a disk controller
were writing data to a disk, it would use a destination-synchronized request (data would be mov-
ing from memory to the disk). The type of synchronization a channel uses is programmable. (See
“Selecting Channel Synchronization” on page 10-18.)
10-4
DIRECT MEMORY ACCESS UNIT
10.1.4.1
Source Synchronization
A typical source-synchronized transfer is shown in Figure 10-3. Most DMA-driven peripherals
deassert their DRQ line only after the DMA transfer has begun. The DRQ signal must be deas-
serted at least four clocks before the end of the DMA transfer (at the T1 state of the deposit phase)
to prevent another DMA cycle from occurring. A source-synchronized transfer provides the
source device at least three clock cycles from the time it is accessed (acknowledged) to deassert
its request line if further transfers are not required.
Deposit Cycle
T2 T3 T4
Fetch Cycle
T2 T3
T1
T4
T1
1
CLKOUT
DRQ (Case 1)
2
DRQ (Case 2)
NOTES:
1. Current source synchronized transfer will not be immediately
followed by another DMA transfer.
2. Current source synchronized transfer will be immediately
followed by another DMA transfer.
A1188-0A
Figure 10-3. Source-Synchronized Transfers
10.1.4.2
Destination Synchronization
A destination-synchronized transfer differs from a source-synchronized transfer by the addition
of two idle states at the end of the deposit cycle (Figure 10-4). The two idle states extend the DMA
cycle to allow the destination device to deassert its DRQ pin four clocks before the end of the
cycle. If the two idle states were not inserted, the destination device would not be able to deassert
its request in time to prevent another DMA cycle from occurring.
The insertion of two idle states at the end of a destination synchronization transfer has an impor-
tant side effect. A destination-synchronized DMA channel gives up the bus during the idle
states, allowing any other bus master to gain ownership. This includes the CPU, the Refresh
Control Unit, an external bus master or another DMA channel.
10-5
DIRECT MEMORY ACCESS UNIT
Deposit Cycle
Fetch Cycle
T2 T3
T1
T4
T1
T2
T3
T4
TI
TI
CLKOUT
DRQ
(Case 1)
1
DRQ
(Case 2)
2
NOTES:
1. Current destination synchronized transfer will not be immediately
followed by another DMA transfer.
2. Current destination synchronized transfer will be immediately
followed by another DMA transfer.
A1189-0A
Figure 10-4. Destination-Synchronized Transfers
10.1.5 Internal Requests
Internal DMA requests can come from either Timer 2 or the system software.
10.1.5.1 Timer 2-Initiated Transfers
When programmed for Timer 2-initiated transfers, the DMA channel performs one DMA transfer
every time that Timer 2 reaches its maximum count. Timer-initiated transfers are useful for ser-
vicing time-based peripherals. For example, an A/D converter would require data every 22 mi-
croseconds in order to produce an audio range waveform. In this case, the DMA source would
point to the waveform data, the destination would point to the A/D converter and Timer 2 would
request a transfer every 22 microseconds. (See “Timed DMA Transfers” on page 10-26.)
10.1.5.2
Unsynchronized Transfers
DMA transfers can be initiated directly by the system software by selecting unsynchronized
transfers. Unsynchronized transfers continue, back-to-back, at the full bus bandwidth, until the
channel’s transfer count reaches zero or DMA transfers are suspended by an NMI.
10-6
DIRECT MEMORY ACCESS UNIT
10.1.6 DMA Transfer Counts
Each DMA Unit maintains a programmable 16-bit transfer count value that controls the total
number of transfers the channel runs. The transfer count is decremented by one after each transfer
(regardless of data size). The DMA channel can be programmed to terminate transfers when the
transfer count reaches zero (also referred to as terminal count).
10.1.7 Termination and Suspension of DMA Transfers
When DMA transfers for a channel are terminated, no further DMA requests for that channel will
be granted until the channel is re-started by direct programming. A suspended DMA transfer tem-
porarily disables transfers in order to perform a specific task. A suspended DMA channel does
not need to be re-started by direct programming.
10.1.7.1
Termination at Terminal Count
When programmed to terminate on terminal count, the DMA channel disarms itself when the
transfer count value reaches zero. No further DMA transfers take place on the channel until it is
re-armed by direct programming. Unsynchronized transfers always terminate when the transfer
count reaches zero, regardless of programming.
10.1.7.2
Software Termination
A DMA channel can be disarmed by direct programming. Any DMA transfer that is in progress
will complete, but no further transfers are run until the channel is re-armed.
10.1.7.3
Suspension of DMA During NMI
DMA transfers are inhibited during the service of Non-Maskable Interrupts (NMI). DMA activity
is halted in order to give the CPU full command of the system bus during the NMI service. Exit
from the NMI via an IRET instruction re-enables the DMA Unit. DMA transfers can be enabled
during an NMI service routine by the system software.
10.1.7.4
Software Suspension
DMA transfers can be temporarily suspended by direct programming. In time-critical sections of
code, such as interrupt handlers, it may be necessary to shut off DMA activity temporarily in or-
der to give the CPU total control of the bus.
10-7
DIRECT MEMORY ACCESS UNIT
10.1.8 DMA Unit Interrupts
Each DMA channel can be programmed to generate an interrupt request when its transfer count
reaches zero.
10.1.9 DMA Cycles and the BIU
The DMA Unit uses the Bus Interface Unit to perform its transfers. When the DMA Unit has a
pending request, it signals the BIU. If the BIU has no other higher-priority request pending, it runs
the DMA cycle. (BIU priority is described in Chapter 3, “Bus Interface Unit.”) The BIU signals
that it is running a bus cycle initiated by a master other than the CPU by driving the S6 status bit
high.
The Chip-Select Unit monitors the BIU addresses to determine which chip-select, if any, to acti-
vate. Because the DMA Unit uses the BIU, chip-selects are active for DMA cycles. If a DMA
channel accesses a region of memory or I/O space within a chip-select’s programmed range, then
that chip-select is asserted during the cycle. The Chip-Select Unit will not recognize DMA cycles
that access I/O space above 64K.
10.1.10 The Two-Channel DMA Unit
Two DMA channels are combined with arbitration logic to form the DMA Unit (see Figure 10-5).
10.1.10.1 DMA Channel Arbitration
Within the two-channel DMA Unit, the arbitration logic decides which channel takes precedence
when both channels simultaneously request transfers. Each channel can be set to either low pri-
ority or high priority. If the two channels are set to the same priority (either both high or both
low), then the channels rotate priority.
10.1.10.1.1 Fixed Priority
Fixed priority results when one channel in a module is programmed to high priority and the other
is set to low priority. If both DMA requests occur simultaneously, the high priority channel per-
forms its transfer (or transfers) first. The high priority channel continues to perform transfers as
long as the following conditions are met:
• the channel’s DMA request is still active
• the channel has not terminated or suspended transfers (through programming or interrupts)
• the channel has not released the bus (through the insertion of idle states for destination-
synchronized transfers)
10-8
DIRECT MEMORY ACCESS UNIT
The last point is extremely important when the two channels use different synchronization. For
example, consider the case in which channel 1 is programmed for high priority and destination
synchronization and channel 0 is programmed for low priority and source synchronization. If a
DMA request occurs for both channels simultaneously, channel 1 performs the first transfer. At
the end of channel 1’s deposit cycle, two idle states are inserted (thus releasing the bus). With the
bus released, channel 0 is free to perform its transfer even though the higher-priority channel
has not completed all of its transfers. Channel 1 regains the bus at the end of channel 0’s trans-
fer. The transfers will alternate as long as both requests remain active.
Module
Timer 2
DMA Request
Internal - DMA
Request
Multiplexer
Inter-module
Arbitration
Logic
Timer 2
Request
Timer 2 Request
Source Pointer
Source Pointer
Destination Pointer
Destination Pointer
Channel 0
Channel 1
Control Logic
Control Logic
DRQ Pin
DRQ Pin
A1540-01
Figure 10-5. Two-Channel DMA Module
A higher-priority DMA channel will interrupt the transfers of a lower-priority channel. Figure
10-6 shows several transfers with different combinations of channel priority and synchronization.
10-9
DIRECT MEMORY ACCESS UNIT
Both Requests Asserted
Channel
Priority
Synch
0
1
Etc.
Low Low
SRC SRC
Channel 1 Channel 0 Channel 1 Channel 0
Channel
Priority High Low
Synch
0
1
Etc.
Etc.
Channel 0 Channel 0
Channel 1 Channel 1
SRC SRC
Channel 0 Completes
All Transfers
Channel
Priority High Low
Synch Dest SRC
0
1
Channel 0
Channel 1 Channel 0
Channel 1
Destination Synch Releases Bus
A1190-0A
Figure 10-6. Examples of DMA Priority
10.1.10.1.2 Rotating Priority
Channel priority rotates when the channels are programmed as both high or both low priority. The
highest priority is initially assigned to channel 1 of the module. After a channel performs a trans-
fer, it is assigned the lower priority. When requests are active for both channels, the transfers al-
ternate between the two. Channel 1 is reassigned high priority whenever the bus is released (that
is, at the end of a destination-synchronized transfer or when DMA requests are no longer active).
10.2 PROGRAMMING THE DMA UNIT
A total of six Peripheral Control Block registers configure each DMA channel.
10.2.1 DMA Channel Parameters
The first step in programming the DMA Unit is to set up the parameters for each channel.
10.2.1.1
Programming the Source and Destination Pointers
The following parameters are programmable for the source and destination pointers:
• pointer address
• address space (memory or I/O)
• automatic pointer indexing (increment, decrement or no change) after transfer
10-10
DIRECT MEMORY ACCESS UNIT
Two 16-bit Peripheral Control Block registers define each of the 20-bit pointers. Figures 10.7 and
10.8 show the layout of the DMA Source Pointer address registers, and Figures 10.9 and 10.10
show the layout of the DMA Destination Pointer address registers. The DSA19:16 and
DDA19:16 (high-order address bits) are driven on the bus even if I/O transfers have been pro-
grammed. When performing I/O transfers within the normal 64K I/O space only, the high-order
bits in the pointer registers must be cleared.
Register Name:
DMA Source Address Pointer (High)
DxSRCH
Register Mnemonic:
Register Function:
Contains the upper 4 bits of the DMA Source pointer.
15
0
D
S
A
1
D
S
A
1
D
S
A
1
D
S
A
1
9
8
7
6
A1185-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
DSA19:16
DMA
Source
Address
XXXXH
DSA19:16 are driven on A19:16 during the
fetch phase of a DMA transfer.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 10-7. DMA Source Pointer (High-Order Bits)
10-11
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Source Address Pointer (Low)
DxSRCL
Register Mnemonic:
Register Function:
Contains the lower 16 bits of the DMA Source pointer.
15
0
D
S
A
1
D
S
A
1
D
S
A
1
D
S
A
1
D
S
A
1
D
S
A
1
D
S
A
9
D
S
A
8
D
S
A
7
D
S
A
6
D
S
A
5
D
S
A
4
D
S
A
3
D
S
A
2
D
S
A
1
D
S
A
0
5
4
3
2
1
0
A1177-0A
Bit
Mnemonic
Reset
State
Bit Name
DMA
Source
Address
Function
DSA15:0
XXXXH
DSA15:0 are driven on the lower 16 bits of the
address bus during the fetch phase of a DMA
transfer.
Figure 10-8. DMA Source Pointer (Low-Order Bits)
The address space referenced by the source and destination pointers is programmed in the DMA
Control Register for the channel (see Figure 10-11 on page 10-15). The SMEM and DMEM bits
control the address space (memory or I/O) for source pointer and destination pointer, respective-
ly.
Automatic pointer indexing is also controlled by the DMA Control Register. Each pointer has two
bits, increment and decrement, that control the indexing. If the increment and decrement bits for
a pointer are programmed to the same value, then the pointer remains constant. The programmed
data width (byte or word) for the channel automatically controls the amount that a pointer is in-
cremented or decremented.
10-12
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Destination Address Pointer (High)
Register Mnemonic:
Register Function:
DxDSTH
Contains the upper 4 bits of the DMA
Destination pointer.
15
0
D
D
A
1
D
D
A
1
D
D
A
1
D
D
A
1
9
8
7
6
A1178-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
DDA19:16
DMA
Destination
Address
XXXXH
DDA19:16 are driven on A19:16 during the
deposit phase of a DMA transfer.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 10-9. DMA Destination Pointer (High-Order Bits)
10-13
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Destination Address Pointer (Low)
DxDSTL
Register Mnemonic:
Register Function:
Contains the lower 16 bits of the DMA Destination
pointer.
15
0
D
D
A
1
D
D
A
1
D
D
A
1
D
D
A
1
D
D
A
1
D
D
A
1
D
D
A
9
D
D
A
8
D
D
A
7
D
D
A
6
D
D
A
5
D
D
A
4
D
D
A
3
D
D
A
2
D
D
A
1
D
D
A
0
5
4
3
2
1
0
A1179-0A
Bit
Mnemonic
Reset
State
Bit Name
DMA
Destination
Address
Function
DDA15:0
XXXXH
DDA15:0 are driven on the lower 16 bits of the
address bus during the deposit phase of a DMA
transfer.
Figure 10-10. DMA Destination Pointer (Low-Order Bits)
Selecting Byte or Word Size Transfers
10.2.1.2
The WORD bit in the DMA Control Register (Figure 10-11) controls the data size for a channel.
When WORD is set, the channel transfers data in 16-bit words. Byte transfers are selected by
clearing the WORD bit. The data size for a channel also affects pointer indexing. Word transfers
modify (increment or decrement) the pointer registers by two for each transfer, while byte trans-
fers modify the pointer registers by one.
10-14
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Control Register
DxCON
Register Mnemonic:
Register Function:
Controls DMA channel parameters.
15
0
D
M
E
D
D
E
C
D
I
N
C
S
M
E
S
D
E
C
S
I
N
C
T
C
I
N
T
S
Y
N
1
S
Y
N
0
P
I
C
H
G
S
T
R
T
W
D
R
Q
O
R
D
M
M
A1180-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
DMEM
Destination
Address
Space
X
Selects memory or I/O space for the destination
pointer. Set DMEM to select memory space; clear
DMEM to select I/O space.
Select
DDEC
DINC
Destination
Decrement
X
X
X
Set DDEC to automatically decrement the destination
pointer after each transfer. (See Note.)
Destination
Increment
Set DINC to automatically increment the destination
pointer after each transfer. (See Note.)
SMEM
Source
Address
Space
Select
Selects memory or I/O space for the source pointer.
Set SMEM to select memory space; clear SMEM to
select I/O space.
SDEC
SINC
Source
Decrement
X
X
Set SDEC to automatically decrement the source
pointer after each transfer. (See Note.)
Source
Increment
Set SINC to automatically increment the source
pointer after each transfer. (See Note.)
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products. A pointer remains constant if
its increment and decrement bits are equal.
Figure 10-11. DMA Control Register
10-15
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Control Register
DxCON
Register Mnemonic:
Register Function:
Controls DMA channel parameters.
15
0
D
M
E
D
D
E
C
D
I
N
C
S
M
E
S
D
E
C
S
I
N
C
T
C
I
N
T
S
Y
N
1
S
Y
N
0
P
I
C
H
G
S
T
R
T
W
D
R
Q
O
R
D
M
M
A1180-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
TC
Terminal
Count
X
Set TC to terminate transfers on Terminal Count. This bit
is ignored for unsynchronized transfers (that is, the DMA
channel behaves as if TC is set, regardless of its
condition).
INT
Interrupt
X
Set INT to generate an interrupt request on Terminal
Count. The TC bit must be set to generate an interrupt.
SYN1:0
Synchron-
XX
Selects channel synchronization:
ization Type
SYN1 SYN0
Synchronization Type
Unsynchronized
0
0
1
1
0
1
0
1
Source-synchronized
Destination-synchronized
Reserved (do not use)
P
Relative
Priority
X
X
Set P to select high priority for the channel; clear P to
select low priority for the channel.
IDRQ
Internal
DMA
Request
Select
Set IDRQ to select internal DMA requests and ignore
the external DRQ pin. Clear IDRQ to select the DRQ pin
as the source of DMA requests. When IDRQ is set, the
channel must be configured for source-synchronized
transfers (SYN1:0 = 01).
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products.
Figure 10-11. DMA Control Register (Continued)
10-16
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Control Register
Register Mnemonic:
Register Function:
DxCON
Controls DMA channel parameters.
15
0
D
M
E
D
D
E
C
D
I
N
C
S
M
E
S
D
E
C
S
I
N
C
T
C
I
N
T
S
Y
N
1
S
Y
N
0
P
I
C
H
G
S
T
R
T
W
D
R
Q
O
R
D
M
M
A1180-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
CHG
Change
Start Bit
X
Set CHG to enable modifying the STRT bit.
STRT
Start DMA
Channel
0
Set STRT to arm the DMA channel. The STRT bit can
be modified only when the CHG bit is set.
WORD
Word
Transfer
Select
X
Set WORD to select word transfers; clear WORD to
select byte transfers. The 8-bit bus versions of the
device ignore the WORD bit.
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written to a
logic zero to ensure compatibility with future Intel products.
Figure 10-11. DMA Control Register (Continued)
10.2.1.3
Selecting the Source of DMA Requests
DMA requests can come from either an internal source (Timer 2) or an external source.
Internal DMA requests are selected by setting the IDRQ bit in the DMA Control Register (see
Figure 10-11 on page 10-15) for the channel. The DMA channel ignores its DRQ pin when inter-
nal requests are programmed. Similarly, the DMA channel responds only to the DRQ pin (and
ignores internal requests) when external requests are selected.
10-17
DIRECT MEMORY ACCESS UNIT
10.2.1.4
Arming the DMA Channel
Each DMA channel must be armed before it can recognize DMA requests. A channel is armed
by setting its STRT (Start) bit in the DMA Control Register (Figure 10-11 on page 10-15). The
STRT bit can be modified only if the CHG (Change Start) bit is set at the same time. The CHG
bit is a safeguard to prevent accidentally arming a DMA channel while modifying other channel
parameters.
software or by the channel itself when it is programmed to terminate on terminal count.
10.2.1.5
Selecting Channel Synchronization
The synchronization method for a channel is controlled by the SYN1:0 bits in the DMA Control
Register (Figure 10-11 on page 10-15).
NOTE
The combination SYN1:0=11 is reserved and will result in unpredictable
operation. When IDRQ is set (internal requests selected) the channel must
always be programmed for source-synchronized transfers (SYN1:0=01).
transfer data as soon as the STRT bit is set.
10.2.1.6
Programming the Transfer Count Options
The Transfer Count Register (Figure 10-12) and the TC bit in the DMA Control Register (Figure
10-11 on page 10-15) are used to stop DMA transfers for a channel after a specified number of
transfers have occurred.
The transfer count (the number of transfers desired) is written to the DMA Transfer Count Reg-
ister. The Transfer Count Register is 16 bits wide, limiting the total number of transfers for a
channel to 65,536 (without reprogramming). The Transfer Count Register is decremented by one
after each transfer (for both byte and word transfers).
10-18
DIRECT MEMORY ACCESS UNIT
Register Name:
DMA Transfer Count
DxTC
Register Mnemonic:
Register Function:
Contains the DMA channel’s transfer count.
15
0
T
C
1
T
C
1
T
C
1
T
C
1
T
C
1
T
C
1
T
C
9
T
C
8
T
C
7
T
C
6
T
C
5
T
C
4
T
C
3
T
C
2
T
C
1
T
C
0
5
4
3
2
1
0
A1172-0A
Bit
Mnemonic
Reset
State
Bit Name
Function
TC15:0
Transfer
Count
XXXXH
Contains the transfer count for a DMA channel.
This value is decremented by one after each
transfer.
Figure 10-12. Transfer Count Register
The TC bit, when set, instructs the DMA channel to disarm itself (by clearing the STRT bit) when
the transfer count reaches zero. If the TC bit is cleared, the channel continues to perform transfers
regardless of the state of the Transfer Count Register. Unsynchronized (software-initiated) trans-
10.2.1.7
Generating Interrupts on Terminal Count
A channel can be programmed to generate an interrupt request whenever the transfer count reach-
10-15) must be set to generate an interrupt request.
10.2.1.8
Setting the Relative Priority of a Channel
The priority of a channel is controlled by the Priority bit in the DMA Control Register (Figure
10-11 on page 10-15). A channel may be assigned either high or low priority. If both channels are
programmed to the same priority (i.e., both high or both low), the channels rotate priority.
10-19
DIRECT MEMORY ACCESS UNIT
10.2.2 Suspension of DMA Transfers
Whenever the CPU receives an NMI, all DMA activity is suspended at the end of the current
transfer. The CPU suspends DMA activity by setting the DHLT bit in the Interrupt Status Regis-
ter (Figure 8-14 on page 8-23). When an IRET instruction is executed, the CPU clears the DHLT
bit and DMA transfers are allowed to resume. Software can read and write the DHLT bit.
NOTE
Do not write to the DHLT bit while Timer/Counter Unit interrupts are enabled.
A conflict with the internal use of the register may cause incorrect processing
of timer interrupts.
The DHLT bit does not function when the interrupt controller is in slave mode.
10.2.3 Initializing the DMA Unit
Use the following sequence when programming the DMA Unit:
1. Program the source and destination pointers for all used channels.
2. Program the DMA Control Registers in order of highest-priority channel to lowest-
priority channel.
10.3 HARDWARE CONSIDERATIONS AND THE DMA UNIT
This section covers hardware interfacing and performance factors for the DMA Unit.
10.3.1 DRQ Pin Timing Requirements
The DRQ pins are sampled on the falling edge of CLKOUT. The DRQ pins must be set up a min-
imum of TINVCL before CLKOUT falling, to guarantee recognition at a specific clock edge. Refer
to the data sheet for specific values.
The DRQ pins have an internal synchronizer. Violating the setup time can cause only a missed
DMA request, not a processor malfunction.
10-20
DIRECT MEMORY ACCESS UNIT
10.3.2 DMA Latency
DMA Latency is the delay between a DMA request being asserted and the DMA cycle being run.
The DMA latency for a channel is controlled by many factors:
• Bus HOLD — Bus HOLD takes precedence over internal DMA requests. Using bus HOLD
will degrade DMA latency.
• LOCKed Instructions — Long LOCKed instructions (e.g., LOCK REP MOVS) will
monopolize the bus, preventing access by the DMA Unit.
• Inter-channel Priority Scheme — Setting a channel at low priority will affect its latency.
The minimum latency in all cases is four CLKOUT cycles. This is the amount of time it takes to
synchronize and prioritize a request.
10.3.3 DMA Transfer Rates
The maximum DMA transfer rate is a function of processor operating frequency and synchroni-
zation mode. For unsynchronized and source-synchronized transfers, the 80C186 Modular Core
can transfer two bytes every eight CLKOUT cycles. For destination-synchronized transfers, the
addition of two idle T-states reduces the bandwidth by two clocks per word.
Maximum DMA transfer rates (in Mbytes per second) for the 80C186 Modular Core are calcu-
lated by the following equations, where FCPU is the CPU operating frequency (in megahertz).
For unsynchronized and source-synchronized transfers:
0.25 × FCPU
For destination-synchronized transfers:
0.20 × FCPU
Because of its 8-bit data bus, the 80C188 Modular Core can transfer only one byte per DMA cy-
cle. Therefore, the maximum transfer rates for the 80C188 Modular Core are half those calculated
by the equations for the 80C186 Modular Core.
10-21
DIRECT MEMORY ACCESS UNIT
10.3.4 Generating a DMA Acknowledge
The DMA channels do not provide a distinct DMA acknowledge signal. A chip-select line can be
programmed to activate for the memory or I/O range that requires the acknowledge. The chip-
can be used as a qualifier to the chip-select for situations in which the chip-select line will be ac-
tive for both DMA and normal data accesses.
10.4 DMA UNIT EXAMPLES
Example 10-1 sets up channel 0 to perform an unsynchronized burst transfer from memory to
memory while channel 1 is used to service an external DMA request from a hard disk controller.
Example 10-2 shows timed DMA transfers. A sawtooth waveform is created using DMA trans-
fers to an A/D converter.
10-22
DIRECT MEMORY ACCESS UNIT
$MOD186
name
DMA_EXAMPLE_1
; This example shows code necessary to set up two DMA channels.
; One channel performs an unsynchronized transfer from memory to memory.
; The second channel is used by a hard disk controller located in
; I/O space.
; It is assumed that the constants for PCB register addresses are
; defined elsewhere with EQUates.
CODE_SEG SEGMENT
ASSUME CS:CODE_SEG
START:
MOV
MOV
AX, DATA_SEG
DS, AX
; DATA SEGMENT POINTER
ASSUME DS:DATA_SEG
; First we must initialize DMA channel 0. DMA0 will perform an
; unsynchronized transfer from SOURCE_DATA_1 to DEST_DATA_1.
; The first step is to calculate the proper values for the
; source and destination pointers.
MOV
AX, SEG SOURCE_DATA_1
ROL
MOV
AND
AX, 4
; GET HIGH 4 BITS
; SAVE ROTATED VALUE
; GET SHIFTED LOW 4 NIBBLES
BX, AX
AX, 0FFF0H
ADD
AX, OFFSET SOURCE_DATA_1
; NOW LOW BYTES OF POINTER ARE IN AX
ADC
BX, 0
; ADD IN THE CARRY
; TO THE HIGH NIBBLE
; GET JUST THE HIGH NIBBLE
AND
MOV
OUT
BX, 000FH
DX, D0SRCL
DX, AL
; AX=LOW 4 BYTES
MOV
MOV
OUT
DX, D0SRCH
AX, BX
DX, AX
; GET HIGH NIBBLE
; SOURCE POINTER DONE. REPEAT FOR DESTINATION.
MOV
AX, SEG DEST_DATA_1
ROL
MOV
AND
AX, 4
; GET HIGH 4 BITS
; SAVE ROTATED VALUE
; GET SHIFTED LOW 4 NIBBLES
BX, AX
AX, 0FFF0H
ADD
AX, OFFSET DEST_DATA_1
; NOW LOW BYTES OF POINTER ARE IN AX
ADC
BX, 0
; ADD IN THE CARRY
; TO THE HIGH NIBBLE
; GET JUST THE HIGH NIBBLE
AND
MOV
OUT
BX, 000FH
DX, D0DSTL
DX, AX
; AX=LOW 4 BYTES
Example 10-1. Initializing the DMA Unit
10-23
DIRECT MEMORY ACCESS UNIT
MOV
MOV
OUT
DX, D0DSTH
AX, BX
DX, AX
; GET HIGH NIBBLE
; THE POINTER ADDRESSES HAVE BEEN SET UP. NOW WE SET UP THE TRANSFER COUNT.
MOV
MOV
OUT
AX, 29
DX, D0TC
DX, AX
; THE MESSAGE IS 29 BYTES LONG.
; XFER COUNT REG
; NOW WE NEED TO SET THE PARAMETERS FOR THE CHANNEL AS FOLLOWS:
;
;
;
;
;
;
DESTINATION
-----------
SOURCE
------
MEMORY SPACE MEMORY SPACE
INCREMENT PTR INCREMENT PTR
; TERMINATE ON TC, NO INTERRUPT, UNSYNCHRONIZED, LOW PRIORITY RELATIVE
; TO CHANNEL 1, BYTE XFERS. WE START THE CHANNEL.
MOV
MOV
OUT
AX, 1011011000000110B
DX, D0CON
DX, AX
; THE UNSYNCHRONIZED BURST IS NOW RUNNING ON THE BUS...
; NOW SET UP CHANNEL 1 TO SERVICE THE DISK CONTROLLER.
; FOR THIS EXAMPLE WE WILL ONLY BE READING FROM THE DISK.
; THE SOURCE IS THE I/O PORT FOR THE DISK CONTROLLER.
MOV
MOV
OUT
AX, DISK_IO_ADDR
DX, D1SRCL
DX, AX
; PROGRAM LOW ADDR
; HI ADDR FOR IO=0
XOR
MOV
OUT
AX, AX
DX, D1SRCH
DX, AX
; THE DESTINATION IS THE DISK BUFFER IN MEMORY
MOV
AX, SEG DISK_BUFF
ROL
MOV
AND
AX, 4
; GET HIGH 4 BITS
; SAVE ROTATED VALUE
; GET SHIFTED LOW 4 NIBBLES
BX, AX
AX, 0FFF0H
ADD
AX, OFFSET DISK_BUFF
; NOW LOW BYTES OF POINTER ARE IN AX
ADC
BX, 0
; ADD IN THE CARRY
; TO THE HIGH NIBBLE
; GET JUST THE HIGH NIBBLE
AND
MOV
OUT
MOV
MOV
OUT
BX, 000FH
DX, D1DSTL
DX, AL
DX, D1DSTH
AX, BX
DX, AX
; AX=LOW 4 BYTES
; GET HIGH NIBBLE
; THE POINTER ADDRESSES HAVE BEEN SET UP. NOW WE SET UP THE TRANSFER COUNT.
Example 10-1. Initializing the DMA Unit (Continued)
10-24
DIRECT MEMORY ACCESS UNIT
MOV
AX, 512
; THE DISK READS IN 512 BYTE
; XFER COUNT REG
SECTORS
MOV
OUT
DX, D1TC
DX, AX
; NOW WE NEED TO SET THE PARAMETERS FOR THE CHANNEL AS FOLLOWS:
;
;
;
;
;
;
DESTINATION
-----------
MEMORY SPACE
INCREMENT PTR
SOURCE
------
I/O SPACE
CONSTANT PTR
; TERMINATE ON TC, INTERRUPT, SOURCE SYNC, HIGH PRIORITY RELATIVE TO
; CHANNEL 0, BYTE XFERS, USE DRQ PIN FOR REQUEST SOURCE. ARM CHANNEL.
MOV
MOV
OUT
AX, 1010001101100110B
DX, D0CON
DX, AX
; REQUESTS ON DRQ1 WILL NOW RESULT IN TRANSFERS
CODE_SEG ENDS
DATA_SEG SEGMENT
SOURCE_DATA_1DB '80C186EC INTEGRATED PROCESSOR'
DEST_DATA_1DB
DISK_BUFF DB
DATA_SEG ENDS
30 DUP('MITCH')
; JUNK DATA FOR TEST
512 DUP(?)
END START
Example 10-1. Initializing the DMA Unit (Continued)
10-25
DIRECT MEMORY ACCESS UNIT
$mod186
name
DMA_EXAMPLE_1
; This example sets up the DMA Unit to perform a transfer from memory to
; I/O space every 22 uS. The data is sent to an A/D converter.
; It is assumed that the constants for PCB register addresses are
; defined elsewhere with EQUates.
CODE_SEG SEGMENT
ASSUME CS:CODE_SEG
START:
MOV
MOV
AX, DATA_SEG
DS, AX
; DATA SEGMENT POINTER
ASSUME DS:DATA_SEG
; First, set up the pointers. The source is in memory.
MOV
AX, SEG WAVEFORM_DATA
ROL
MOV
AND
AX, 4
; GET HIGH 4 BITS
; SAVE ROTATED VALUE
; GET SHIFTED LOW 4 NIBBLES
BX, AX
AX, 0FFF0H
ADD
AX, OFFSET WAVEFORM_DATA
; NOW LOW BYTES OF POINTER ARE IN AX.
ADC
BX, 0
; ADD IN THE CARRY
; TO THE HIGH NIBBLE
; GET JUST THE HIGH NIBBLE
AND
MOV
OUT
BX, 000FH
DX, D0SRCL
DX, AX
; AX=LOW 4 BYTES
MOV
MOV
OUT
DX, D0SRCH
AX, BX
; GET HIGH NIBBLE
; I/O ADDRESS OF D/A
DX, AX
MOV
MOV
OUT
AX, DA_CNVTR
DX, D0DSTL
DX, AX
MOV
XOR
OUT
DX, D0DSTH
AX, AX
; CLEAR HIGH NIBBLE
DX, AX
; THE POINTER ADDRESSES HAVE BEEN SET UP. NOW WE SET UP THE TRANSFER COUNT.
MOV
MOV
OUT
AX, 255
DX, D0TC
DX, AX
; 8-BIT D/A, SO WE SEND 256 BYTES
; TO GET A FULL SCALE
; PROGRAM IDRQ MUX
MOV
MOV
DX, DMAPRI
AX, 00H
; TIMER2 IS IDRQ SOURCE
; MODULES HAVE EQUAL PRIORITY
OUT
DX, AX
Example 10-2. Timed DMA Transfers
10-26
DIRECT MEMORY ACCESS UNIT
; NOW WE NEED TO SET THE PARAMETERS FOR THE CHANNEL AS FOLLOWS:
;
;
;
;
;
;
DESTINATION
-----------
I/O SPACE
SOURCE
------
MEMORY SPACE
CONSTANT PTR INCREMENT PTR
; TERMINATE ON TC, INTERRUPT, SOURCE SYNCHRONIZE, INTERNAL REQUESTS,
; LOW PRIORITY RELATIVE TO CHANNEL 1, BYTE XFERS.
MOV
MOV
OUT
AX, 0001011101010110B
DX, D0CON
DX, AX
; NOW WE ASSUME THAT TIMER 2 HAS BEEN PROPERLY PROGRAMMED FOR A 22uS DELAY.
; WHEN THE TIMER IS STARTED, A DMA TRANSFER WILL OCCUR EVERY 22uS.
CODE_SEG ENDS
DATA_SEG SEGMENT
WAVEFORM_DATADB 0,1,2,3,4,5,6,7,8,9,10,11,12,13
DB
14,15,16,17,18,19,20,21,22,23,24
; ETC., UP TO 255
DATA_SEG ENDS
END START
Example 10-2. Timed DMA Transfers (Continued)
10-27
11
Math Coprocessing
CHAPTER 11
MATH COPROCESSING
The 80C186 Modular Core Family meets the need for a general-purpose embedded microproces-
sor. In most data control applications, efficient data movement and control instructions are fore-
most and arithmetic performed on the data is simple. However, some applications do require
more powerful arithmetic instructions and more complex data types than those provided by the
80C186 Modular Core.
11.1 OVERVIEW OF MATH COPROCESSING
Applications needing advanced mathematics capabilities have the following characteristics.
• Numeric data values are non-integral or vary over a wide range
• Algorithms produce very large or very small intermediate results
• Computations must be precise (i.e., calculations must retain several significant digits)
• Computations must be reliable without dependence on programmed algorithms
• Overall math performance exceeds that afforded by a general-purpose processor and
software alone
For the 80C186 Modular Core family, the 80C187 math coprocessor satisfies the need for pow-
erful mathematics. The 80C187 can increase the math performance of the microprocessor system
by 50 to 100 times.
11.2 AVAILABILITY OF MATH COPROCESSING
The 80C186 Modular Core supports the 80C187 with a hardware interface under microcode con-
trol. However, not all proliferations support the 80C187. Some package types have insufficient
leads to support the required external handshaking requirements. The 3-volt versions of the pro-
cessor do not specify math coprocessing because the 80C187 has only a 5-volt rating. Please refer
to the current data sheets for details.
To execute numerics instructions, the 80C186XL must exit reset in Enhanced Mode. The pro-
cessor checks its TEST pin at reset and automatically enters Enhanced Mode if the math copro-
cessor is present.
11-1
MATH COPROCESSING
The core has an Escape Trap (ET) bit in the PCB Relocation Register (Figure 4-1 on page 4-2) to
control the availability of math coprocessing. If the ET bit is set, an attempted numerics execution
results in a Type 7 interrupt. The 80C187 will not work with the 8-bit bus version of the processor
because all 80C187 accesses must be 16-bit. The 80C188 Modular Core automatically traps ESC
(numerics) opcodes to the Type 7 interrupt, regardless of Relocation Register programming.
11.3 THE 80C187 MATH COPROCESSOR
The 80C187’s high performance is due to its 80-bit internal architecture. It contains three units:
a Floating Point Unit, a Data Interface and Control Unit and a Bus Control Logic Unit. The foun-
dation of the Floating Point Unit is an 8-element register file, which can be used either as indi-
vidually addressable registers or as a register stack. The register file allows storage of
intermediate results in the 80-bit format. The Floating Point Unit operates under supervision of
the Data Interface and Control Unit. The Bus Control Logic Unit maintains handshaking and
communications with the host microprocessor. The 80C187 has built-in exception handling.
The 80C187 executes code written for the Intel387™ DX and Intel387 SX math coprocessors.
The 80C187 conforms to ANSI/IEEE Standard 754-1985.
11.3.1 80C187 Instruction Set
80C187 instructions fall into six functional groups: data transfer, arithmetic, comparison, tran-
scendental, constant and processor control. Typical 80C187 instructions accept one or two oper-
ands and produce a single result. Operands are usually located in memory or the 80C187 stack.
Some operands are predefined; for example, FSQRT always takes the square root of the number
in the top stack element. Other instructions allow or require the programmer to specify the oper-
and(s) explicitly along with the instruction mnemonic. Still other instructions accept one explicit
operand and one implicit operand (usually the top stack element).
As with the basic (non-numerics) instruction set, there are two types of operands for coprocessor
instructions, source and destination. Instruction execution does not alter a source operand. Even
when an instruction converts the source operand from one format to another (for example, real to
integer), the coprocessor performs the conversion in a work area to preserve the source operand.
A destination operand differs from a source operand because the 80C187 can alter the register
when it receives the result of the operation. For most destination operands, the coprocessor usu-
ally replaces the destinations with results.
11-2
MATH COPROCESSING
11.3.1.1
Data Transfer Instructions
Data transfer instructions move operands between elements of the 80C187 register stack or be-
tween stack top and memory. Instructions can convert any data type to temporary real and load it
onto the stack in a single operation. Conversely, instructions can convert a temporary real oper-
and on the stack to any data type and store it to memory in a single operation. Table 11-1 sum-
marizes the data transfer instructions.
Table 11-1. 80C187 Data Transfer Instructions
Real Transfers
FLD
Load real
FST
Store real
FSTP
FXCH
Store real and pop
Exchange registers
Integer Transfers
FILD
Integer load
FIST
Integer store
FISTP
Integer store and pop
Packed Decimal Transfers
FBLD
Packed decimal (BCD) load
FBSTP
Packed decimal (BCD) store and pop
11.3.1.2
Arithmetic Instructions
divide operations and several other useful functions. Examples include a simple absolute value
and a square root instruction that executes faster than ordinary division. Other arithmetic instruc-
tions perform exact modulo division, round real numbers to integers and scale values by powers
of two.
Table 11-2 summarizes the available operation and operand forms for basic arithmetic. In addi-
tion to the four normal operations, “reversed” instructions make subtraction and division “sym-
metrical” like addition and multiplication. In summary, the arithmetic instructions are highly
flexible for these reasons:
• the 80C187 uses register or memory operands
• the 80C187 can save results in a choice of registers
11-3
MATH COPROCESSING
Available data types include temporary real, long real, short real, short integer and word integer.
The 80C187 performs automatic type conversion to temporary real.
Table 11-2. 80C187 Arithmetic Instructions
Addition
Add real
Division
Divide real
FADD
FDIV
FADDP
FIADD
Add real and pop
Integer add
FDIVP
FIDIV
FDIVR
Divide real and pop
Integer divide
Divide real reversed
Subtraction
FSUB
Subtract real
FDIVRP
FIDIVR
Divide real reversed and pop
Integer divide reversed
FSUBP
FISUB
Subtract real and pop
Integer subtract
Other Operations
FSUBR
FSUBRP
FISUBR
Subtract real reversed
FSQRT
Square root
Scale
Subtract real reversed and pop
Integer subtract reversed
FSCALE
FPREM
FRNDINT
Partial remainder
Round to integer
Multiplication
FMUL
Multiply real
FXTRACT
FABS
Extract exponent and significand
Absolute value
FMULP
FIMUL
Multiply real and pop
Integer multiply
FCHS
Change sign
FPREMI
Partial remainder (IEEE)
11-4
MATH COPROCESSING
11.3.1.3
Comparison Instructions
Each comparison instruction (see Table 11-3) analyzes the stack top element, often in relationship
to another operand. Then it reports the result in the Status Word condition code. The basic oper-
ations are compare, test (compare with zero) and examine (report tag, sign and normalization).
Table 11-3. 80C187 Comparison Instructions
FCOM
Compare real
FCOMP
FCOMPP
FICOM
Compare real and pop
Compare real and pop twice
Integer compare
FICOMP
FTST
Integer compare and pop
Test
FXAM
Examine
FUCOM
FUCOMP
FUCOMPP
Unordered compare
Unordered compare and pop
Unordered compare and pop twice
11.3.1.4
Transcendental Instructions
Transcendental instructions (see Table 11-4) perform the core calculations for common trigono-
metric, hyperbolic, inverse hyperbolic, logarithmic and exponential functions. Use prologue code
to reduce arguments to a range accepted by the instruction. Use epilogue code to adjust the result
to the range of the original arguments. The transcendentals operate on the top one or two stack
elements and return their results to the stack.
Table 11-4. 80C187 Transcendental Instructions
FPTAN
FPATAN
F2XM1
FYL2X
Partial tangent
Partial arctangent
X
2 – 1
Y log2 X
FYL2XP1
FCOS
Y log2 (X+1)
Cosine
FSIN
Sine
FSINCOS
Sine and Cosine
11-5
MATH COPROCESSING
11.3.1.5
Constant Instructions
Each constant instruction (see Table 11-5) loads a commonly used constant onto the stack. The
values have full 80-bit precision and are accurate to about 19 decimal digits. Since a temporary
real constant occupies 10 memory bytes, the constant instructions, only 2 bytes long, save mem-
ory space.
Table 11-5. 80C187 Constant Instructions
FLDZ
Load + 0.1
Load +1.0
Load
FLD1
FLDPI
FLDL2T
FLDL2E
FLDLG2
FLDLN2
Load log2 10
Load log2 e
Load log10
2
Load loge 2
11.3.1.6
Processor Control Instructions
Computations do not use the processor control instructions; these instructions are available for
activities at the operating system level. This group (see Table 11-6) includes initialization, excep-
tion handling and task switching instructions.
Table 11-6. 80C187 Processor Control Instructions
FINIT/FNINIT
Initialize processor
Disable interrupts
Enable interrupts
Load control word
Store control word
Store status word
Clear exceptions
Store environment
FLDENV
Load environment
FDISI/FNDISI
FENI/FNENI
FSAVE/FNSAVE Save state
FRSTOR
FINCSTP
FDECSTP
FFREE
Restore state
FLDCW
Increment stack pointer
Decrement stack pointer
Free register
FSTCW/FNSTCW
FSTSW/FNSTSW
FCLEX/FNCLEX
FSTENV/FNSTENV
FNOP
No operation
FWAIT
CPU wait
11-6
MATH COPROCESSING
11.3.2 80C187 Data Types
The microprocessor/math coprocessor combination supports seven data types:
• Word Integer — A signed 16-bit numeric value. All operations assume a 2’s complement
representation.
• Short Integer — A signed 32-bit numeric value (double word). All operations assume a 2’s
complement representation.
• Long Integer — A signed 64-bit numeric value (quad word). All operations assume a 2’s
complement representation.
• Packed Decimal — A signed numeric value contained in an 80-bit BCD format.
• Long Real — A signed 64-bit floating point numeric value.
• Temporary Real — A signed 80-bit floating point numeric value. Temporary real is the
native 80C187 format.
Figure 11-1 graphically represents these data types.
11.4 MICROPROCESSOR AND COPROCESSOR OPERATION
The 80C187 interfaces directly to the microprocessor (as shown in Figure 11-2) and operates as
an I/O-mapped slave peripheral device. Hardware handshaking requires connections between the
80C187 and four special pins on the processor: NCS, BUSY, PEREQ and ERROR. These pins
are multiplexed with MCS3, TEST, MCS0, and MCS1, respectively. When the processor leaves
reset, the presence of the 80C187 automatically places the processor in Enhanced Mode and con-
figures the pins correctly. MCS2 retains its function as a chip-select and the processor retains the
wait state and ready programming for the entire mid-range memory block, even though MCS0,
MCS1 and MCS3 are no longer available.
11-7
MATH COPROCESSING
Increasing Significance
Word
Integer
(Two's Complement)
S Magnitude
15
0
Short
(Two's Complement)
Magnitude
S
Integer
31
0
(Two's
Complement)
Long
Integer
Magnitude
S
63
0
Magnitude
Packed
Decimal
S
X
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
15
10
3
17 16
14 13
11
7
6
5
4
2
1
0
12
9
8
79
72
Short
Real
Biased
S
Significand
I
Exponent
31
0
23
Long
Real
Biased
Exponent
Significand
S
63
52
0
I
Temporary
Real
Biased
Exponent
Significand
S
I
64 63
79
0
NOTES:
S
d
= Sign bit (0 = positive, 1 = negative)
= Decimal digit (two per byte)
n
X
= Bits have no significance; 80C187 ignores when loading, zeros when storing.
= Position of implicit binary point
I
= Integer bit of significand; stored in temporary real, implicit in short and long real
Exponent Bias (normalized values):
Short Real: 127 (7FH)
Long Real: 1023 (3FFH)
Temporary Real: 16383 (FFFH)
A1257-0A
Figure 11-1. 80C187-Supported Data Types
11-8
MATH COPROCESSING
11.4.1 Clocking the 80C187
The microprocessor and math coprocessor operate asynchronously, and their clock rates may dif-
fer. The 80C187 has a CKM pin that determines whether it uses the input clock directly or divided
by two. Direct clocking works up to 12.5 MHz, which makes it convenient to feed the clock input
from the microprocessor’s CLKOUT pin. Beyond 12.5 MHz, the 80C187 must use a multiply-
have correct timing relationships, even with operation at different frequencies.
11.4.2 Processor Bus Cycles Accessing the 80C187
Data transfers between the microprocessor and the 80C187 occur through the dedicated, 16-bit
I/O ports shown in Table 11-7. When the processor encounters a numerics opcode, it first writes
the opcode to the 80C187. The 80C187 decodes the instruction and passes elementary instruction
information (Opcode Status Word) back to the processor. Since the 80C187 is a slave processor,
the Modular Core processor performs all loads and stores to memory. Including the overhead in
the microprocessor’s microcode, each data transfer between memory and the 80C187 (via the mi-
croprocessor) takes at least 17 processor clocks.
Table 11-7. 80C187 I/O Port Assignments
I/O Address
Read Definition
Write Definition
00F8H
00FAH
00FCH
00FEH
Status/Control
Data
Opcode
Data
Reserved
CS:IP, DS:EA
Reserved
Opcode Status
The microprocessor cannot process any numerics (ESC) opcodes alone. If the CPU encounters a
numerics opcode when the Escape Trap (ET) bit in the Relocation Register is a zero and the
80C187 is not present, its operation is indeterminate. Even the FINIT/FNINIT initialization in-
struction (used in the past to test the presence of a coprocessor) fails without the 80C187. If an
application offers the 80C187 as an option, problems can be prevented in one of three ways:
• Remove all numerics (ESC) instructions, including code that checks for the presence of the
80C187.
• Use a jumper or switch setting to indicate the presence of the 80C187. The program can
interrogate the jumper or switch setting and branch away from numerics instructions when
the 80C187 socket is empty.
• Trick the microprocessor into predictable operation when the 80C187 socket is empty. The
fix is placing pull-up or pull-down resistors on certain data and handshaking lines so the
CPU reads a recognizable Opcode Status Word. This solution requires a detailed knowledge
of the interface.
11-10
MATH COPROCESSING
Bus cycles involving the 80C187 Math Coprocessor behave exactly like other I/O bus cycles with
respect to the processor’s control pins. See “System Design Tips” for information on integrating
the 80C187 into the overall system.
11.4.3 System Design Tips
All 80C187 operations require that bus ready be asserted. The simplest way to return the ready
indication is through hardware connected to the processor’s external ready pin. If you program a
chip-select to cover the math coprocessor port addresses, its ready programming is in force and
can provide bus ready for coprocessor accesses. The user must verify that there are no conflicts
from other hardware connected to that chip-select pin.
A chip-select pin goes active on 80C187 accesses if you program it for a range including the math
coprocessor I/O ports. The converse is not true — a non-80C187 access cannot activate NCS (nu-
merics coprocessor select), regardless of programming.
In a buffered system, it is customary to place the 80C187 on the local bus. Since DTR and DEN
function normally during 80C187 transfers, you must qualify DEN with NCS (see Figure 11-3).
Otherwise, contention between the 80C187 and the transceivers occurs on read cycles to the
80C187.
The microprocessor’s local bus is available to the integrated peripherals during numerics execu-
tion whenever the CPU is not communicating with the 80C187. The idle bus allows the processor
to intersperse DRAM refresh cycles and DMA cycles with accesses to the 80C187.
The microprocessor’s local bus is available to alternate bus masters during execution of numerics
instructions when the CPU does not need it. Bus cycles driven by alternate masters (via the
HOLD/HLDA protocol) can suspend coprocessor bus cycles for an indefinite period.
The programmer can lock 80C187 instructions. The CPU asserts the LOCK pin for the entire du-
ration of a numerics instruction, monopolizing the bus for a very long time.
11-11
MATH COPROCESSING
Latch
External
A15:0
A2
Oscillator
Buffer
T OE
A1
D15:8
1
2
AD15:0
ALE
EN
CKM
CLK
CLKOUT
80C187
80C186
Modular
Core
RESET
NPWR
Buffer
T OE
RESET
D7:0
WR
RD
NPRD
BUSY
BUSY
ERROR
ERROR
PEREQ
NPS1
PEREQ
NCS
CS
DEN
NPS2
DT/R
D15:0
A1530-0A
Figure 11-3. 80C187 Configuration with a Partially Buffered Bus
11-12
MATH COPROCESSING
11.4.4 Exception Trapping
The 80C187 detects six error conditions that can occur during instruction execution. The 80C187
can apply default fix-ups or signal exceptions to the microprocessor’s ERROR pin. The processor
tests ERROR at the beginning of numerics instructions, so it traps an exception on the next at-
tempted numerics instruction after it occurs. When ERROR tests active, the processor executes a
Type 16 interrupt.
There is no automatic exception-trapping on the last numerics instruction of a series. If the last
numerics instruction writes an invalid result to memory, subsequent non-numerics instructions
can use that result as if it is valid, further compounding the original error. Insert the FNOP in-
struction at the end of the 80C187 routine to force an ERROR check. If the program is written in
a high-level language, it is impossible to insert FNOP. In this case, route the error signal through
an inverter to an interrupt pin on the microprocessor (see Figure 11-4). With this arrangement,
tion-handler routine. Use an additional flip-flop to latch PEREQ to maintain the correct hand-
shaking sequence with the microprocessor.
11.5 EXAMPLE MATH COPROCESSOR ROUTINES
Example 11-1 shows the initialization sequence for the 80C187. Example 11-2 is an example of
a floating point routine using the 80C187. The FSINCOS instruction yields both sine and cosine
in one operation.
11-13
MATH COPROCESSING
80C186
Modular Core
ERROR
RESET
CSx
INTx
Latch
BUSY
PEREQ
NCS
ALE
EN
A19:A16
AD15:0
RD
WR
C
CLKOUT
A
d
d
r
e
s
s
D
Q
Q
'74
D15:0
D15:0
S
CLK
A2
A1
NPWR
CMD1
CMD0
NPRD
80C187
A19:0
C
NPS1
D
Q
'74
PEREQ
CKM
S
BUSY
ERROR
RESET
NPS2
A1531-0A
Figure 11-4. 80C187 Exception Trapping via Processor Interrupt Pin
11-14
MATH COPROCESSING
$mod186
name
;
example_80C187_init
;FUNCTION:
This function initializes the 80C187 numerics coprocessor.
;
;SYNTAX:
extern unsigned char far 187_init(void);
None
;
;INPUTS:
;
;OUTPUTS:
unsigned char - 0000h -> False -> coprocessor not initialized
ffffh -> True -> coprocessor initialized
;
;
;NOTE:
Parameters are passed on the stack as required by
high-level languages.
;
;
lib_80186
segment public ’code’
assume cs:lib_80186
public _187_init
_187_initproc far
push
mov
bp
bp, sp
;save caller’s bp
;get current top of stack
cli
;disable maskable interrupts
fninit
fnstcw [bp-2]
;init 80C187 processor
;get current control word
sti
;enable interrupts
mov
and
cmp
je
xor
pop
ret
ax, [bp-2]
ax, 0300h
ax, 0300h
Ok
;mask off unwanted control bits
;PC bits = 11
;yes: processor ok
ax, ax
bp
;return false (80C187 not ok)
;restore caller’s bp
Ok: and
[bp-2], 0fffeh
;unmask possible exceptions
fldcw [bp-2]
mov
pop
ret
ax,0ffffh
bp
;return true (80C187 ok)
;restore caller’s bp
_187_initendp
lib_80186ends
end
Example 11-1. Initialization Sequence for 80C187 Math Coprocessor
11-15
MATH COPROCESSING
$mod186
$modc187
name
example_80C187_proc
;DESCRIPTION: This code section uses the 80C187 FSINCOS transcendental
;
instruction to convert the locus of a point from polar
to Cartesian coordinates.
;
;
;VARIABLES:
The variables consist of the radius, r, and the angle, theta.
Both are expressed as 32-bit reals and 0 <= theta <= pi/4.
;
;
;RESULTS:
The results of the computation are the coordinates x and y
expressed as 32-bit reals.
;
;
;NOTES:
This routine is coded for Intel ASM86. It is not set up as an
HLL-callable routine.
;
;
;
;
This code assumes that the 80C187 has already been initialized.
assume cs:code, ds:data
data
segment at 0100h
r
dd x.xxxx
;substitute real operand
;substitute real operand
theta dd x.xxxx
x
dd ?
dd ?
y
data
code
ends
segment at 0080h
convert
proc far
ax, data
ds, ax
mov
mov
fld
fld
r
;load radius
;load angle
theta
fsincos
fmul
fstp
fmul
fstp
;st=cos, st(1)=sin
;compute x
st, st(2)
x
;store to memory and pop
;compute y
y
;store to memory and pop
convertendp
code
ends
end
Example 11-2. Floating Point Math Routine Using FSINCOS
11-16
12
ONCE Mode
CHAPTER 12
ONCE MODE
ONCE (pronounced “ahnce”) Mode provides the ability to three-state all output, bidirectional, or
weakly held high/low pins except OSCOUT. To allow device operation with a crystal network,
OSCOUT does not three-state.
ONCE Mode electrically isolates the device from the rest of the board logic. This isolation allows
a bed-of-nails tester to drive the device pins directly for more accurate and thorough testing. An
in-circuit emulation probe uses ONCE Mode to isolate a surface-mounted device from board log-
ic and essentially “take over” operation of the board (without removing the soldered device from
the board).
12.1 ENTERING/LEAVING ONCE MODE
Forcing UCS and LCS low while RES is asserted (low) enables ONCE Mode (see Figure 12-1).
Maintaining UCS and LCS and RES low continues to keep ONCE Mode active. Returning UCS
and LCS high exits ONCE Mode.
However, it is possible to keep ONCE Mode always active by deasserting RES while keeping
UCS and LCS low. Removing RES “latches” ONCE Mode and allows UCS and LCS to be driv-
en to any level. UCS and LCS must remain low for at least one clock beyond the time RES is
driven high. Asserting RES exits ONCE Mode, assuming UCS and LCS do not also remain low
(see Figure 12-1).
12-1
A
80C186 Instruction
Set Additions and
Extensions
APPENDIX A
80C186 INSTRUCTION SET
ADDITIONS AND EXTENSIONS
The 80C186 Modular Core family instruction set differs from the original 8086/8088 instruction
set in two ways. First, several instructions that were not available in the 8086/8088 instruction set
have been added. Second, several 8086/8088 instructions have been enhanced for the 80C186
Modular Core family instruction set.
A.1 80C186 INSTRUCTION SET ADDITIONS
This section describes the seven instructions that were added to the base 8086/8088 instruction
set to make the instruction set for the 80C186 Modular Core family. These instructions did not
exist in the 8086/8088 instruction set.
• Data transfer instructions
— PUSHA
— POPA
• String instructions
— INS
— OUTS
• High-level instructions
— ENTER
— LEAVE
— BOUND
A.1.1 Data Transfer Instructions
PUSHA/POPA
PUSHA (push all) and POPA (pop all) allow all general-purpose registers to be stacked and un-
stacked. The PUSHA instruction pushes all CPU registers (except as noted below) onto the stack.
The POPA instruction pops all registers pushed by PUSHA off of the stack. The registers are
pushed onto the stack in the following order: AX, CX, DX, BX, SP, BP, SI, DI. The Stack Pointer
(SP) value pushed is the Stack Pointer value before the AX register was pushed. When POPA is
executed, the Stack Pointer value is popped, but ignored. Note that this instruction does not save
segment registers (CS, DS, SS, ES), the Instruction Pointer (IP), the Processor Status Word or
any integrated peripheral registers.
A-1
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
A.1.2 String Instructions
INS source_string, port
INS (in string) performs block input from an I/O port to memory. The port address is placed in
the DX register. The memory address is placed in the DI register. This instruction uses the ES
segment register (which cannot be overridden). After the data transfer takes place, the pointer reg-
ister (DI) increments or decrements, depending on the value of the Direction Flag (DF). The
pointer register changes by one for byte transfers or by two for word transfers.
OUTS port, destination_string
OUTS (out string) performs block output from memory to an I/O port. The port address is placed
in the DX register. The memory address is placed in the SI register. This instruction uses the DS
segment register, but this may be changed with a segment override instruction. After the data
transfer takes place, the pointer register (SI) increments or decrements, depending on the value
of the Direction Flag (DF). The pointer register changes by one for byte transfers or by two for
word transfers.
A.1.3 High-Level Instructions
ENTER size, level
ENTER creates the stack frame required by most block-structured high-level languages. The first
parameter, size, specifies the number of bytes of dynamic storage to be allocated for the procedure
being entered (16-bit value). The second parameter, level, is the lexical nesting level of the pro-
cedure (8-bit value). Note that the higher the lexical nesting level, the lower the procedure is in
the nesting hierarchy.
The lexical nesting level determines the number of pointers to higher level stack frames copied
into the current stack frame. This list of pointers is called the display. The first word of the display
points to the previous stack frame. The display allows access to variables of higher level (lower
lexical nesting level) procedures.
After ENTER creates a display for the current procedure, it allocates dynamic storage space. The
in the procedure use this value of the Stack Pointer as a base.
Two forms of ENTER exist: non-nested and nested. A lexical nesting level of 0 specifies the non-
nested form. In this situation, BP is pushed, then the Stack Pointer is copied to BP and decrement-
ed by the size of the frame. If the lexical nesting level is greater than 0, the nested form is used.
Figure A-1 gives the formal definition of ENTER.
A-2
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
The following listing gives the formal definition of the
ENTER instruction for all cases.
LEVEL denotes the value of the second operand.
Push BP
Set a temporary value FRAME_PTR: = SP
If LEVEL > 0 then
Repeat (LEVEL - 1) times:
BP:=BP - 2
Push the word pointed to by BP
End Repeat
Push FRAME_PTR
End if
BP:=FRAME_PTR
SP:=SP - first operand
Figure A-1. Formal Definition of ENTER
level. A reentrant procedure can address only its own variables and variables of higher-level call-
ing procedures. ENTER ensures this by copying only stack frame pointers from higher-level pro-
cedures.
previously nested procedures. For example, assume for Figure A-2 that Procedure A calls Proce-
dure B, which calls Procedure C, which calls Procedure D. Procedure C will have access to the
variables of Main and Procedure A, but not to those of Procedure B because Procedures C and B
operate at the same lexical nesting level.
The following is a summary of the variable access for Figure A-2.
1. Main has variables at fixed locations.
2. Procedure A can access only the fixed variables of Main.
3. Procedure B can access only the variables of Procedure A and Main.
Procedure B cannot access the variables of Procedure C or Procedure D.
4. Procedure C can access only the variables of Procedure A and Main.
Procedure C cannot access the variables of Procedure B or Procedure D.
5. Procedure D can access the variables of Procedure C, Procedure A and Main.
Procedure D cannot access the variables of Procedure B.
A-3
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
Main Program (Lexical Level 1)
Procedure A (Lexical Level 2)
Procedure B (Lexical Level 3)
Procedure C (Lexical Level 3)
Procedure D (Lexical Level 4)
A1001-0A
Figure A-2. Variable Access in Nested Procedures
The first ENTER, executed in the Main Program, allocates dynamic storage space for Main, but
no pointers are copied. The only word in the display points to itself because no previous value
exists to return to after LEAVE is executed (see Figure A-3).
0
15
Old BP
BPM
BP
SP
Display Main
Dynamic
Storage
Main
*BPM = BP Value for MAIN
A1002-0A
Figure A-3. Stack Frame for Main at Level 1
After Main calls Procedure A, ENTER creates a new display for Procedure A. The first word
points to the previous value of BP (BPM). The second word points to the current value of BP
(BPA). BPM contains the base for dynamic storage in Main. All dynamic variables for Main will
be at a fixed offset from this value (see Figure A-4).
A-4
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
15
0
Old BP
BPM
BPM
BP
SP
Display A
BPM
BPA*
Dynamic
Storage A
*BPA = BP Value for Procedure A
A1003-0A
After Procedure A calls Procedure B, ENTER creates the display for Procedure B. The first word
of the display points to the previous value of BP (BPA). The second word points to the value of
BP for MAIN (BPM). The third word points to the BP for Procedure A (BPA). The last word
points to the current BP (BPB). Procedure B can access variables in Procedure A or Main via the
appropriate BP in the display (see Figure A-5).
of the display points to the previous value of BP (BPB). The second word points to the value of
BP for MAIN (BPM). The third word points to the value of BP for Procedure A (BPA). The
fourth word points to the current BP (BPC). Because Procedure B and Procedure C have the same
lexical nesting level, Procedure C cannot access variables in Procedure B. The only pointer to
Procedure B in the display of Procedure C exists to allow the LEAVE instruction to collapse the
Procedure C stack frame (see Figure A-6).
A-5
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
15
0
Old BP
BPM
BPM
BPM
BPA
BPA
BPM
BPA
BPB
BPB
BPM
BP
Display C
BPA
BPC
Dynamic
Storage C
SP
A1005-0A
Figure A-6. Stack Frame for Procedure C at Level 3 Called from B
LEAVE
LEAVE reverses the action of the most recent ENTER instruction. It collapses the last stack
frame created. First, LEAVE copies the current BP to the Stack Pointer, releasing the stack space
allocated to the current procedure. Second, LEAVE pops the old value of BP from the stack, to
return to the calling procedure's stack frame. A RET instruction will remove arguments stacked
by the calling procedure for use by the called procedure.
A-7
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
BOUND register, address
BOUND verifies that the signed value in the specified register lies within specified limits. If the
value does not lie within the bounds, an array bounds exception (type 5) occurs. BOUND is useful
for checking array bounds before attempting to access an array element. This prevents the pro-
gram from overwriting information outside the limits of the array.
BOUND has two operands. The first, register, specifies the register being tested. The second, ad-
dress, contains the effective relative address of the two signed boundary values. The lower limit
word is at this address and the upper limit word immediately follows. The limit values cannot be
register operands (if they are, an invalid opcode exception occurs).
A.2 80C186 INSTRUCTION SET ENHANCEMENTS
This section describes ten instructions that were available with the 8086/8088 but have been en-
hanced for the 80C186 Modular Core family.
• Data transfer instructions
— PUSH
• Arithmetic instructions
— IMUL
• Bit manipulation instructions (shifts and rotates)
— SAL
— SHL
— SAR
— SHR
— ROL
— ROR
— RCL
— RCR
A.2.1 Data Transfer Instructions
PUSH data
PUSH (push immediate) allows an immediate argument, data, to be pushed onto the stack. The
value can be either a byte or a word. Byte values are sign extended to word size before being
pushed.
A-8
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
A.2.2 Arithmetic Instructions
IMUL destination, source, data
IMUL (integer immediate multiply, signed) allows a value to be multiplied by an immediate op-
erand. IMUL requires three operands. The first, destination, is the register where the result will
be placed. The second, source, is the effective address of the multiplier. The source may be the
same register as the destination, another register or a memory location. The third, data, is an im-
mediate value used as the multiplicand. The data operand may be a byte or word. If data is a byte,
it is sign extended to 16 bits. Only the lower 16 bits of the result are saved. The result must be
placed in a general-purpose register.
A.2.3 Bit Manipulation Instructions
This section describes the eight enhanced bit-manipulation instructions.
A.2.3.1
Shift Instructions
SAL destination, count
SAL (immediate shift arithmetic left) shifts the destination operand left by an immediate value.
SAL has two operands. The first, destination, is the effective address to be shifted. The second,
count, is an immediate byte value representing the number of shifts to be made. The CPU will
AND count with 1FH before shifting, to allow no more than 32 shifts. Zeros shift in on the right.
SHL destination, count
SHL (immediate shift logical left) is physically the same instruction as SAL (immediate shift
arithmetic left).
SAR destination, count
SAR (immediate shift arithmetic right) shifts the destination operand right by an immediate val-
ue. SAL has two operands. The first, destination, is the effective address to be shifted. The sec-
ond, count, is an immediate byte value representing the number of shifts to be made. The CPU
will AND count with 1FH before shifting, to allow no more than 32 shifts. The value of the orig-
inal sign bit shifts into the most-significant bit to preserve the initial sign.
SHR destination, count
SHR (immediate shift logical right) is physically the same instruction as SAR (immediate shift
arithmetic right).
A-9
80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS
A.2.3.2
Rotate Instructions
ROL destination, count
ROL (immediate rotate left) rotates the destination byte or word left by an immediate value. ROL
has two operands. The first, destination, is the effective address to be rotated. The second, count,
is an immediate byte value representing the number of rotations to be made. The most-significant
bit of destination rotates into the least-significant bit.
ROR destination, count
ROR (immediate rotate right) rotates the destination byte or word right by an immediate value.
ROR has two operands. The first, destination, is the effective address to be rotated. The second,
count, is an immediate byte value representing the number of rotations to be made. The least-sig-
nificant bit of destination rotates into the most-significant bit.
RCL destination, count
RCL (immediate rotate through carry left) rotates the destination byte or word left by an imme-
diate value. RCL has two operands. The first, destination, is the effective address to be rotated.
The second, count, is an immediate byte value representing the number of rotations to be made.
The Carry Flag (CF) rotates into the least-significant bit of destination. The most-significant bit
of destination rotates into the Carry Flag.
RCR destination, count
RCR (immediate rotate through carry right) rotates the destination byte or word right by an im-
mediate value. RCR has two operands. The first, destination, is the effective address to be rotated.
The second, count, is an immediate byte value representing the number of rotations to be made.
The Carry Flag (CF) rotates into the most-significant bit of destination. The least-significant bit
of destination rotates into the Carry Flag.
A-10
B
Input
Synchronization
APPENDIX B
Many input signals to an embedded processor are asynchronous. Asynchronous signals do not re-
quire a specified setup or hold time to ensure the device does not incur a failure. However, asyn-
chronous setup and hold times are specified in the data sheet to ensure recognition. Associated
with each of these inputs is a synchronizing circuit (see Figure B-1) that samples the asynchro-
nous signal and synchronizes it to the internal operating clock. The output of the synchronizing
circuit is then safely routed to the logic units.
Synchronized
Output
Asynchronous
Input
D
Q
D
Q
First
Latch
Second
Latch
2
1
NOTES:
1. First latch sample clock, can be phase 1 or phase 2 depending on pin function.
2. Second latch sample clock, opposite phase of first latch sample clock
(e.g., if first latch is sampled with phase 1, the second latch is sampled with phase 2).
A1007-0A
Figure B-1. Input Synchronization Circuit
B.1 WHY SYNCHRONIZERS ARE REQUIRED
Every data latch requires a specific setup and hold time to operate properly. The duration of the
setup and hold time defines a window during which the device attempts to latch the data. If the
input makes a transition within this window, the output may not attain a stable state. The data
sheet specifies a setup and hold window larger than is actually required. However, variations in
all conditions.
Should the input to the data latch make a transition during the sample and hold window, the out-
put of the latch eventually attains a stable state. This stable state must be attained before the sec-
ond stage of synchronization requires a valid input. To synchronize an asynchronous signal, the
circuit in Figure B-1 samples the input into the first latch, allows the output to stabilize, then sam-
ples the stabilized value into a second latch. With the asynchronous signal resolved in this way,
the input signal cannot cause an internal device failure.
B-1
INPUT SYNCHRONIZATION
A synchronization failure can occur when the output of the first latch does not meet the setup and
hold requirements of the input of the second latch. The rate of failure is determined by the actual
size of the sampling window of the data latch and by the amount of time between the strobe sig-
nals of the two latches. As the sampling window gets smaller, the number of times an asynchro-
nous transition occurs during the sampling window drops.
B.2 ASYNCHRONOUS PINS
The 80C186XL/80C188XL inputs that use the two-stage synchronization circuit are TMR IN 0,
TMR IN 1, NMI, TEST/BUSY, INT3:0, HOLD, DRQ0 and DRQ1.
B-2
C
Instruction Set
Descriptions
APPENDIX C
INSTRUCTION SET DESCRIPTIONS
This appendix provides reference information for the 80C186 Modular Core family instruction
set. Tables C-1 through C-3 define the variables used in Table C-4, which lists the instructions
with their descriptions and operations.
Table C-1. Instruction Format Variables
Variable
Description
dest
src
A register or memory location that may contain data operated on by the instruction,
and which receives (is replaced by) the result of the operation.
A register, memory location or immediate value that is used in the operation, but is not
altered by the instruction
target
disp8
A label to which control is to be transferred directly, or a register or memory location
whose content is the address of the location to which control is to be transferred
indirectly.
A label to which control is to be conditionally transferred; must lie within –128 to +127
bytes of the first byte of the next instruction.
accum
port
Register AX for word transfers, AL for bytes.
An I/O port number; specified as an immediate value of 0–255, or register DX (which
contains port number in range 0–64K).
src-string
Name of a string in memory that is addressed by register SI; used only to identify
string as byte or word and specify segment override, if any. This string is used in the
operation, but is not altered.
dest-string
count
Name of string in memory that is addressed by register DI; used only to identify string
as byte or word. This string receives (is replaced by) the result of the operation.
Specifies number of bits to shift or rotate; written as immediate value 1 or register CL
(which contains the count in the range 0–255).
interrupt-type
Immediate value of 0–255 identifying interrupt pointer number.
optional-pop-value Number of bytes (0–64K, ordinarily an even number) to discard from the stack.
external-opcode
Immediate value (0–63) that is encoded in the instruction for use by an external
processor.
C-1
INSTRUCTION SET DESCRIPTIONS
Table C-2. Instruction Operands
Operand
Description
reg
An 8- or 16-bit general register.
An 16-bit general register.
A segment register.
reg16
seg-reg
accum
Register AX or AL
immed
A constant in the range 0–FFFFH.
A constant in the range 0–FFH.
An 8- or 16-bit memory location.
A 16-bit memory location.
A 32-bit memory location.
Name of 256-byte translate table.
immed8
mem
mem16
mem32
src-table
src-string
dest-string
short-label
near-label
far-label
near-proc
far-proc
memptr16
Name of string addressed by register SI.
Name of string addressed by register DI.
A label within the –128 to +127 bytes of the end of the instruction.
A lavel in current code segment.
A label in another code segment.
A procedure in current code segment.
A procedure in another code segment.
A word containing the offset of the location in the current code segment to which
control is to be transferred.
memptr32
regptr16
repeat
A doubleword containing the offset and the segment base address of the location in
another code segment to which control is to be transferred.
A 16-bit general register containing the offset of the location in the current code
segment to which control is to be transferred.
A string instruction repeat prefix.
C-2
INSTRUCTION SET DESCRIPTIONS
Table C-3. Flag Bit Functions
Name
Function
AF
Auxiliary Flag:
Set on carry from or borrow to the low order four bits of AL; cleared otherwise.
CF
DF
Carry Flag:
Set on high-order bit carry or borrow; cleared otherwise.
Direction Flag:
Causes string instructions to auto decrement the appropriate index register
when set. Clearing DF causes auto increment.
IF
Interrupt-enable Flag:
When set, maskable interrupts will cause the CPU to transfer control to an
interrupt vector specified location.
OF
PF
Overflow Flag:
Set if the signed result cannot be expressed within the number of bits in the
destination operand; cleared otherwise.
Parity Flag:
Set if low-order 8 bits of result contain an even number of 1 bits; cleared
otherwise.
SF
TF
Sign Flag:
Set equal to high-order bit of result (0 if positive, 1 if negative).
Single Step Flag:
Once set, a single step interrupt occurs after the next instruction executes. TF
is cleared by the single step interrupt.
ZF
Zero Flag:
Set if result is zero; cleared otherwise.
C-3
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set
Flags
Affected
Name
AAA
Description
Operation
ASCII Adjust for Addition:
if
AF
((AL) and 0FH) > 9 or (AF) = 1
then
(AL) ← (AL) + 6
CF
DF –
IF
OF ?
PF ?
SF ?
TF –
ZF ?
AAA
Changes the contents of register AL to
a valid unpacked decimal number; the
high-order half-byte is zeroed.
–
(AH) ← (AH) + 1
(AF) ← 1
(CF) ← (AF)
Instruction Operands:
(AL) ← (AL) and 0FH
none
AAD
ASCII Adjust for Division:
(AL) ← (AH) × 0AH + (AL)
(AH) ← 0
AF ?
CF ?
DF –
AAD
Modifies the numerator in AL before
dividing two valid unpacked decimal
operands so that the quotient
produced by the division will be a valid
unpacked decimal number. AH must
be zero for the subsequent DIV to
produce the correct result. The
IF
–
OF ?
PF
SF
TF –
ZF
quotient is returned in AL, and the
remainder is returned in AH; both high-
order half-bytes are zeroed.
Instruction Operands:
none
AAM
ASCII Adjust for Multiply:
(AH) ← (AL) / 0AH
(AL) ← (AL) % 0AH
AF ?
CF ?
DF –
AAM
Corrects the result of a previous multi-
plication of two valid unpacked
IF
–
OF ?
PF
SF
TF –
ZF
decimal operands. A valid 2-digit
unpacked decimal number is derived
from the content of AH and AL and is
returned to AH and AL. The high-order
half-bytes of the multiplied operands
must have been 0H for AAM to
produce a correct result.
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-4
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
AAS
Description
Operation
ASCII Adjust for Subtraction:
if
AF
((AL) and 0FH) > 9 or (AF) = 1
then
(AL) ← (AL) – 6
CF
AAS
DF –
IF –
OF ?
PF ?
SF ?
TF –
ZF ?
Corrects the result of a previous
subtraction of two valid unpacked
decimal operands (the destination
operand must have been specified as
register AL). Changes the content of
AL to a valid unpacked decimal
number; the high-order half-byte is
zeroed.
(AH) ← (AH) – 1
(AF) ← 1
(CF) ← (AF)
(AL) ← (AL) and 0FH
Instruction Operands:
none
ADC
Add with Carry:
ADC dest, src
if
AF
CF
(CF) = 1
then
(dest) ← (dest) + (src) + 1
else
(dest) ← (dest) + (src)
DF –
IF –
OF
PF
SF
Sums the operands, which may be
bytes or words, adds one if CF is set
and replaces the destination operand
with the result. Both operands may be
signed or unsigned binary numbers
(see AAA and DAA). Since ADC incor-
porates a carry from a previous
operation, it can be used to write
routines to add numbers longer than
16 bits.
TF –
ZF
Instruction Operands:
ADC reg, reg
ADC reg, mem
ADC mem, reg
ADC reg, immed
ADC mem, immed
ADC accum, immed
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-5
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
ADD
Description
Operation
Addition:
(dest) ← (dest) + (src)
AF
CF
DF –
ADD dest, src
Sums two operands, which may be
bytes or words, replaces the
destination operand. Both operands
may be signed or unsigned binary
numbers (see AAA and DAA).
IF
–
OF
PF
SF
TF –
ZF
Instruction Operands:
ADD reg, reg
ADD reg, mem
ADD mem, reg
ADD reg, immed
ADD mem, immed
ADD accum, immed
AND
And Logical:
(dest) ← (dest) and (src)
(CF) ← 0
(OF) ← 0
AF ?
CF
DF –
AND dest, src
Performs the logical "and" of the two
operands (byte or word) and returns
the result to the destination operand. A
bit in the result is set if both corre-
sponding bits of the original operands
are set; otherwise the bit is cleared.
IF
–
OF
PF
SF
TF –
ZF
Instruction Operands:
AND reg, reg
AND reg, mem
AND mem, reg
AND reg, immed
AND mem, immed
AND accum, immed
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-6
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
Description
Operation
BOUND
Detect Value Out of Range:
if
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
((dest) < (src) or (dest) > ((src) + 2)
then
BOUND dest, src
Provides array bounds checking in
hardware. The calculated array index
is placed in one of the general purpose
registers, and the upper and lower
bounds of the array are placed in two
consecutive memory locations. The
contents of the register are compared
with the memory location values, and if
the register value is less than the first
location or greater than the second
memory location, a trap type 5 is
generated.
(SP) ← (SP) – 2
((SP) + 1 : (SP)) ← FLAGS
(IF) ← 0
(TF) ← 0
(SP) ← (SP) – 2
((SP) + 1 : (SP)) ← (CS)
(CS) ← (1EH)
(SP) ← (SP) – 2
((SP) + 1 : (SP)) ← (IP)
(IP) ← (1CH)
Instruction Operands:
BOUND reg, mem
CALL
Call Procedure:
if
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Inter-segment
then
CALL procedure-name
Activates an out-of-line procedure,
saving information on the stack to
permit a RET (return) instruction in the
procedure to transfer control back to
the instruction following the CALL. The
assembler generates a different type
of CALL instruction depending on
whether the programmer has defined
the procedure name as NEAR or FAR.
(SP) ← (SP) – 2
((SP) +1:(SP)) ← (CS)
(CS) ← SEG
(SP) ← (SP) – 2
((SP) +1:(SP)) ← (IP)
(IP) ← dest
Instruction Operands:
CALL near-proc
CALL far-proc
CALL memptr16
CALL regptr16
CALL memptr32
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-7
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
CBW
Description
Operation
Convert Byte to Word:
if
AF –
CF –
DF –
(AL) < 80H
then
(AH) ← 0
else
(AH) ← FFH
CBW
Extends the sign of the byte in register
AL throughout register AH. Use to
produce a double-length (word)
dividend from a byte prior to
IF
–
OF –
PF –
SF –
TF –
ZF –
performing byte division.
Instruction Operands:
none
CLC
Clear Carry flag:
CLC
(CF) ← 0
AF –
CF
DF –
Zeroes the carry flag (CF) and affects
no other flags. Useful in conjunction
with the rotate through carry left (RCL)
and the rotate through carry right
(RCR) instructions.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
none
CLD
Clear Direction flag:
(DF) ← 0
AF –
CF –
DF
CLD
Zeroes the direction flag (DF) causing
the string instructions to auto-
increment the source index (SI) and/or
destination index (DI) registers.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-8
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
CLI
Description
Operation
Clear Interrupt-enable Flag:
(IF) ← 0
AF –
CF –
DF –
IF
OF –
PF –
SF –
TF –
ZF –
CLI
Zeroes the interrupt-enable flag (IF).
When the interrupt-enable flag is
cleared, the 8086 and 8088 do not
recognize an external interrupt request
that appears on the INTR line; in other
words maskable interrupts are
disabled. A non-maskable interrupt
appearing on NMI line, however, is
honored, as is a software interrupt.
Instruction Operands:
none
CMC
Complement Carry Flag:
if
AF –
CF
(CF) = 0
then
(CF) ← 1
else
CMC
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Toggles complement carry flag (CF) to
its opposite state and affects no other
flags.
(CF) ← 0
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-9
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
CMP
Description
Operation
Compare:
(dest) – (src)
AF
CF
DF –
CMP dest, src
Subtracts the source from the desti-
nation, which may be bytes or words,
but does not return the result. The
operands are unchanged, but the flags
are updated and can be tested by a
subsequent conditional jump
IF
–
OF
PF
SF
TF –
ZF
instruction. The comparison reflected
in the flags is that of the destination to
the source. If a CMP instruction is
followed by a JG (jump if greater)
instruction, for example, the jump is
taken if the destination operand is
greater than the source operand.
Instruction Operands:
CMP reg, reg
CMP reg, mem
CMP mem, reg
CMP reg, immed
CMP mem, immed
CMP accum, immed
CMPS
Compare String:
(dest-string) – (src-string)
if
(DF) = 0
AF
CF
DF –
IF
OF
PF
SF
TF –
ZF
CMPS dest-string, src-string
Subtracts the destination byte or word
from the source byte or word. The
destination byte or word is addressed
by the destination index (DI) register
and the source byte or word is
addresses by the source index (SI)
register. CMPS updates the flags to
reflect the relationship of the
then
–
(SI) ← (SI) + DELTA
(DI) ← (DI) + DELTA
else
(SI) ← (SI) – DELTA
(DI) ← (DI) – DELTA
destination element to the source
element but does not alter either
operand and updates SI and DI to
point to the next string element.
Instruction Operands:
CMP dest-string, src-string
CMP (repeat) dest-string, src-string
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-10
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
CWD
Description
Operation
Convert Word to Doubleword:
if
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
(AX) < 8000H
then
(DX) ← 0
else
(DX) ← FFFFH
CWD
Extends the sign of the word in register
AX throughout register DX. Use to
produce a double-length (doubleword)
dividend from a word prior to
performing word division.
Instruction Operands:
none
DAA
Decimal Adjust for Addition:
if
AF
((AL) and 0FH) > 9 or (AF) = 1
then
(AL) ← (AL) + 6
(AF) ← 1
CF
DAA
DF –
IF –
OF ?
PF
SF
TF –
ZF
Corrects the result of previously
adding two valid packed decimal
operands (the destination operand
must have been register AL). Changes
the content of AL to a pair of valid
packed decimal digits.
if
(AL) > 9FH or (CF) = 1
then
(AL) ← (AL) + 60H
Instruction Operands:
(CF) ← 1
none
DAS
Decimal Adjust for Subtraction:
if
AF
((AL) and 0FH) > 9 or (AF) = 1
then
(AL) ← (AL) – 6
CF
DAS
DF –
IF –
OF ?
PF
SF
TF –
ZF
Corrects the result of a previous
subtraction of two valid packed
decimal operands (the destination
operand must have been specified as
register AL). Changes the content of
AL to a pair of valid packed decimal
digits.
(AF) ← 1
if
(AL) > 9FH or (CF) = 1
then
(AL) ← (AL) – 60H
(CF) ← 1
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-11
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
DEC
Description
Operation
(dest) ← (dest) – 1
Decrement:
AF
CF –
DF –
DEC dest
Subtracts one from the destination
operand. The operand may be a byte
or a word and is treated as an
unsigned binary number (see AAA and
DAA).
IF
–
OF
PF
SF
TF –
ZF
Instruction Operands:
DEC reg
DEC mem
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-12
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
DIV
Description
Operation
Divide:
When Source Operand is a Byte:
AF ?
CF ?
DF –
IF –
OF ?
PF ?
SF ?
TF –
ZF ?
DIV src
(temp) ← (byte-src)
if
Performs an unsigned division of the
accumulator (and its extension) by the
source operand.
(temp) / (AX) > FFH
then (type 0 interrupt is generated)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← FLAGS
(IF) ← 0
If the source operand is a byte, it is
divided into the two-byte dividend
assumed to be in registers AL and AH.
The byte quotient is returned in AL,
and the byte remainder is returned in
AH.
(TF) ← 0
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (CS)
(CS) ← (2)
If the source operand is a word, it is
divided into the two-word dividend in
registers AX and DX. The word
quotient is returned in AX, and the
word remainder is returned in DX.
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (IP)
(IP) ← (0)
else
(AL) ← (temp) / (AX)
(AH) ← (temp) % (AX)
If the quotient exceeds the capacity of
its destination register (FFH for byte
source, FFFFH for word source), as
when division by zero is attempted, a
type 0 interrupt is generated, and the
quotient and remainder are undefined.
Nonintegral quotients are truncated to
integers.
When Source Operand is a Word:
(temp) ← (word-src)
if
(temp) / (DX:AX) > FFFFH
then (type 0 interrupt is generated)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← FLAGS
(IF) ← 0
Instruction Operands:
(TF) ← 0
DIV reg
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (CS)
(CS) ← (2)
DIV mem
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (IP)
(IP) ← (0)
else
(AX) ← (temp) / (DX:AX)
(DX) ← (temp) % (DX:AX)
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-13
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
Description
Operation
ENTER
Procedure Entry:
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (BP)
(FP) ← (SP)
AF –
CF –
DF –
ENTER locals, levels
Executes the calling sequence for a
high-level language. It saves the
current frame pointer in BP, copies the
frame pointers from procedures below
the current call (to allow access to
local variables in these procedures)
and allocates space on the stack for
the local variables of the current
procedure invocation.
if
IF
–
level > 0
then
OF –
PF –
SF –
TF –
ZF –
repeat (level – 1) times
(BP) ← (BP) – 2
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (BP)
end repeat
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (FP)
end if
Instruction Operands:
ENTER locals, level
(BP) ← (FP)
(SP) ← (SP) – (locals)
ESC
Escape:
if
AF –
CF –
DF –
mod ≠ 11
then
ESC
Provides a mechanism by which other
processors (coprocessors) may
receive their instructions from the 8086
or 8088 instruction stream and make
use of the 8086 or 8088 addressing
modes. The CPU (8086 or 8088) does
a no operation (NOP) for the ESC
instruction other than to access a
memory operand and place it on the
bus.
data bus ← (EA)
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
ESC immed, mem
ESC immed, reg
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-14
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
HLT
Description
Operation
Halt:
None
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
HLT
Causes the CPU to enter the halt
state. The processor leaves the halt
state upon activation of the RESET
line, upon receipt of a non-maskable
interrupt request on NMI, or upon
receipt of a maskable interrupt request
on INTR (if interrupts are enabled).
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-15
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
IDIV
Description
Integer Divide:
Operation
When Source Operand is a Byte:
AF ?
CF ?
DF –
IDIV src
(temp) ← (byte-src)
if
Performs a signed division of the
accumulator (and its extension) by the
source operand. If the source operand
is a byte, it is divided into the double-
length dividend assumed to be in
registers AL and AH; the single-length
quotient is returned in AL, and the
single-length remainder is returned in
AH. For byte integer division, the
maximum positive quotient is +127
(7FH) and the minimum negative
quotient is –127 (81H).
IF
–
(temp) / (AX) > 0 and
(temp) / (AX) > 7FH or
(temp) / (AX) < 0 and
(temp) / (AX) < 0 – 7FH – 1
then (type 0 interrupt is generated)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← FLAGS
(IF) ← 0
OF ?
PF ?
SF ?
TF –
ZF ?
(TF) ← 0
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (CS)
(CS) ← (2)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (IP)
(IP) ← (0)
If the source operand is a word, it is
divided into the double-length dividend
in registers AX and DX; the single-
length quotient is returned in AX, and
the single-length remainder is returned
in DX. For word integer division, the
maximum positive quotient is +32,767
(7FFFH) and the minimum negative
quotient is –32,767 (8001H).
else
(AL) ← (temp) / (AX)
(AH) ← (temp) % (AX)
When Source Operand is a Word:
(temp) ← (word-src)
if
If the quotient is positive and exceeds
the maximum, or is negative and is
less than the minimum, the quotient
and remainder are undefined, and a
type 0 interrupt is generated. In
particular, this occurs if division by 0 is
attempted. Nonintegral quotients are
truncated (toward 0) to integers, and
the remainder has the same sign as
the dividend.
(temp) / (DX:AX) > 0 and
(temp) / (DX:AX) > 7FFFH or
(temp) / (DX:AX) < 0 and
(temp) / (DX:AX) < 0 – 7FFFH – 1
then (type 0 interrupt is generated)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← FLAGS
(IF) ← 0
(TF) ← 0
(SP) ← (SP) – 2
Instruction Operands:
((SP) + 1:(SP)) ← (CS)
(CS) ← (2)
(SP) ← (SP) – 2
IDIV reg
IDIV mem
((SP) + 1:(SP)) ← (IP)
(IP) ← (0)
else
(AX) ← (temp) / (DX:AX)
(DX) ← (temp) % (DX:AX)
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-16
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
IMUL
Description
Integer Multiply:
Operation
When Source Operand is a Byte:
AF ?
CF
DF –
IF –
OF
PF ?
SF ?
TF –
ZF ?
IMUL src
(AX) ← (byte-src) × (AL)
if
Performs a signed multiplication of the
source operand and the accumulator.
If the source is a byte, then it is
(AH) = sign-extension of (AL)
then
(CF) ← 0
else
multiplied by register AL, and the
double-length result is returned in AH
and AL. If the source is a word, then it
is multiplied by register AX, and the
double-length result is returned in
registers DX and AX. If the upper half
of the result (AH for byte source, DX
for word source) is not the sign
(CF) ← 1
(OF) ← (CF)
When Source Operand is a Word:
(DX:AX) ← (word-src) × (AX)
if
(DX) = sign-extension of (AX)
extension of the lower half of the
result, CF and OF are set; otherwise
they are cleared. When CF and OF are
set, they indicate that AH or DX
then
(CF) ← 0
else
(CF) ← 1
(OF) ← (CF)
contains significant digits of the result.
Instruction Operands:
IMUL reg
IMUL mem
IMUL immed
IN
Input Byte or Word:
When Source Operand is a Byte:
(AL) ← (port)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
IN accum, port
Transfers a byte or a word from an
input port to the AL register or the AX
register, respectively. The port number
may be specified either with an
immediate byte constant, allowing
access to ports numbered 0 through
255, or with a number previously
placed in the DX register, allowing
variable access (by changing the value
in DX) to ports numbered from 0
through 65,535.
When Source Operand is a Word:
(AX) ← (port)
Instruction Operands:
IN AL, immed8
IN AX, DX
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-17
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
INC
Description
Operation
Increment:
(dest) ← (dest) + 1
AF
CF –
DF –
INC dest
Adds one to the destination operand.
The operand may be byte or a word
and is treated as an unsigned binary
number (see AAA and DAA).
IF
–
OF
PF
SF
Instruction Operands:
TF –
ZF
INC reg
INC mem
INS
In String:
(dest) ← (src)
AF –
CF –
DF –
INS dest-string, port
Performs block input from an I/O port
to memory. The port address is placed
in the DX register. The memory
address is placed in the DI register.
This instruction uses the ES register
(which cannot be overridden). After the
data transfer takes place, the DI
register increments or decrements,
depending on the value of the direction
flag (DF). The DI register changes by 1
for byte transfers or 2 for word
transfers.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
INS dest-string, port
INS (repeat) dest-string, port
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-18
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
INT
Description
Operation
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← FLAGS
(IF) ← 0
(TF) ← 0
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (CS)
(CS) ← (interrupt-type × 4 + 2)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (IP)
(IP) ← (interrupt-type × 4)
Interrupt:
AF –
CF –
DF –
IF
OF –
PF –
SF –
TF
INT interrupt-type
Activates the interrupt procedure
specified by the interrupt-type
operand. Decrements the stack pointer
by two, pushes the flags onto the
stack, and clears the trap (TF) and
interrupt-enable (IF) flags to disable
single-step and maskable interrupts.
The flags are stored in the format used
by the PUSHF instruction. SP is
decremented again by two, and the CS
register is pushed onto the stack.
ZF –
The address of the interrupt pointer is
calculated by multiplying interrupt-
type by four; the second word of the
interrupt pointer replaces CS. SP
again is decremented by two, and IP is
pushed onto the stack and is replaced
by the first word of the interrupt pointer.
If interrupt-type = 3, the assembler
generates a short (1 byte) form of the
instruction, known as the breakpoint
interrupt.
Instruction Operands:
INT immed8
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-19
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
INTO
Description
Operation
Interrupt on Overflow:
if
AF –
CF –
DF –
(OF) = 1
then
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← FLAGS
(IF) ← 0
INTO
Generates a software interrupt if the
overflow flag (OF) is set; otherwise
control proceeds to the following
instruction without activating an
interrupt procedure. INTO addresses
the target interrupt procedure (its type
is 4) through the interrupt pointer at
location 10H; it clears the TF and IF
flags and otherwise operates like INT.
INTO may be written following an
arithmetic or logical operation to
activate an interrupt procedure if
overflow occurs.
IF
–
OF –
PF –
SF –
TF –
ZF –
(TF) ← 0
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (CS)
(CS) ← (12H)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (IP)
(IP) ← (10H)
Instruction Operands:
none
IRET
Interrupt Return:
IRET
(IP) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
(CS) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
FLAGS ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
AF
CF
DF
IF
OF
PF
SF
TF
ZF
Transfers control back to the point of
interruption by popping IP, CS, and the
flags from the stack. IRET thus affects
all flags by restoring them to previously
saved values. IRET is used to exit any
interrupt procedure, whether activated
by hardware or software.
Instruction Operands:
none
JA
JNBE
Jump on Above:
Jump on Not Below or Equal:
if
AF –
CF –
DF –
((CF) = 0) or ((ZF) = 0)
then
disp8
JA
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
JNBE disp8
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition ((CF=0) or
(ZF=0)) is true.
Instruction Operands:
JA short-label
JNBE short-label
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-20
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
JAE
Description
Operation
Jump on Above or Equal:
Jump on Not Below:
if
AF –
CF –
DF –
JNB
(CF) = 0
then
JAE disp8
JNB disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition (CF = 0) is true.
Instruction Operands:
JAE short-label
JNB short-label
JB
Jump on Below:
if
AF –
JNAE
Jump on Not Above or Equal:
(CF) = 1
then
CF –
DF –
JB disp8
JNAE disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition (CF = 1) is true.
Instruction Operands:
JB short-label
JNAE short-label
JBE
JNA
Jump on Below or Equal:
Jump on Not Above:
if
AF –
CF –
DF –
((CF) = 1) or ((ZF) = 1)
then
JBE disp8
JNA disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition ((C =1) or
(ZF=1)) is true.
Instruction Operands:
JBE short-label
JNA short-label
JC
Jump on Carry:
if
AF –
(CF) = 1
then
CF –
DF –
JC disp8
Transfers control to the target location
if the tested condition (CF=1) is true.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
JC short-label
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-21
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
JCXZ
Description
Operation
Jump if CX Zero:
if
AF –
CF –
DF –
(CX) = 0
then
JCXZ disp8
Transfers control to the target location
if CX is 0. Useful at the beginning of a
loop to bypass the loop if CX has a
zero value, i.e., to execute the loop
zero times.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
JCXZ short-label
JE
JZ
Jump on Equal:
Jump on Zero:
if
AF –
CF –
DF –
(ZF) = 1
then
JE disp8
JZ disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the condition tested (ZF = 1) is true.
Instruction Operands:
JE short-label
JZ short-label
JG
JNLE
Jump on Greater Than:
Jump on Not Less Than or Equal:
if
AF –
CF –
DF –
((SF) = (OF)) and ((ZF) = 0)
then
JG disp8
JNLE disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the condition tested (SF = OF) and
(ZF=0) is true.
Instruction Operands:
JG short-label
JNLE short-label
JGE
JNL
Jump on Greater Than or Equal:
Jump on Not Less Than:
if
AF –
CF –
DF –
(SF) = (OF)
then
JGE disp8
JNL disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the condition tested (SF=OF) is true.
Instruction Operands:
JGE short-label
JNL short-label
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-22
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
JL
Description
Jump on Less Than:
Operation
if
AF –
CF –
DF –
≠
JNGE
Jump on Not Greater Than or Equal:
(SF) (OF)
then
JL disp8
JNGE disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the condition tested (SF≠OF) is true.
Instruction Operands:
JL short-label
JNGE short-label
JLE
Jump on Less Than or Equal:
if
AF –
≠
JNG
Jump on Not Greater Than:
((SF) (OF)) or ((ZF) = 1)
then
CF –
DF –
JGE disp8
JNL disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
≠
If the condition tested ((SF OF) or
(ZF=0)) is true.
Instruction Operands:
JGE short-label
JNL short-label
JMP
Jump Unconditionally:
JMP target
if
AF –
Inter-segment
then
(CS) ← SEG
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location.
Instruction Operands:
(IP) ← dest
JMP short-label
JMP near-label
JMP far-label
JMP memptr
JMP regptr
JNC
Jump on Not Carry:
if
AF –
CF –
DF –
(CF) = 0
JNC disp8
then
Transfers control to the target location
if the tested condition (CF=0) is true.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
JNC short-label
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-23
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
JNE
Description
Operation
Jump on Not Equal:
Jump on Not Zero:
if
AF –
CF –
DF –
JNZ
(ZF) = 0
then
JNE disp8
JNZ disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition (ZF = 0) is true.
Instruction Operands:
JNE short-label
JNZ short-label
JNO
Jump on Not Overflow:
if
AF –
CF –
DF –
(OF) = 0
then
JNO disp8
Transfers control to the target location
if the tested condition (OF = 0) is true.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
JNO short-label
JNS
Jump on Not Sign:
if
AF –
CF –
DF –
(SF) = 0
then
JNS disp8
Transfers control to the target location
if the tested condition (SF = 0) is true.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
JNS short-label
JNP
JPO
Jump on Not Parity:
Jump on Parity Odd:
if
AF –
CF –
DF –
(PF) = 0
then
JNO disp8
JPO disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF
–
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition (PF=0) is true.
Instruction Operands:
JNO short-label
JPO short-label
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-24
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
JO
Description
Jump on Overflow:
Operation
if
AF –
CF –
DF –
(OF) = 1
then
JO disp8
Transfers control to the target location
if the tested condition (OF = 1) is true.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
JO short-label
JP
Jump on Parity:
if
AF –
JPE
Jump on Parity Equal:
(PF) = 1
then
CF –
DF –
JP disp8
JPE disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Transfers control to the target location
if the tested condition (PF = 1) is true.
Instruction Format:
JP short-label
JPE short-label
JS
Jump on Sign:
if
AF –
(SF) = 1
then
CF –
DF –
JS disp8
Transfers control to the target location
if the tested condition (SF = 1) is true.
(IP) ← (IP) + disp8 (sign-ext to 16 bits) IF –
OF –
PF –
SF –
TF –
ZF –
Instruction Format:
JS short-label
LAHF
Load Register AH From Flags:
(AH) ← (SF):(ZF):X:(AF):X:(PF):X:(CF)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
LAHF
Copies SF, ZF, AF, PF and CF (the
8080/8085 flags) into bits 7, 6, 4, 2 and
0, respectively, of register AH. The
content of bits 5, 3, and 1 are
undefined. LAHF is provided primarily
for converting 8080/8085 assembly
language programs to run on an 8086
or 8088.
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-25
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
LDS
Description
Operation
Load Pointer Using DS:
(dest) ← (EA)
AF –
CF –
DF –
(DS) ← (EA + 2)
LDS dest, src
Transfers a 32-bit pointer variable from
the source operand, which must be a
memory operand, to the destination
operand and register DS. The offset
word of the pointer is transferred to the
destination operand, which may be
any 16-bit general register. The
segment word of the pointer is
IF
–
OF –
PF –
SF –
TF –
ZF –
transferred to register DS.
Instruction Operands:
LDS reg16, mem32
Load Effective Address:
LEA dest, src
LEA
(dest) ← EA
AF –
CF –
DF –
Transfers the offset of the source
operand (rather than its value) to the
destination operand.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
LEA reg16, mem16
LEAVE
Leave:
(SP) ← (BP)
(BP) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
AF –
CF –
DF –
LEAVE
Reverses the action of the most recent
ENTER instruction. Collapses the last
stack frame created. First, LEAVE
copies the current BP to the stack
pointer releasing the stack space
allocated to the current procedure.
Second, LEAVE pops the old value of
BP from the stack, to return to the
calling procedure's stack frame. A
return (RET) instruction will remove
arguments stacked by the calling
procedure for use by the called
procedure.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-26
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
LES
Description
Operation
Load Pointer Using ES:
(dest) ← (EA)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
(ES) ← (EA + 2)
LES dest, src
Transfers a 32-bit pointer variable from
the source operand to the destination
operand and register ES. The offset
word of the pointer is transferred to the
destination operand. The segment
word of the pointer is transferred to
register ES.
Instruction Operands:
LES reg16, mem32
Lock the Bus:
LOCK
LOCK
none
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Causes the 8088 (configured in
maximum mode) to assert its bus
LOCK signal while the following
instruction executes. The instruction
most useful in this context is an
exchange register with memory.
The LOCK prefix may be combined
with the segment override and/or REP
prefixes.
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-27
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
LODS
Description
Operation
Load String (Byte or Word):
When Source Operand is a Byte:
AF –
CF –
DF –
LODS src-string
(AL) ← (src-string)
if
Transfers the byte or word string
element addressed by SI to register AL
or AX and updates SI to point to the
next element in the string. This
instruction is not ordinarily repeated
since the accumulator would be
overwritten by each repetition, and
only the last element would be
retained.
IF
–
(DF) = 0
OF –
PF –
SF –
TF –
ZF –
then
(SI) ← (SI) + DELTA
else
(SI) ← (SI) – DELTA
When Source Operand is a Word:
(AX) ← (src-string)
if
Instruction Operands:
(DF) = 0
then
LODS src-string
(SI) ← (SI) + DELTA
else
LODS (repeat) src-string
(SI) ← (SI) – DELTA
LOOP
Loop:
(CX) ← (CX) – 1
AF –
CF –
DF –
if
LOOP disp8
(CX) ≠ 0
then
Decrements CX by 1 and transfers
control to the target location if CX is
not 0; otherwise the instruction
following LOOP is executed.
IF
–
(IP) ← (IP) + disp8 (sign-ext to 16 bits) OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
LOOP short-label
LOOPE
LOOPZ
Loop While Equal:
Loop While Zero:
(CX) ← (CX) – 1
AF –
CF –
DF –
if
(ZF) = 1 and (CX) ≠ 0
then
(IP)←(IP) + disp8 (sign-ext to 16 bits)
LOOPE disp8
LOOPZ disp8
IF
–
OF –
PF –
SF –
TF –
ZF –
Decrements CX by 1 and transfers
control is to the target location if CX is
not 0 and if ZF is set; otherwise the
next sequential instruction is executed.
Instruction Operands:
LOOPE short-label
LOOPZ short-label
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-28
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
Description
Operation
LOOPNE Loop While Not Equal:
LOOPNZ Loop While Not Zero:
(CX) ← (CX) – 1
if
(ZF) = 0 and (CX) ≠ 0
then
AF –
CF –
DF –
IF –
LOOPNE disp8
LOOPNZ disp8
(IP) ← (IP) + disp8 (sign-ext to 16 bits) OF –
Decrements CX by 1 and transfers
PF –
SF –
TF –
ZF –
control to the target location if CX is
not 0 and if ZF is clear; otherwise the
next sequential instruction is executed.
Instruction Operands:
LOOPNE short-label
LOOPNZ short-label
MOV
Move (Byte or Word):
(dest)←(src)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
MOV dest, src
Transfers a byte or a word from the
source operand to the destination
operand.
Instruction Operands:
MOV mem, accum
MOV accum, mem
MOV reg, reg
MOV reg, mem
MOV mem, reg
MOV reg, immed
MOV mem, immed
MOV seg-reg, reg16
MOV seg-reg, mem16
MOV reg16, seg-reg
MOV mem16, seg-reg
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-29
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
Description
Operation
MOVS
Move String:
(dest-string) ← (src-string)
AF –
CF –
DF –
MOVS dest-string, src-string
Transfers a byte or a word from the
source string (addressed by SI) to the
destination string (addressed by DI)
and updates SI and DI to point to the
next string element. When used in
conjunction with REP, MOVS
performs a memory-to-memory block
transfer.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
MOVS dest-string, src-string
MOVS (repeat) dest-string, src-string
MUL
Multiply:
When Source Operand is a Byte:
AF ?
CF
DF –
MUL src
(AX) ← (AL) × (src)
if
(AH) = 0
then
(CF) ← 0
else
(CF) ← 1
(OF) ← (CF)
Performs an unsigned multiplication of
the source operand and the accumu-
lator. If the source is a byte, then it is
multiplied by register AL, and the
double-length result is returned in AH
and AL. If the source operand is a
word, then it is multiplied by register
AX, and the double-length result is
returned in registers DX and AX. The
operands are treated as unsigned
binary numbers (see AAM). If the
upper half of the result (AH for byte
source, DX for word source) is non-
zero, CF and OF are set; otherwise
they are cleared.
IF
–
OF
PF ?
SF ?
TF –
ZF ?
When Source Operand is a Word:
(DX:AX) ← (AX) × (src)
if
(DX) = 0
then
(CF) ← 0
else
(CF) ← 1
(OF) ← (CF)
Instruction Operands:
MUL reg
MUL mem
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-30
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
NEG
Description
Operation
Negate:
When Source Operand is a Byte:
AF
CF
NEG dest
(dest) ← FFH – (dest)
(dest) ← (dest) + 1 (affecting flags)
DF –
IF –
OF
PF
SF
Subtracts the destination operand,
which may be a byte or a word, from 0 When Source Operand is a Word:
and returns the result to the desti-
nation. This forms the two's
complement of the number, effectively
reversing the sign of an integer. If the
operand is zero, its sign is not
(dest) ← FFFFH – (dest)
(dest) ← (dest) + 1 (affecting flags)
TF –
ZF
changed. Attempting to negate a byte
containing –128 or a word containing –
32,768 causes no change to the
operand and sets OF.
Instruction Operands:
NEG reg
NEG mem
NOP
No Operation:
NOP
None
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Causes the CPU to do nothing.
Instruction Operands:
none
NOT
Logical Not:
When Source Operand is a Byte:
(dest) ← FFH – (dest)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
NOT dest
Inverts the bits (forms the one's
complement) of the byte or word
operand.
When Source Operand is a Word:
(dest) ← FFFFH – (dest)
Instruction Operands:
NOT reg
NOT mem
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-31
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
OR
Description
Operation
Logical OR:
(dest) ← (dest) or (src)
(CF) ← 0
AF ?
CF
DF –
OR dest,src
(OF) ← 0
Performs the logical "inclusive or" of
the two operands (bytes or words) and
returns the result to the destination
operand. A bit in the result is set if
either or both corresponding bits in the
original operands are set; otherwise
the result bit is cleared.
IF
–
OF
PF
SF
TF –
ZF
Instruction Operands:
OR reg, reg
OR reg, mem
OR mem, reg
OR accum, immed
OR reg, immed
OR mem, immed
OUT
Output:
(dest) ← (src)
AF –
CF –
DF –
OUT port, accumulator
Transfers a byte or a word from the AL
register or the AX register, respec-
tively, to an output port. The port
number may be specified either with
an immediate byte constant, allowing
access to ports numbered 0 through
255, or with a number previously
placed in register DX, allowing variable
access (by changing the value in DX)
to ports numbered from 0 through
65,535.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
OUT immed8, AL
OUT DX, AX
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-32
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
OUTS
Description
Operation
Out String:
(dst) ← (src)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
OUTS port, src_string
Performs block output from memory to
an I/O port. The port address is placed
in the DX register. The memory
address is placed in the SI register.
This instruction uses the DS segment
register, but this may be changed with
a segment override instruction. After
the data transfer takes place, the
pointer register (SI) increments or
decrements, depending on the value
of the direction flag (DF). The pointer
register changes by 1 for byte
transfers or 2 for word transfers.
Instruction Operands:
OUTS port, src_string
OUTS (repeat) port, src_string
POP
Pop:
(dest) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
POP dest
Transfers the word at the current top of
stack (pointed to by SP) to the
destination operand and then
increments SP by two to point to the
new top of stack.
Instruction Operands:
POP reg
POP seg-reg (CS illegal)
POP mem
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-33
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
POPA
Description
Operation
Pop All:
(DI) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
(SI) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
(BP) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
(BX) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
AF –
CF –
DF –
POPA
Pops all data, pointer, and index
registers off of the stack. The SP value
popped is discarded.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
none
(DX) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
(CX) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
(AX) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
POPF
Pop Flags:
Flags ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
AF
CF
DF
IF
OF
PF
SF
TF
ZF
POPF
Transfers specific bits from the word at
the current top of stack (pointed to by
register SP) into the 8086/8088 flags,
replacing whatever values the flags
previously contained. SP is then
incremented by two to point to the new
top of stack.
Instruction Operands:
none
PUSH
Push:
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (src)
AF –
CF –
DF –
PUSH src
Decrements SP by two and then
transfers a word from the source
operand to the top of stack now
pointed to by SP.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
PUSH reg
PUSH seg-reg (CS legal)
PUSH mem
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-34
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
Description
Operation
PUSHA
Push All:
temp ← (SP)
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
(SP) ← (SP) – 2
PUSHA
((SP) + 1:(SP)) ← (AX)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (CX)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (DX)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (BX)
(SP) ← (SP) – 2
Pushes all data, pointer, and index
registers onto the stack . The order in
which the registers are saved is: AX,
CX, DX, BX, SP, BP, SI, and DI. The
SP value pushed is the SP value
before the first register (AX) is pushed.
Instruction Operands:
((SP) + 1:(SP)) ← (temp)
(SP) ← (SP) – 2
none
((SP) + 1:(SP)) ← (BP)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (SI)
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← (DI)
PUSHF
Push Flags:
(SP) ← (SP) – 2
((SP) + 1:(SP)) ← Flags
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
PUSHF
Decrements SP by two and then
transfers all flags to the word at the top
of stack pointed to by SP.
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-35
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
RCL
Description
Operation
Rotate Through Carry Left:
(temp) ← count
do while (temp) ≠ 0
(tmpcf) ← (CF)
(CF) ← high-order bit of (dest)
(dest) ← (dest) × 2 + (tmpcf)
AF –
CF
DF –
RCL dest, count
Rotates the bits in the byte or word
destination operand to the left by the
number of bits specified in the count
operand. The carry flag (CF) is treated
as "part of" the destination operand;
that is, its value is rotated into the low-
order bit of the destination, and itself is
replaced by the high-order bit of the
destination.
IF
–
OF
(temp) ← (temp) – 1
PF –
SF –
TF –
ZF –
if
count = 1
then
if
high-order bit of (dest) ≠ (CF)
then
Instruction Operands:
(OF) ← 1
else
RCL reg, n
(OF) ← 0
else
(OF) undefined
RCL mem, n
RCL reg, CL
RCL mem, CL
RCR
Rotate Through Carry Right:
(temp) ← count
AF –
CF
DF –
do while (temp) ≠ 0
RCR dest, count
(tmpcf) ← (CF)
Operates exactly like RCL except that
the bits are rotated right instead of left.
(CF) ← low-order bit of (dest)
IF
–
(dest) ← (dest) / 2
OF
Instruction Operands:
high-order bit of (dest) ← (tmpcf)
PF –
SF –
TF –
ZF –
(temp) ← (temp) – 1
RCR reg, n
if
RCR mem, n
RCR reg, CL
RCR mem, CL
count = 1
then
if
high-order bit of (dest) ≠
next-to-high-order bit of (dest)
then
(OF) ← 1
else
(OF) ← 0
else
(OF) undefined
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-36
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
REP
REPE
REPZ
Description
Operation
≠
Repeat:
do while (CX)
0
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Repeat While Equal:
Repeat While Zero:
Repeat While Not Equal:
Repeat While Not Zero:
service pending interrupts (if any)
execute primitive string
Operation in succeeding byte
(CX) ← (CX) – 1
REPNE
REPNZ
if
Controls subsequent string instruction
repetition. The different mnemonics
are provided to improve program
clarity.
primitive operation is CMPB,
CMPW, SCAB, or SCAW and
(ZF)
then
exit from while loop
≠ 0
REP is used in conjunction with the
MOVS (Move String) and STOS (Store
String) instructions and is interpreted
as "repeat while not end-of-string" (CX
not 0).
REPE and REPZ operate identically
and are physically the same prefix byte
as REP. These instructions are used
with the CMPS (Compare String) and
SCAS (Scan String) instructions and
require ZF (posted by these instruc-
tions) to be set before initiating the
next repetition.
REPNE and REPNZ are mnemonics
for the same prefix byte. These
instructions function the same as
REPE and REPZ except that the zero
flag must be cleared or the repetition is
terminated. ZF does not need to be
initialized before executing the
repeated string instruction.
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-37
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
RET
Description
Operation
Return:
(IP) ← ((SP) = 1:(SP))
(SP) ← (SP) + 2
if
AF –
CF –
DF –
RET optional-pop-value
Transfers control from a procedure
back to the instruction following the
CALL that activated the procedure.
The assembler generates an intra-
segment RET if the programmer has
defined the procedure near, or an
intersegment RET if the procedure has
been defined as far. RET pops the
word at the top of the stack (pointed to
by register SP) into the instruction
pointer and increments SP by two. If
RET is intersegment, the word at the
new top of stack is popped into the CS
register, and SP is again incremented
by two. If an optional pop value has
been specified, RET adds that value to
SP.
inter-segment
then
(CS) ← ((SP) + 1:(SP))
(SP) ← (SP) + 2
if
add immed8 to SP
then
(SP) ← (SP) + data
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
RET immed8
ROL
Rotate Left:
(temp) ← count
AF –
CF
DF –
do while (temp) ≠ 0
ROL dest, count
(CF) ← high-order bit of (dest)
(dest) ← (dest) × 2 + (CF)
Rotates the destination byte or word
left by the number of bits specified in
the count operand.
IF
–
(temp) ← (temp) – 1
OF
if
PF –
SF –
TF –
ZF –
Instruction Operands:
count = 1
then
if
high-order bit of (dest) ≠ (CF)
then
ROL reg, n
ROL mem, n
ROL reg, CL
ROL mem CL
(OF) ← 1
else
(OF) ← 0
else
(OF) undefined
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-38
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
ROR
Description
Rotate Right:
Operation
(temp) ← count
do while (temp) ≠ 0
AF –
CF
DF –
IF –
OF
PF –
SF –
TF –
ZF –
ROR dest, count
(CF) ← low-order bit of (dest)
Operates similar to ROL except that
the bits in the destination byte or word
are rotated right instead of left.
(dest) ← (dest) / 2
high-order bit of (dest) ← (CF)
(temp) ← (temp) – 1
Instruction Operands:
if
count = 1
ROR reg, n
then
ROR mem, n
ROR reg, CL
ROR mem, CL
if
high-order bit of (dest) ≠
next-to-high-order bit of (dest)
then
(OF) ← 1
else
(OF) ← 0
else
(OF) undefined
SAHF
Store Register AH Into Flags:
(SF):(ZF):X:(AF):X:(PF):X:(CF) ← (AH)
AF
CF
SAHF
DF –
IF –
OF –
PF
Transfers bits 7, 6, 4, 2, and 0 from
registerAH intoSF, ZF, AF, PF,andCF,
respectively, replacing whatever
values these flags previously had.
SF
Instruction Operands:
TF –
ZF
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-39
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
Description
Operation
SHL
SAL
Shift Logical Left:
Shift Arithmetic Left:
(temp) ← count
do while (temp)
AF ?
CF
≠
0
(CF) ← high-order bit of (dest)
DF –
SHL dest, count
SAL dest, count
(dest) ← (dest) × 2
IF
–
(temp) ← (temp) – 1
OF
PF
SF
TF –
ZF
Shifts the destination byte or word left
by the number of bits specified in the
count operand. Zeros are shifted in on
the right. If the sign bit retains its
original value, then OF is cleared.
if
count = 1
then
if
≠
high-order bit of (dest) (CE)
then
(OF) ← 1
else
(OF) ← 0
else
Instruction Operands:
SHL reg, n
SHL mem, n
SHL reg, CL
SAL reg, n
SAL mem, n
SAL reg, CL
SHL mem, CL SAL mem, CL
(OF) undefined
SAR
Shift Arithmetic Right:
(temp) ← count
AF ?
CF
DF –
do while (temp) ≠ 0
SAR dest, count
(CF) ← low-order bit of (dest)
(dest) ← (dest) / 2
Shifts the bits in the destination
operand (byte or word) to the right by
the number of bits specified in the
count operand. Bits equal to the
original high-order (sign) bit are shifted
in on the left, preserving the sign of the
original value. Note that SAR does not
produce the same result as the
dividend of an "equivalent" IDIV
instruction if the destination operand is
negative and 1 bits are shifted out. For
example, shifting –5 right by one bit
yields –3, while integer division –5 by 2
yields –2. The difference in the instruc-
tions is that IDIV truncates all numbers
toward zero, while SAR truncates
positive numbers toward zero and
negative numbers toward negative
infinity.
IF
–
(temp) ← (temp) – 1
OF
PF
SF
TF –
ZF
if
count = 1
then
if
high-order bit of (dest) ≠
next-to-high-order bit of (dest)
then
(OF) ← 1
else
(OF) ← 0
else
(OF) ← 0
Instruction Operands:
SAR reg, n
SAR mem, n
SAR reg, CL
SAR mem, CL
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-40
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
SBB
Description
Operation
Subtract With Borrow:
if
AF
(CF) = 1
then
(dest) = (dest) – (src) – 1
else
(dest) ← (dest) – (src)
CF
SBB dest, src
DF –
IF –
OF
PF
SF
Subtracts the source from the desti-
nation, subtracts one if CF is set, and
returns the result to the destination
operand. Both operands may be bytes
or words. Both operands may be
signed or unsigned binary numbers
(see AAS and DAS)
TF –
ZF
Instruction Operands:
SBB reg, reg
SBB reg, mem
SBB mem, reg
SBB accum, immed
SBB reg, immed
SBB mem, immed
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-41
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
SCAS
Description
Operation
Scan String:
When Source Operand is a Byte:
AF
CF
DF –
IF
OF
PF
SF
TF –
ZF
SCAS dest-string
(AL) – (byte-string)
if
(DF) = 0
then
(DI) ← (DI) + DELTA
else
(DI) ← (DI) – DELTA
Subtracts the destination string
–
element (byte or word) addressed by
DI from the content of AL (byte string)
or AX (word string) and updates the
flags, but does not alter the destination
string or the accumulator. SCAS also
updates DI to point to the next string
element and AF, CF, OF, PF, SF and
ZF to reflect the relationship of the
scan value in AL/AX to the string
element. If SCAS is prefixed with
REPE or REPZ, the operation is
interpreted as "scan while not end-of-
string (CX not 0) and string-element =
scan-value (ZF = 1)." This form may be
used to scan for departure from a
given value. If SCAS is prefixed with
REPNE or REPNZ, the operation is
interpreted as "scan while not end-of-
string (CX not 0) and string-element is
not equal to scan-value (ZF = 0)."
When Source Operand is a Word:
(AX) – (word-string)
if
(DF) = 0
then
(DI) ← (DI) + DELTA
else
(DI) ← (DI) – DELTA
Instruction Operands:
SCAS dest-string
SCAS (repeat) dest-string
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-42
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
SHR
Description
Shift Logical Right:
Operation
(temp) ← count
AF ?
CF
DF –
IF –
OF
PF
SF
TF –
ZF
≠
do while (temp)
0
SHR dest, src
(CF) ← low-order bit of (dest)
(dest) ← (dest) / 2
Shifts the bits in the destination
operand (byte or word) to the right by
the number of bits specified in the
count operand. Zeros are shifted in on
the left. If the sign bit retains its original
value, then OF is cleared.
(temp) ← (temp) – 1
if
count = 1
then
if
Instruction Operands:
≠
high-order bit of (dest)
next-to-high-order bit of (dest)
then
(OF) ← 1
else
(OF) ← 0
else
SHR reg, n
SHR mem, n
SHR reg, CL
SHR mem, CL
(OF) undefined
STC
Set Carry Flag:
STC
(CF) ← 1
AF –
CF
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
Sets CF to 1.
Instruction Operands:
none
STD
Set Direction Flag:
(DF) ← 1
AF –
CF –
DF
IF –
OF –
PF –
SF –
TF –
ZF –
STD
Sets DF to 1 causing the string instruc-
tions to auto-decrement the SI and/or
DI index registers.
Instruction Operands:
none
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-43
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
STI
Description
Operation
Set Interrupt-enable Flag:
(IF) ← 1
AF –
CF –
DF –
IF
OF –
PF –
SF –
TF –
ZF –
STI
Sets IF to 1, enabling processor
recognition of maskable interrupt
requests appearing on the INTR line.
Note however, that a pending interrupt
will not actually be recognized until the
instruction following STI has executed.
Instruction Operands:
none
STOS
Store (Byte or Word) String:
STOS dest-string
When Source Operand is a Byte:
AF –
CF –
DF –
(DEST) ← (AL)
if
(DF) = 0
then
(DI) ← (DI) + DELTA
else
(DI) ← (DI) – DELTA
Transfers a byte or word from register
AL or AX to the string element
addressed by DI and updates DI to
point to the next location in the string.
As a repeated operation.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
When Source Operand is a Word:
STOS dest-string
STOS (repeat) dest-string
(DEST) ← (AX)
if
(DF) = 0
then
(DI) ← (DI) + DELTA
else
(DI) ← (DI) – DELTA
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-44
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
SUB
Description
Operation
(dest) ← (dest) – (src)
Subtract:
AF
CF
DF –
IF –
OF
PF
SF
TF –
ZF
SUB dest, src
The source operand is subtracted from
the destination operand, and the result
replaces the destination operand. The
operands may be bytes or words. Both
operands may be signed or unsigned
binary numbers (see AAS and DAS).
Instruction Operands:
SUB reg, reg
SUB reg, mem
SUB mem, reg
SUB accum, immed
SUB reg, immed
SUB mem, immed
TEST
Test:
(dest) and (src)
(CF ) ← 0
(OF) ← 0
AF ?
CF
DF –
IF –
OF
PF
SF
TF –
ZF
TEST dest, src
Performs the logical "and" of the two
operands (bytes or words), updates
the flags, but does not return the
result, i.e., neither operand is
changed. If a TEST instruction is
followed by a JNZ (jump if not zero)
instruction, the jump will be taken if
there are any corresponding one bits
in both operands.
Instruction Operands:
TEST reg, reg
TEST reg, mem
TEST accum, immed
TEST reg, immed
TEST mem, immed
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-45
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
WAIT
Description
Operation
Wait:
None
AF –
CF –
DF –
WAIT
Causes the CPU to enter the wait state
while its test line is not active.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
none
XCHG
Exchange:
(temp) ← (dest)
(dest) ← (src)
(src) ← (temp)
AF –
CF –
DF –
XCHG dest, src
Switches the contents of the source
and destination operands (bytes or
words). When used in conjunction with
the LOCK prefix, XCHG can test and
set a semaphore that controls access
to a resource shared by multiple
processors.
IF
–
OF –
PF –
SF –
TF –
ZF –
Instruction Operands:
XCHG accum, reg
XCHG mem, reg
XCHG reg, reg
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-46
INSTRUCTION SET DESCRIPTIONS
Table C-4. Instruction Set (Continued)
Flags
Affected
Name
XLAT
Description
Operation
AL ← ((BX) + (AL))
Translate:
AF –
CF –
DF –
IF –
OF –
PF –
SF –
TF –
ZF –
XLAT translate-table
Replaces a byte in the AL register with
a byte from a 256-byte, user-coded
translation table. Register BX is
assumed to point to the beginning of
the table. The byte in AL is used as an
index into the table and is replaced by
the byte at the offset in the table corre-
sponding to AL's binary value. The first
byte in the table has an offset of 0. For
example, if AL contains 5H, and the
sixth element of the translation table
contains 33H, then AL will contain 33H
following the instruction. XLAT is
useful for translating characters from
one code to another, the classic
example being ASCII to EBCDIC or
the reverse.
Instruction Operands:
XLAT src-table
XOR
Exclusive Or:
(dest) ← (dest) xor (src)
(CF) ← 0
(OF) ← 0
AF ?
CF
DF –
IF –
OF
PF
SF
TF –
ZF
XOR dest, src
Performs the logical "exclusive or" of
the two operands and returns the
result to the destination operand. A bit
in the result is set if the corresponding
bits of the original operands contain
opposite values (one is set, the other
is cleared); otherwise the result bit is
cleared.
Instruction Operands:
XOR reg, reg
XOR reg, mem
XOR mem, reg
XOR accum, immed
XOR reg, immed
XOR mem, immed
NOTE: The three symbols used in the Flags Affected column are defined as follows:
– the contents of the flag remain unchanged after the instruction is executed
? the contents of the flag is undefined after the instruction is executed
the flag is updated after the instruction is executed
C-47
D
Instruction Set
Opcodes and Clock
Cycles
APPENDIX D
This appendix provides reference information for the 80C186 Modular Core family instruction
set. Table D-1 defines the variables used in Table D-2, which lists the instructions with their for-
mats and execution times. Table D-3 is a guide for decoding machine instructions. Table D-4 is
a guide for encoding instruction mnemonics, and Table D-5 defines Table D-4 abbreviations.
Table D-1. Operand Variables
Variable
Description
mod
r/m
mod and r/m determine the Effective Address (EA).
r/m and mod determine the Effective Address (EA).
reg represents a register.
reg
MMM
PPP
TTT
MMM and PPP are opcodes to the math coprocessor.
PPP and MMM are opcodes to the math coprocessor.
TTT defines which shift or rotate instruction is executed.
r/m
EA Calculation
mod
Effect on EA Calculation
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
(BX) + (SI) + DISP
(BX) + (DI) + DISP
0 0
0 0
0 1
1 0
1 1
if r/m ≠ 110, DISP = 0; disp-low and disp-high are absent
if r/m = 110, EA = disp-high:disp-low
(BP) + (SI) + DISP
(BP) + (DI) + DISP
(SI) + DISP
DISP = disp-low, sign-extended to 16 bits; disp-high is absent
DISP = disp-high:disp-low
r/m is treated as a reg field
(DI) + DISP
DISP follows the second byte of the instruction (before any required data).
(BP) + DISP, if mod ≠ 00
Physical addresses of operands addressed by the BP register are computed
using the SS segment register. Physical addresses of destination operands of
string primitives (addressed by the DI register) are computed using the ES seg-
ment register, which cannot be overridden.
disp-high:disp-low, if mod =00
1 1 1
(BX) + DISP
reg
16-bit (w=1)
8-bit (w=0)
TTT
Instruction
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
AX
CX
DX
BP
SP
BP
SI
AL
CL
DL
BL
AH
CH
DH
BH
0 0 0
ROL
ROR
RCL
RCR
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
SHL/SAL
SHR
—
DI
SAR
D-1
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary
Function
Format
Clocks
Notes
DATA TRANSFER INSTRUCTIONS
MOV = Move
register to register/memory
1 0 0 0 1 0 0 w
1 0 0 0 1 0 1 w
1 1 0 0 0 1 1 w
1 0 1 1 w reg
1 0 1 0 0 0 0 w
1 0 1 0 0 0 1 w
1 0 0 0 1 1 1 0
1 0 0 0 1 1 0 0
mod reg r/m
2/12
2/9
12/13
3/4
9
register/memory to register
immediate to register/memory
immediate to register
mod reg r/m
mod 000 r/m
data
data
data if w=1
(1)
(1)
data if w=1
addr-high
addr-high
memory to accumulator
addr-low
accumulator to memory
addr-low
8
register/memory to segment register
segment register to register/memory
mod 0 reg r/m
mod 0 reg r/m
2/9
2/11
PUSH = Push
memory
1 1 1 1 1 1 1 1
0 1 0 1 0 reg
0 0 0 reg 1 1 0
0 1 1 0 1 0 s 0
mod 110 r/m
16
10
9
register
segment register
immediate
data
data if s=0
10
POP = Pop
memory
1 0 0 0 1 1 1 1
0 1 0 1 1 reg
0 0 0 reg 1 1 1
0 1 1 0 0 0 0 0
0 1 1 0 0 0 0 1
mod 000 r/m
(reg ?01)
20
10
8
register
segment register
PUSHA = Push all
36
51
POPA = Pop all
XCHG = Exchange
register/memory with register
1 0 0 0 0 1 1 w
1 0 0 1 0 reg
1 1 0 1 0 1 1 1
mod reg r/m
4/17
3
register with accumulator
XLAT = Translate byte to AL
11
IN = Input from
fixed port
1 1 1 0 0 1 0 w
1 1 1 0 1 1 0 w
port
port
10
8
variable port
OUT = Output from
fixed port
1 1 1 0 0 1 0 w
1 1 1 0 1 1 0 w
9
7
variable port
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-2
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
DATA TRANSFER INSTRUCTIONS (Continued)
LEA = Load EA to register
LDS = Load pointer to DS
LES = Load pointer to ES
1 0 0 0 1 1 0 1
mod reg r/m
6
1 1 0 0 0 1 0 1
1 1 0 0 0 1 0 0
1 1 0 0 1 0 0 0
mod reg r/m
mod reg r/m
data-low
(mod ?11)
(mod ?11)
data-high
18
18
ENTER = Build stack frame
L
L = 0
15
L = 1
L > 1
25
22+16(n-1)
LEAVE = Tear down stack frame
LAHF = Load AH with flags
SAHF = Store AH into flags
PUSHF = Push flags
1 1 0 0 1 0 0 1
1 0 0 1 1 1 1 1
1 0 0 1 1 1 1 0
1 0 0 1 1 1 0 0
1 0 0 1 1 1 0 1
8
2
3
9
8
POPF = Pop flags
ARITHMETIC INSTRUCTIONS
ADD = Add
reg/memory with register to either
0 0 0 0 0 0 d w
1 0 0 0 0 0 s w
0 0 0 0 0 1 0 w
mod reg r/m
mod 000 r/m
data
3/10
4/16
3/4
immediate to register/memory
immediate to accumulator
data
data if sw=01
data if sw=01
data if w=1
(1)
(1)
ADC = Add with carry
reg/memory with register to either
0 0 0 1 0 0 d w
1 0 0 0 0 0 s w
0 0 0 1 0 1 0 w
mod reg r/m
mod 010 r/m
data
3/10
4/16
3/4
immediate to register/memory
immediate to accumulator
data
data if w=1
INC = Increment
register/memory
1 1 1 1 1 1 1 w
0 1 0 0 0 reg
0 0 1 1 0 1 1 1
0 0 1 0 0 1 1 1
mod 000 r/m
3/15
register
3
8
4
AAA = ASCII adjust for addition
DAA = Decimal adjust for addition
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-3
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
ARITHMETIC INSTRUCTIONS (Continued)
SUB = Subtract
reg/memory with register to either
0 0 1 0 1 0 d w
1 0 0 0 0 0 s w
0 0 0 1 1 1 0 w
mod reg r/m
3/10
4/16
3/4
immediate from register/memory
immediate from accumulator
mod 101 r/m
data
data
data if sw=01
data if sw=01
data if w=1
(1)
(1)
SBB = Subtract with borrow
reg/memory with register to either
0 0 0 1 1 0 d w
1 0 0 0 0 0 s w
0 0 0 1 1 1 0 w
mod reg r/m
mod 011 r/m
data
3/10
4/16
3/4
immediate from register/memory
immediate from accumulator
data
data if w=1
DEC = Decrement
register/memory
1 1 1 1 1 1 1 w
0 1 0 0 1 reg
1 1 1 1 0 1 1 w
mod 001 r/m
mod reg r/m
3/15
3
register
NEG = Change sign
3
CMP = Compare
register/memory with register
0 0 1 1 1 0 1 w
0 0 1 1 1 0 0 w
1 0 0 0 0 0 s w
0 0 1 1 1 1 0 w
0 0 1 1 1 1 1 1
0 0 1 0 1 1 1 1
1 1 1 1 0 1 1 w
mod reg r/m
mod reg r/m
mod 111 r/m
data
3/10
3/10
3/10
3/4
7
register with register/memory
immediate with register/memory
immediate with accumulator
data
data if sw=01
data if w=1
(1)
AAS = ASCII adjust for subtraction
DAS = Decimal adjust for subtraction
4
MUL = multiply (unsigned)
mod 100 r/m
mod 101 r/m
mod reg r/m
register-byte
26-28
35-37
32-34
41-43
register-word
memory-byte
memory-word
IMUL = Integer multiply (signed)
1 1 1 1 0 1 1 w
register-byte
25-28
34-37
31-34
40-43
22-25/
29-32
register-word
memory-byte
memory-word
integer immediate multiply (signed)
0 1 1 0 1 0 s 1
data
data if s=0
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-4
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
ARITHMETIC INSTRUCTIONS (Continued)
AAM = ASCII adjust for multiply
1 1 0 1 0 1 0 0
1 1 1 1 0 1 1 w
0 0 0 0 1 0 1 0
19
DIV = Divide (unsigned)
mod 110 r/m
mod 111 r/m
0 0 0 0 1 0 1 0
mod 010 r/m
register-byte
29
38
35
44
register-word
memory-byte
memory-word
IDIV = Integer divide (signed)
1 1 1 1 0 1 1 w
register-byte
29
38
35
44
15
2
register-word
memory-byte
memory-word
AAD = ASCII adjust for divide
CBW = Convert byte to word
1 1 0 1 0 1 0 1
1 0 0 1 1 0 0 0
1 0 0 1 1 0 0 1
CWD = Convert word to double-word
4
BIT MANIPULATION INSTRUCTIONS
NOT= Invert register/memory
1 1 1 1 0 1 1 w
3
AND = And
reg/memory and register to either
0 0 1 0 0 0 d w
1 0 0 0 0 0 0 w
0 0 1 0 0 1 0 w
mod reg r/m
mod 100 r/m
data
3/10
4/16
3/4
immediate to register/memory
immediate to accumulator
data
data if w=1
data if w=1
(1)
OR = Or
reg/memory and register to either
0 0 0 0 1 0 d w
1 0 0 0 0 0 0 w
0 0 0 0 1 1 0 w
mod reg r/m
mod 001 r/m
data
3/10
4/10
3/4
immediate to register/memory
immediate to accumulator
data
data if w=1
data if w=1
data if w=1
(1)
(1)
XOR = Exclusive or
reg/memory and register to either
0 0 1 1 0 0 d w
1 0 0 0 0 0 0 w
0 0 1 1 0 1 0 w
mod reg r/m
mod 110 r/m
data
3/10
4/10
3/4
immediate to register/memory
immediate to accumulator
data
data if w=1
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-5
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
BIT MANIPULATION INSTRUCTIONS (Continued)
TEST= And function to flags, no result
register/memory and register
1 0 0 0 0 1 0 w
mod reg r/m
3/10
4/10
3/4
immediate data and register/memory
immediate data and accumulator
1 1 1 1 0 1 1 w
1 0 1 0 1 0 0 w
mod 000 r/m
data
data
data if w=1
data if w=1
(1)
Shifts/Rotates
register/memory by 1
1 1 0 1 0 0 0 w
1 1 0 1 0 0 1 w
1 1 0 0 0 0 0 w
mod TTT r/m
mod TTT r/m
mod TTT r/m
2/15
register/memory by CL
5+n/17+n
5+n/17+n
register/memory by Count
count
STRING MANIPULATION INSTRUCTIONS
MOVS = Move byte/word
1 0 1 0 0 1 0 w
0 1 1 0 1 1 0 w
0 1 1 0 1 1 1 w
1 0 1 0 0 1 1 w
1 0 1 0 1 1 1 w
14
14
14
22
15
INS = Input byte/word from DX port
OUTS = Output byte/word to DX port
CMPS = Compare byte/word
SCAS = Scan byte/word
STRING MANIPULATION INSTRUCTIONS (Continued)
LODS = Load byte/word to AL/AX
1 0 1 0 1 1 0 w
1 0 1 0 1 0 1 w
12
10
STOS = Store byte/word from AL/AX
Repeated by count in CX:
MOVS = Move byte/word
1 1 1 1 0 0 1 0
1 1 1 1 0 0 1 0
1 1 1 1 0 0 1 0
1 1 1 1 0 0 1 z
1 1 1 1 0 0 1 z
1 1 1 1 0 0 1 0
1 1 1 1 0 1 0 0
1 0 1 0 0 1 0 w
0 1 1 0 1 1 0 w
0 1 1 0 1 1 1 w
1 0 1 0 0 1 1 w
1 0 1 0 1 1 1 w
0 1 0 1 0 0 1 w
0 1 0 1 0 0 1 w
8+8n
8-8n
INS = Input byte/word from DX port
OUTS = Output byte/word to DX port
CMPS = Compare byte/word
SCAS = Scan byte/word
8+8n
5+22n
5+15n
6+11n
6+9n
LODS = Load byte/word to AL/AX
STOS = Store byte/word from AL/AX
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-6
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
PROGRAM TRANSFER INSTRUCTIONS
Conditional Transfers — jump if:
JE/JZ= equal/zero
0 1 1 1 0 1 0 0
0 1 1 1 1 1 0 0
0 1 1 1 1 1 1 0
0 1 1 1 0 0 1 0
0 1 1 1 0 0 1 0
0 1 1 1 0 1 1 0
0 1 1 1 1 0 1 0
0 1 1 1 0 0 0 0
0 1 1 1 1 0 0 0
0 1 1 1 0 1 0 1
disp
disp
disp
disp
disp
disp
disp
disp
disp
disp
4/13
4/13
4/13
4/13
4/13
4/13
4/13
4/13
4/13
4/13
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
JL/JNGE = less/not greater or equal
JLE/JNG = less or equal/not greater
JB/JNAE = below/not above or equal
JC = carry
JBE/JNA = below or equal/not above
JP/JPE = parity/parity even
JO = overflow
JS = sign
JNE/JNZ = not equal/not zero
JNL/JGE = not less/greater or equal
JNLE/JG = not less or equal/greater
JNB/JAE = not below/above or equal
JNC = not carry
0 1 1 1 1 1 0 1
0 1 1 1 1 1 1 1
0 1 1 1 0 0 1 1
0 1 1 1 0 0 1 1
0 1 1 1 0 1 1 1
0 1 1 1 1 0 1 1
0 1 1 1 0 0 0 1
0 1 1 1 1 0 0 1
disp
disp
disp
disp
disp
disp
disp
disp
4/13
4/13
4/13
4/13
4/13
4/13
4/13
5/15
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
JNBE/JA = not below or equal/above
JNP/JPO = not parity/parity odd
JNO = not overflow
JNS = not sign
Unconditional Transfers
CALL = Call procedure
direct within segment
1 1 1 0 1 0 0 0
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 0 0 1 1 0 1 0
disp-low
disp-high
15
13/19
38
reg/memory indirect within segment
indirect intersegment
mod 010 r/m
mod 011 r/m
segment offset
selector
(mod ?11)
direct intersegment
23
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-7
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
PROGRAM TRANSFER INSTRUCTIONS (Continued)
RET = Return from procedure
within segment
1 1 0 0 0 0 1 1
16
18
22
25
within segment adding immed to SP
1 1 0 0 0 0 1 0
data-low
data-low
data-high
intersegment
1 1 0 0 1 0 1 1
intersegment adding immed to SP
1 1 0 0 1 0 1 0
data-high
JMP = Unconditional jump
short/long
1 1 1 0 1 0 1 1
1 1 1 0 1 0 0 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 0 1 0 1 0
disp-low
14
14
direct within segment
disp-low
disp-high
reg/memory indirect within segment
indirect intersegment
mod 100 r/m
mod 101 r/m
segment offset
selector
26
(mod ?11)
11/17
14
direct intersegment
Iteration Control
LOOP = Loop CX times
1 1 1 0 0 0 1 0
disp
disp
disp
6/16
5/16
5/16
(2)
(2)
(2)
LOOPZ/LOOPE =Loop while zero/equal 1 1 1 0 0 0 0 1
LOOPNZ/LOOPNE =
1 1 1 0 0 0 0 0
Loop while not zero/not equal
JCXZ = Jump if CX = zero
1 1 1 0 0 0 1 1
disp
6/16
(2)
Interrupts
INT = Interrupt
Type specified
1 1 0 0 1 1 0 1
1 1 0 0 1 1 0 0
1 1 0 0 1 1 1 0
0 1 1 0 0 0 1 0
1 1 0 0 1 1 1 1
type
47
45
Type 3
INTO = Interrupt on overflow
BOUND = Detect value out of range
IRET = Interrupt return
48/4
33-35
28
(3)
mod reg r/m
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
D-8
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-2. Instruction Set Summary (Continued)
Function
Format
Clocks
Notes
PROCESSOR CONTROL INSTRUCTIONS
CLC = Clear carry
1 1 1 1 1 0 0 0
1 1 1 1 0 1 0 1
1 1 1 1 1 0 0 1
1 1 1 1 1 1 0 0
1 1 1 1 1 1 0 1
1 1 1 1 1 0 1 0
1 1 1 1 1 0 1 1
1 1 1 1 0 1 0 0
1 0 0 1 1 0 1 1
1 1 1 1 0 0 0 0
1 1 0 1 1 M M M
1 0 0 1 0 0 0 0
2
2
2
2
2
2
2
2
6
2
6
3
CMC = Complement carry
STC = Set carry
CLD = Clear direction
STD = Set direction
CLI = Clear interrupt
STI = Set interrupt
HLT = Halt
WAIT = Wait
(4)
LOCK = Bus lock prefix
ESC = Math coprocessor escape
NOP = No operation
mod PPP r/m
SEGMENT OVERRIDE PREFIX
CS
0 0 1 0 1 1 1 0
0 0 1 1 0 1 1 0
0 0 1 1 1 1 1 0
0 0 1 0 0 1 1 0
2
2
2
2
SS
DS
ES
NOTES:
1. Clock cycles are given for 8-bit/16-bit operations.
2. Clock cycles are given for jump not taken/jump taken.
3. Clock cycles are given for interrupt taken/interrupt not taken.
4. If TEST = 0
Shading indicates additions and enhancements to the 8086/8088 instruction set. See Appendix A, “80C186
Instruction Set Additions and Extensions,” for details.
Table D-3. Machine Instruction Decoding Guide
Byte 1
Byte 2
Bytes 3–6
(disp-lo),(disp-hi)
ASM-86 Instruction Format
Hex
00
Binary
0000 0000
0000 0001
0000 0010
0000 0011
0000 0100
0000 0101
0000 0110
0000 0111
0000 0100
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
add
add
add
add
add
add
push
pop
or
reg8/mem8, reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
01
02
03
04
05
06
07
08
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
data-lo
data-hi
AX,immed16
ES
ES
mod reg r/m
(disp-lo),(disp-hi)
reg8/mem8,reg8
D-9
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
(disp-lo),(disp-hi)
ASM-86 Instruction Format
Hex
09
0000 1001
0000 1010
0000 1011
0000 1100
0000 1101
0000 1110
0000 1111
0001 0000
0001 0001
0001 0010
0001 0011
0001 0100
0001 0101
0001 0110
0001 0111
0001 1000
0001 1001
0001 1010
0001 1011
0001 1100
0001 1101
0001 1110
0001 1111
0010 0000
0010 0001
0010 0010
0010 0011
0010 0100
0010 0101
0010 0110
0010 0111
0010 1000
0010 1001
0010 1010
0010 1011
0010 1100
0010 1101
mod reg r/m
mod reg r/m
mod reg r/m
data-8
or
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL, immed8
0A
0B
0C
0D
0E
0F
10
11
12
13
14
15
16
17
18
19
1A
1B
1C
1D
1E
1F
20
21
22
23
24
25
26
27
28
29
2A
2B
2C
2D
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
or
or
or
data-lo
data-hi
or
AX,immed16
CS
push
—
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
adc
adc
adc
adc
adc
adc
push
pop
sbb
sbb
sbb
sbb
sbb
sbb
push
pop
and
and
and
and
and
and
ES:
daa
sub
sub
sub
sub
sub
sub
reg8/mem8,reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
data-lo
data-hi
AX,immed16
SS
SS
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
reg8/mem8,reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
data-lo
data-hi
AX,immed16
DS
DS
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
reg8/mem8,reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
data-lo
data-hi
AX,immed16
(segment override prefix)
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
reg8/mem8,reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
data-lo
data-hi
AX,immed16
D-10
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
ASM-86 Instruction Format
Hex
2E
0010 1110
0010 1111
0011 0000
0011 0001
0011 0010
0011 0011
0011 0100
0011 0101
0011 0110
0011 0111
0011 1000
0011 1001
0011 1010
0011 1011
0011 1100
0011 1101
0011 1110
0011 1111
0100 0000
0100 0001
0100 0010
0100 0011
0100 0100
0100 0101
0100 0110
0100 0111
0100 1000
0100 1001
0100 1010
0100 1011
0100 1100
0100 1101
0100 1110
0100 1111
0101 0000
0101 0001
0101 0010
DS:
das
xor
xor
xor
xor
xor
xor
SS:
aaa
xor
xor
xor
xor
xor
xor
DS:
aas
inc
(segment override prefix)
2F
30
31
32
33
34
35
36
37
38
39
3A
3B
3C
3D
3E
3F
40
41
42
43
44
45
46
47
48
49
4A
4B
4C
4D
4E
4F
50
51
52
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
(disp-lo),(disp-hi)
reg8/mem8,reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
data-lo
data-hi
AX,immed16
(segment override prefix)
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
data-8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
reg8/mem8,reg8
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
AL,immed8
data-lo
data-hi
AX,immed16
(segment override prefix)
AX
CX
DX
BX
SP
BP
SI
inc
inc
inc
inc
inc
inc
inc
DI
dec
dec
dec
dec
dec
dec
dec
dec
push
push
push
AX
CX
DX
BX
SP
BP
SI
DI
AX
CX
DX
D-11
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
ASM-86 Instruction Format
Hex
53
0101 0011
0101 0100
0101 0101
0101 0110
0101 0111
0101 1000
0101 1001
0101 1010
0101 1011
0101 1100
0101 1101
0101 1110
0101 1111
0110 0000
0110 0001
0110 0010
0110 0011
0110 0100
0110 0101
0110 0110
0110 0111
0110 1000
0110 1001
0111 0000
0111 0001
0111 0010
0111 0011
0111 0100
0111 0101
0111 0110
0111 0111
0111 1000
0111 1001
0111 1010
0111 1011
0111 1100
0111 1101
push
push
push
push
push
pop
pop
pop
pop
pop
pop
pop
pop
pusha
popa
bound
—
BX
SP
BP
SI
54
55
56
57
58
59
5A
5B
5C
5D
5E
5F
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
7A
7B
7C
7D
DI
AX
CX
DX
BX
SP
BP
SI
DI
mod reg r/m
reg16,mem16
—
—
—
—
data-lo
data-hi
data-lo, data-hi
push
imul
jo
immed16
immed16
short-label
short-label
short-label
mod reg r/m
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
IP-inc-8
jno
jb/jnae/jc
jnb/jae/jnc
je/jz
short-label
short-label
short-label
short-label
short-label
short-label
short-label
short-label
short-label
short-label
short-label
jne/jnz
jbe/jna
jnbe/ja
js
jns
jp/jpe
jnp/jpo
jl/jnge
jnl/jge
D-12
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
IP-inc-8
Bytes 3–6
ASM-86 Instruction Format
Hex
7E
0111 1110
0111 1111
1000 0000
jle/jng
jnle/jg
add
or
short-label
7F
80
IP-inc-8
short-label
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod reg r/m
mod reg r/m
mod reg r/m
(disp-lo),(disp-hi), data-8
reg8/mem8,immed8
reg8/mem8,immed8
reg8/mem8,immed8
reg8/mem8,immed8
reg8/mem8,immed8
reg8/mem8,immed8
reg8/mem8,immed8
reg8/mem8,immed8
reg16/mem16,immed16
reg16/mem16,immed16
reg16/mem16,immed16
reg16/mem16,immed16
reg16/mem16,immed16
reg16/mem16,immed16
reg16/mem16,immed16
reg16/mem16,immed16
reg8/mem8,immed8
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-8
adc
sbb
and
sub
xor
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-8
cmp
add
or
81
1000 0001
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-lo,data-hi
(disp-lo),(disp-hi), data-8
adc
sbb
and
sub
xor
81
82
1000 0001
1000 0010
cmp
add
—
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-8
adc
sbb
—
reg8/mem8,immed8
reg8/mem8,immed8
(disp-lo),(disp-hi), data-8
sub
—
reg8/mem8,immed8
(disp-lo),(disp-hi), data-8
(disp-lo),(disp-hi), data-SX
cmp
add
—
reg8/mem8,immed8
83
1000 0011
reg16/mem16,immed8
(disp-lo),(disp-hi), data-SX
(disp-lo),(disp-hi), data-SX
adc
sbb
—
reg16/mem16,immed8
reg16/mem16,immed8
(disp-lo),(disp-hi), data-SX
sub
—
reg16/mem16,immed8
(disp-lo),(disp-hi), data-SX
(disp-lo),(disp-hi)
cmp
test
test
xchg
reg16/mem16,immed8
reg8/mem8,reg8
84
85
86
1000 0100
1000 0101
1000 0110
(disp-lo),(disp-hi)
reg16/mem16,reg16
reg8,reg8/mem8
(disp-lo),(disp-hi)
D-13
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
(disp-lo),(disp-hi)
ASM-86 Instruction Format
Hex
87
1000 0111
1000 0100
1000 1001
1000 1010
1000 1011
1000 1100
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
mod reg r/m
mod OSR r/m
mod 1 - r/m
mod reg r/m
mod OSR r/m
mod 1 - r/m
xchg
mov
mov
mov
mov
mov
—
reg16,reg16/mem16
reg8/mem8,reg8
88
89
8A
8B
8C
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
reg16/mem16,reg16
reg8,reg8/mem8
reg16,reg16/mem16
reg16/mem16,SEGREG
8D
8E
1000 1101
1000 1110
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
lea
reg16,mem16
mov
—
SEGREG,reg16/mem16
8F
90
91
92
93
94
95
96
97
98
99
9A
9B
9C
9D
9E
9F
A0
A1
A2
A3
A4
A5
A6
A7
A8
A9
1000 1111
1001 0000
1001 0001
1001 0010
1001 0011
1001 0100
1001 0101
1001 0110
1001 0111
1001 1000
1001 1001
1001 1010
1001 1011
1001 1100
1001 1101
1001 1110
1001 1111
1010 0000
1010 0001
1010 0010
1010 0011
1010 0100
1010 0101
1010 0110
1010 0111
1010 1000
1010 1001
pop
mem16
(xchg AX,AX)
AX,CX
AX,DX
AX,BX
nop
xchg
xchg
xchg
xchg
xchg
xchg
xchg
cbw
cwd
call
AX,SP
AX,BP
AX,SI
AX,DI
disp-lo
disp-hi,seg-lo,seg-hi
far-proc
wait
pushf
popf
sahf
lahf
addr-lo
addr-lo
addr-lo
addr-lo
addr-hi
addr-hi
addr-hi
addr-hi
mov
mov
mov
mov
movs
movs
cmps
cmps
test
AL,mem8
AX,mem16
mem8,AL
mem16,AL
dest-str8,src-str8
dest-str16,src-str16
dest-str8,src-str8
dest-str16,src-str16
AL,immed8
data-8
data-lo
data-hi
test
AX,immed16
D-14
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
ASM-86 Instruction Format
Hex
AA
1010 1010
1010 1011
1010 1100
1010 1101
1010 1110
1010 1111
1011 0000
1011 0001
1011 0010
1011 0011
1011 0100
1011 0101
1011 0110
1011 0111
1011 1000
1011 1001
1011 1010
1011 1011
1011 1100
1011 1101
1011 1110
1011 1111
1100 0000
stos
stos
lods
lods
scas
scas
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
mov
rol
dest-str8
AB
AC
AD
AE
AF
B0
B1
B2
B3
B4
B5
B6
B7
B8
B9
BA
BB
BC
BD
BE
BF
C0
dest-str16
src-str8
src-str16
dest-str8
dest-str16
data-8
AL,immed8
data-8
CL,immed8
data-8
DL,immed8
data-8
BL,immed8
data-8
AH,immed8
data-8
CH,immed8
data-8
DH,immed8
data-8
BH,immed8
data-lo
data-hi
data-hi
data-hi
data-hi
data-hi
data-hi
data-hi
data-hi
data-8
data-8
data-8
data-8
data-8
data-8
AX,immed16
CX,immed16
DX,immed16
BX,immed16
SP,immed16
BP,immed16
SI,immed16
data-lo
data-lo
data-lo
data-lo
data-lo
data-lo
data-lo
DI,immed16
reg8/mem8, immed8
reg8/mem8, immed8
reg8/mem8, immed8
reg8/mem8, immed8
reg8/mem8, immed8
reg8/mem8, immed8
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
ror
rcl
rcr
shl/sal
shr
—
data-8
data-8
data-8
data-8
data-8
data-8
data-8
sar
reg8/mem8, immed8
C1
1100 0001
rol
reg16/mem16, immed8
reg16/mem16, immed8
reg16/mem16, immed8
reg16/mem16, immed8
reg16/mem16, immed8
reg16/mem16, immed8
ror
rcl
rcr
shl/sal
shr
—
D-15
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
ASM-86 Instruction Format
Hex
mod 111 r/m
data-lo
data-8
data-hi
sar
ret
ret
les
lds
mov
—
reg16/mem16, immed8
immed16 (intrasegment)
(intrasegment)
C2
1100 0010
1100 0011
1100 0100
1100 0101
1100 0110
C3
C4
C5
C6
mod reg r/m
mod reg r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
data-lo
(disp-lo),(disp-hi)
reg16,mem16
(disp-lo),(disp-hi)
reg16,mem16
(disp-lo),(disp-hi),data-8
mem8,immed8
—
—
—
—
—
C6
C7
1100 0110
1100 0111
—
(disp-lo),(disp-hi),data-lo,data-hi
mov
—
mem16,immed16
—
—
—
—
—
—
C8
C9
CA
CB
CC
CD
CE
CF
D0
1100 1000
1100 1001
1100 1010
1100 1011
1100 1100
1100 1101
1100 1110
1100 1111
1101 0000
data-hi, level
data-hi
enter
leave
ret
immed16, immed8
data-lo
data-8
immed16 (intersegment)
ret
int
(intersegment)
3
int
immed8
into
iret
rol
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
reg8/mem8,1
reg8/mem8,1
reg8/mem8,1
reg8/mem8,1
reg8/mem8,1
reg8/mem8,1
ror
rcl
rcr
sal/shl
shr
—
(disp-lo),(disp-hi)
sar
reg8/mem8,1
D-16
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
(disp-lo),(disp-hi)
ASM-86 Instruction Format
Hex
D1
1101 0001
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
0000 1010
rol
reg16/mem16,1
reg16/mem16,1
reg16/mem16,1
reg16/mem16,1
reg16/mem16,1
reg16/mem16,1
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
ror
D1
1101 0001
rcl
rcr
sal/shl
shr
—
(disp-lo),(disp-hi)
sar
rol
reg16/mem16,1
reg8/mem8,CL
reg8/mem8,CL
reg8/mem8,CL
reg8/mem8,CL
reg8/mem8,CL
reg8/mem8,CL
D2
1101 0010
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
ror
rcl
rcr
sal/shl
shr
—
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
sar
rol
reg8/mem8,CL
D3
1101 0011
reg16/mem16,CL
reg16/mem16,CL
reg16/mem16,CL
reg16/mem16,CL
reg16/mem16,CL
reg16/mem16,CL
ror
rcl
rcr
sal/shl
shr
—
(disp-lo),(disp-hi)
sar
aam
aad
—
reg16/mem16,CL
D4
D5
D6
D7
D8
D9
DA
DB
DC
DD
DE
DF
E0
1101 0100
1101 0101
1101 0110
1101 0111
1101 1000
1101 1001
1101 1010
1101 1011
1101 1100
1101 1101
1101 1110
1101 1111
1110 0000
0000 1010
xlat
esc
esc
esc
esc
esc
esc
esc
esc
source-table
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
IP-inc-8
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
opcode,source
opcode,source
opcode,source
opcode,source
opcode,source
opcode,source
opcode,source
opcode,source
loopne/loopnz
short-label
D-17
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
IP-inc-8
Bytes 3–6
ASM-86 Instruction Format
Hex
E1
1110 0001
1110 0010
1110 0011
1110 0100
1110 0101
1110 0110
1110 0111
1110 1000
1110 1001
1110 1010
1110 1011
1110 1100
1110 1101
1110 1110
1110 1111
1111 0000
1111 0001
1111 0010
1111 0011
1111 0100
1111 0101
1111 0110
loope/loopz
short-label
short-label
short-label
AL,immed8
AX,immed8
AL,immed8
AX,immed8
near-proc
near-label
far-label
E2
E3
E4
E5
E6
E7
E8
E9
EA
EB
EC
ED
EE
EF
F0
F1
F2
F3
F4
F5
F6
IP-inc-8
IP-inc-8
data-8
data-8
loop
jcxz
in
in
data-8
data-8
IP-inc-lo
IP-inc-lo
IP-lo
out
out
IP-inc-hi
IP-inc-hi
call
jmp
jmp
jmp
in
IP-hi,CS-lo,CS-hi
IP-inc-8
short-label
AL,DX
in
AX,DX
out
AL,DX
out
AX,DX
lock
—
(prefix)
repne/repnz
rep/repe/repz
hlt
cmc
test
—
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
(disp-lo),(disp-hi),data-8
reg8/mem8,immed8
(disp-lo),(disp-hi)
not
reg8/mem8
(disp-lo),(disp-hi)
neg
mul
imul
div
reg8/mem8
(disp-lo),(disp-hi)
reg8/mem8
(disp-lo),(disp-hi)
reg8/mem8
(disp-lo),(disp-hi)
reg8/mem8
(disp-lo),(disp-hi)
idiv
test
—
reg8/mem8
F7
1111 0111
(disp-lo),(disp-hi),data-lo,data-hi
reg16/mem16,immed16
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
not
reg16/mem16
reg16/mem16
reg16/mem16
reg16/mem16
reg16/mem16
reg16/mem16
neg
mul
imul
div
idiv
D-18
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-3. Machine Instruction Decoding Guide (Continued)
Byte 1
Binary
Byte 2
Bytes 3–6
ASM-86 Instruction Format
Hex
F8
1111 1000
1111 1001
1111 1010
1111 1011
1111 1100
1111 1101
1111 1110
clc
stc
cli
F9
FA
FB
FC
FD
FE
sti
cld
std
inc
dec
—
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
(disp-lo),(disp-hi)
mem16
mem16
(disp-lo),(disp-hi)
FE
1111 1110
—
—
—
—
—
FF
1111 1111
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
(disp-lo),(disp-hi)
inc
dec
call
call
jmp
jmp
push
—
mem16
mem16
reg16/mem16 (intrasegment)
mem16 (intersegment)
reg16/mem16 (intrasegment)
mem16 (intersegment)
mem16
D-19
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-4. Mnemonic Encoding Matrix (Left Half)
x0
x1
x2
x3
x4
x5
x6
x7
ADD
ADD
ADD
ADD
ADD
ADD
PUSH
POP
0x
1x
2x
3x
4x
5x
6x
7x
8x
9x
Ax
Bx
Cx
Dx
Ex
Fx
b,f,r/m
ADC
w,f,r/m
ADC
b,t,r/m
ADC
w,t,r/m
ADC
b,ia
ADC
w,ia
ADC
ES
PUSH
ES
POP
b,f,r/m
AND
w,f,r/m
AND
b,t,r/m
AND
w,t,r/m
AND
b,i
AND
w,i
AND
SS
SEG
SS
DAA
b,f,r/m
XOR
w,f,r/m
XOR
b,t,r/m
XOR
w,t,r/m
XOR
b,i
XOR
w,i
XOR
=ES
SEG
AAA
INC
b,f,r/m
INC
w,f,r/m
INC
b,t,r/m
INC
w,t,r/m
INC
b,i
INC
w,i
INC
=SS
INC
AX
PUSH
CX
PUSH
DX
PUSH
BX
PUSH
SP
PUSH
BP
PUSH
SI
PUSH
DI
PUSH
AX
PUSHA
CX
POPA
DX
BOUND
BX
SP
BP
SI
DI
w,f,r/m
JB/
JNAE/
JC
JO
JNO
JNB/
JAE/
JNC
JE/
JZ
JNE/
JNZ
JBE/
JNA
JNBE/
JA
Immed
Immed
Immed
Immed
TEST
TEST
XCHG
XCHG
b,r/m
NOP
(XCHG)
AX
w,r/m
XCHG
b,r/m
XCHG
is,r/m
XCHG
b,r/m
XCHG
w,r/m
XCHG
b,r/m
XCHG
w,r/m
XCHG
CX
MOV
DX
MOV
BX
MOV
SP
MOVS
BP
MOVS
SI
CMPS
DI
CMPS
MOV
m→AL
MOV
m→AX
MOV
AL→m
MOV
AX→m
MOV
MOV
MOV
MOV
MOV
i→AL
Shift
i→CL
Shift
i→DL
RET
i→BL
RET
i→AH
LES
i→CH
LDS
i→DH
MOV
i→BH
MOV
b,i
Shift
w,i
Shift
(i+SP)
Shift
b,i,r/m
OUT
w,i,r/m
XLAT
Shift
AAM
IN
AAD
IN
b
w
b,v
LOOP
w,v
JCXZ
LOOPNZ/
LOOPNE
LOOPZ/
LOOPE
OUT
LOCK
REP
REP
z
HLT
CMC
Grp1
b,r/m
Grp1
w,r/m
NOTE: Table D-5 defines abbreviations used in this matrix. Shading indicates reserved opcodes.
D-20
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-4. Mnemonic Encoding Matrix (Right Half)
x8
x9
xA
xB
xC
xD
xE
xF
OR
OR
OR
OR
OR
OR
PUSH
0x
1x
2x
3x
4x
5x
6x
7x
8x
9x
Ax
Bx
Cx
Dx
Ex
Fx
b,f,r/m
SBB
w,f,r/m
SBB
b,t,r/m
SBB
w,t,r/m
SBB
b,i
SBB
w,i
SBB
CS
PUSH
POP
b,f,r/m
SUB
w,f,r/m
SUB
b,t,r/m
SUB
w,t,r/m
SUB
b,i
SUB
w,i
SUB
DS
SEG
DS
DAS
b,f,r/m
CMP
w,f,r/m
CMP
b,t,r/m
CMP
w,t,r/m
CMP
b,i
CMP
w,i
CMP
=CS
SEG
AAS
DEC
b,f,r/m
DEC
w,f,r/m
DEC
b,t,r/m
DEC
w,t,r/m
DEC
b,i
DEC
w,i
DEC
=DS
DEC
AX
POP
CX
POP
DX
POP
BX
POP
SP
POP
BP
POP
SI
POP
DI
POP
AX
PUSH
CX
IMUL
DX
PUSH
BX
IMUL
SP
INS
BP
INS
SI
OUTS
DI
OUTS
w,i
JS
w,i
JNS
b,i
JP/
JPE
w,i
JNP/
JPO
b
JL/
JNGE
w
JNL/
JGE
b
JLE/
JNG
w
JNLE/
JG
MOV
MOV
MOV
MOV
MOV
LEA
POPF
LODS
MOV
MOV
POP
b,f,r/m
CBW
w,f,r/m
CWD
b,t,r/m
CALL
w,t,r/m
WAIT
sr,f,r/m
PUSHF
sr,t,r/m
SAHF
r/m
LAHF
L,D
STOS
TEST
TEST
STOS
MOV
LODS
MOV
SCAS
MOV
SCAS
MOV
b,ia
MOV
w,ia
MOV
MOV
i→AX
ENTER
i→CX
LEAVE
i→DX
RET
i→BX
RET
i→SP
INT
i→BP
INT
i→SI
INTO
i→DI
IRET
l(i+SP)
ESC
l
type 3
ESC
(any)
ESC
ESC
ESC
ESC
ESC
ESC
0
1
JMP
2
JMP
3
JMP
4
IN
5
IN
6
OUT
7
OUT
CALL
CLC
STC
CLI
STI
CLS
STD
Grp2
b,r/m
Grp2
w,r/m
NOTE: Table D-5 defines abbreviations used in this matrix. Shading indicates reserved opcodes.
D-21
INSTRUCTION SET OPCODES AND CLOCK CYCLES
Table D-5. Abbreviations for Mnemonic Encoding Matrix
Abbr
Definition
byte operation
direct
Abbr
ia
Definition
immediate to accumulator
indirect
Abbr
m
Definition
memory
Abbr
Definition
to CPU register
variable
b
d
t
id
r/m
EA is second byte
v
f
i
from CPU register
immediate
is
l
immediate byte, sign extended
long (intersegment)
si
short intrasegment
segment register
w
z
word operation
zero
sr
Byte 2
Immed
Shift
ROL
Grp1
TEST
—
Grp2
INC
mod 000 r/m
mod 001 r/m
mod 010 r/m
mod 011 r/m
mod 100 r/m
mod 101 r/m
mod 110 r/m
mod 111 r/m
ADD
OR
ROR
RCL
DEC
ADC
SBB
AND
SUB
XOR
CMP
NOT
NEG
MUL
IMUL
DIV
CALL id
CALL l, id
JMP id
JMP i, id
PUSH
—
RCR
SHL/SAL
SHR
—
SAR
IDIV
mod and r/m determine the Effective Address (EA) calculation. See Table D-1 for definitions.
D-22
Index
INDEX
80C187 Math Coprocessor, 10-2–10-8
accessing, 10-10–10-11
based index, 2-34, 2-35
direct, 2-29
arithmetic instructions, 10-3–10-4
bus cycles, 10-11
clocking, 10-10
immediate operands, 2-28
indexed, 2-32, 2-33
indirect, 2-36
code examples, 10-13–10-16
comparison instructions, 10-5
constant instructions, 10-6
data transfer instructions, 10-3
data types, 10-7–10-8
memory operands, 2-28
register indirect, 2-30, 2-31
register operands, 2-27
AH register, 2-5
AL register, 2-5, 2-18, 2-23
design considerations, 10-10–10-11
example floating point routine, 10-16
exceptions, 10-13
ApBUILDER files, obtaining from BBS, 1-5
Application BBS, 1-5
Architecture
I/O port assignments, 10-10
initialization example, 10-13–10-16
instruction set, 10-2
CPU block diagram, 2-2
device feature comparisons, 1-2
family introduction, 1-1
interface, 10-7–10-13
overview, 1-1, 2-1
and chip-selects, 6-17, 10-11
and PCB location, 4-7
ARDY, See READY
Arithmetic
exception trapping, 10-13
generating READY, 10-11
processor control instructions, 10-6
testing for presence, 10-10
transcendental instructions, 10-5
8259A Programmable Interrupt Controllers, 8-1
and special fully nested mode, 8-8
cascading, 8-7, 8-8
instructions, 2-19–2-20
interpretation of 8-bit numbers, 2-20
Arithmetic Logic Unit (ALU), 2-1
Array bounds trap (Type 5 exception), 2-44
ASCII, defined, 2-37
Auxiliary Flag (AF), 2-7, 2-9
AX register, 2-1, 2-5, 2-18, 2-23, 3-6
interrupt type, 8-9
priority structure, 8-8
B
Base Pointer (BP)‚ See BP register
BBS, 1-5
BCD, defined, 2-37
82C59A Programmable Interrupt Controller
interfacing with, 3-25–3-27
Bit manipulation instructions, 2-21–2-22
BOUND instruction, 2-44, A-8
BP register, 2-1, 2-13, 2-30, 2-34
Breakpoint interrupt (Type 3 exception), 2-44
Bulletin board system (BBS), 1-5
Bus cycles, 3-20–3-45
address/status phase, 3-10–3-12
and 80C187, 10-11
A
Address and data bus, 3-1–3-6
16-bit, 3-1–3-5
considerations, 3-7
8-bit, 3-5–3-6
considerations, 3-7
See also Bus cycles‚ Data transfers
Address bus, See Address and data bus
Address space, See Memory space‚ I/O space
Addressing modes, 2-27–2-36
and string instructions, 2-34
based, 2-30, 2-31, 2-32
and CSU, 6-17
and PCB accesses, 4-4
and T-states, 3-9
data phase, 3-13
HALT cycle, 3-28–3-33
Index-1
INDEX
and chip-selects, 6-5
HALT state, exiting, 3-30
idle states, 3-18
instruction prefetch, 3-20
interrupt acknowledge (INTA) cycles, 3-6,
3-25–3-26, 8-9
examples, 6-18–6-22
features and benefits, 6-1
functional overview, 6-2–6-5
programming, 6-6–6-17
registers, 6-6–6-12
system diagram, 6-19
See also Chip selects
and chip-selects, 6-5
interrupt acknowledge cycles, 8-29
operation, 3-7–3-20
Chip-selects
activating, 6-5
priorities, 3-44–3-45, 7-2
read cycles, 3-20–3-21
refresh cycles, 3-22, 7-4, 7-5
control signals, 7-5, 7-6
during HOLD, 3-41–3-43, 7-12–7-13
wait states, 3-13–3-18
and 80C187 interface, 6-17, 10-11
and bus hold protocol, 6-18
and DMA acknowledge signal, 10-22
and DRAM controllers, 7-1
and reserved I/O locations, 6-17
initializing, 6-6–6-18
write cycles, 3-22–3-25
See also Data transfers
Bus hold protocol, 3-39–3-44
and CLKOUT, 5-6
methods for generating, 6-1
overlapping, 6-16–6-17
programming considerations, 6-17
start address, 6-17
and CSU, 6-18
timing, 6-4
and refresh cycles, 3-41–3-43, 7-12–7-13
and reset, 5-9
CL register, 2-5, 2-21, 2-22
CLKOUT
latency, 3-40–3-41
and bus hold, 5-6
Bus Interface Unit (BIU), 2-1, 2-3, 2-11, 3-1–3-45
and DMA, 10-8
and power management modes, 5-6
and reset, 5-6
and DRAM refresh requests, 7-4
and TCU, 9-1
Clock divider, 5-11
control register, 5-12
buffering the data bus, 3-34–3-36
modifying interface, 3-33–3-36, 3-36
relationship to RCU, 7-1
synchronizing software and hardware events,
3-36–3-37
Clock generator, 5-6–5-10
and system reset, 5-6–5-7
output, 5-6
synchronizing CLKOUT and RESOUT, 5-6–
5-7
using a locked bus, 3-37–3-38
using multiple bus masters, 3-39–3-44
using the queue status signals, 3-38–3-39
BX register, 2-1, 2-5, 2-30
Clock sources, TCU, 9-12
Code (programs)‚ See Software
Code segment, 2-5
CompuServe forums, 1-6
Counters‚ See Timer Counter Unit (TCU)
CPU, block diagram, 2-2
Crystal‚ See Oscillator
CS register, 2-1, 2-5, 2-6, 2-13, 2-23, 2-39, 2-41
Customer service, 1-4
CX register, 2-1, 2-5, 2-23, 2-25, 2-26
C
Carry Flag (CF), 2-7, 2-9
Chip-Select Unit (CSU), 6-1
and DMA, 10-8
and DMA acknowledge signal, 10-22
and HALT bus cycles, 3-28
and READY, 6-15–6-16
and wait states, 6-15–6-16
block diagram, 6-3
D
Data, 3-6
Data bus, See Address and data bus
Data segment, 2-5
bus cycle decoding, 6-17
Index-2
INDEX
Data sheets, obtaining from BBS, 1-5
Data transfers, 3-1–3-6
instructions, 2-18
synchronization
destination-synchronized, 10-5
selecting, 10-18
PCB considerations, 4-5
PSW flag storage formats, 2-19
See also Bus cycles
source-synchronized, 10-5
unsynchronized, 10-6
timed DMA transfer example, 10-22–10-27
transfers, 10-1–10-27
Data types, 2-37–2-38
DI register, 2-1, 2-5, 2-13, 2-22, 2-23, 2-30, 2-32,
2-34
Digital one-shot, code example, 9-17–9-23
Direct Memory Access (DMA) Unit, 10-1–10-27
and BIU, 10-8
count, 10-7
programming, 10-18–10-19
direction, 10-3
rates, 10-21
size, 10-3
and CSU, 10-8
selecting, 10-14
and PCB, 10-3
suspending, 10-7, 10-20
terminating, 10-7
Direction Flag (DF), 2-7, 2-9, 2-23
Display, defined, A-2
arming channel, 10-18
DMA acknowledge signal, 10-2, 10-22
DRQ timing, 10-20
examples, 10-22–10-27
HALT bit, 8-22, 8-23, 10-20
hardware considerations, 10-20–10-22
initialization code, 10-22–10-27
initializing, 10-20
Divide Error trap (Type 0 exception), 2-43
DMA Control Register (DxCON), 10-15
DMA Destination Pointer Register, 10-13, 10-14
DMA Source Pointer Register, 10-11, 10-12
Documents, related, 1-3
interrupts, 10-8
DRAM controllers
generating on terminal count, 10-19
introduction, 10-1
and wait state control, 7-5
clocked, 7-5
latency, 10-21
design guidelines, 7-5
modules, 10-8–10-9
unclocked, 7-5
overview, 10-1–10-10
pointers, programming, 10-10–10-14
priority
See also Refresh Control Unit
DS register, 2-1, 2-5, 2-6, 2-13, 2-30, 2-34, 2-43
DX register, 2-1, 2-5, 2-36, 3-6
channel, 10-8–10-9, 10-19
fixed, 10-8–10-10
rotating, 10-10
E
Effective Address (EA), 2-13
calculation, 2-28
Emulation mode, 11-1
programming, 10-17–10-20
arming channel, 10-18
channel priority, 10-19
initializing, 10-20
End-of-Interrupt (EOI)
command, 8-21
interrupts, 10-19
register, 8-21, 8-22, 8-27, 8-28
ENTER instruction, A-2
ES register, 2-1, 2-5, 2-6, 2-13, 2-30, 2-34
Escape opcode fault (Type 7 exception), 2-44, 10-2
Exceptions, 2-43–2-44
priority, 2-46–2-49
suspending transfers, 10-20
synchronization, 10-18
transfer count, 10-18–10-19
programming, pointers, 10-10–10-14
requests, 10-3
external, 10-4
internal, 10-6
Execution Unit (EU), 2-1, 2-2
Extra segment, 2-5
software, 10-6
Timer 2, 10-6
selecting source, 10-17
Index-3
INDEX
rotate instructions, A-10
shift instructions, A-9
string instructions, 2-22–2-23, A-2
INT instruction, single-byte‚ See Breakpoint
interrupt
F
Fault exceptions, 2-43
FaxBack service, 1-4
F-Bus
and PCB, 4-5
operation, 4-5
INT0 instruction, 2-44
INTA bus cycle‚ See Bus cycles
Integer, defined, 2-37, 10-7
Interrupt Control register, 8-28
Interrupt Control registers, 8-12
for external pins, 8-14, 8-15
for internal sources, 8-13
Flags‚ See Processor Status Word (PSW)
Floating Point, defined, 2-37
H
HALT bus cycle‚ See Bus cycles
HOLD/HLDA protocol‚ See Bus hold protocol
Hypertext manuals, obtaining from BBS, 1-5
Interrupt Control Unit (ICU), 8-1–8-31
block diagram, 8-2, 8-24
cascade mode, 8-7
initializing, 8-30, 8-31
I
I/O devices
interfacing with an 82C59A Programmable
Interrupt Controller, 3-25–3-27
master mode, 8-1, 8-2–8-23
initializing, 8-30
operation in slave mode, 8-29
operation with nesting, 8-4
programming, 8-11, 8-25
interfacing with, 3-6–3-7
memory-mapped, 3-6
I/O ports
addressing, 2-36
I/O space, 3-1–3-7
accessing, 3-6
reserved locations, 2-15, 6-17
Idle states
registers, 8-11, 8-25
master mode, 8-11
and bus cycles, 3-18
slave mode, 8-1, 8-23–8-30
special fully nested mode, 8-8
with cascade mode, 8-8
Immediate operands, 2-28
IMUL instruction, A-9
Inputs, asynchronous, synchronizing, B-1
INS instruction, A-2
In-Service register, 8-5, 8-7, 8-18, 8-19, 8-28
Instruction Pointer (IP), 2-1, 2-6, 2-13, 2-23, 2-39,
2-41
without cascade mode, 8-8
typical interrupt sequence, 8-5
Interrupt Enable Flag (IF), 2-7, 2-9, 2-41
Interrupt Mask register, 8-16, 8-17, 8-28
Interrupt Request register, 8-16, 8-28
Interrupt Status register, 8-7, 8-22, 8-23, 8-28
Interrupt Vector register, 8-26, 8-27
Interrupt Vector Table, 2-39, 2-40
Interrupt-on-overflow trap (Type 4 exception),
2-44
reset status, 2-6
Instruction prefetch bus cycle‚ See Bus cycles
Instruction set, 2-17, A-1, D-1
additions, A-1
arithmetic instructions, 2-19–2-20, A-9
bit manipulation instructions, 2-21–2-22, A-9
data transfer instructions, 2-18–2-20, A-1,
A-8
data types, 2-37–2-38
enhancements, A-8
high-level instructions, A-2
nesting, A-2
processor control instructions, 2-27
program transfer instructions, 2-23–2-24
reentrant procedures, A-2
Interrupts, 2-39–2-43
and CSU initialization, 6-6
controlling priority, 8-12
edge- and level-sensitive, 8-10
and external 8259As, 8-10
enabling cascade mode, 8-12
enabling special fully nested mode, 8-12
latency, 2-45
reducing, 3-28
latency and response times, 8-10, 8-11, 8-30
Index-4
INDEX
maskable, 2-43
masking, 8-3, 8-12, 8-16
priority-based, 8-17
logical, 2-10, 2-12
offset value, 2-10, 2-13
overriding, 2-11, 2-13
multiplexed, 8-7
nesting, 8-4
NMI, 2-42
physical, 2-3, 2-10, 2-12
and dynamic code relocation, 2-13
Memory space, 3-1–3-6
nonmaskable, 2-45
overview, 8-1, 8-2
priority, 2-46–2-49, 8-3
default, 8-3
resolution, 8-5, 8-6
processing, 2-39–2-42
reserved, 2-39
N
Normally not-ready signal‚ See Ready
Normally ready signal‚ See Ready
Numerics coprocessor fault (Type 16 exception),
2-44, 10-13
response time, 2-46
O
selecting edge- or level-triggering, 8-12
slave mode sources, 8-25
software, 2-45
ONCE mode, 11-1
One-shot, code example, 9-17–9-23
Ordinal, defined, 2-37
Oscillator
timer interrupts, 9-16
types, 8-9, 8-26, 8-27
See also Exceptions, Interrupt Control Unit
INTn instruction, 2-45
Invalid opcode trap (Type 6 exception), 2-44
IRET instruction, 2-41
external
selecting crystal, 5-5
using canned, 5-6
internal crystal, 5-1–5-10
operation, 5-2–5-3
selecting C and L components, 5-3–
1
1
L
5-6
Latency‚ See Bus hold protocol‚ Direct Memory
Access (DMA) Unit‚ Interrupts
LEAVE instruction, A-7
Local bus, 3-1, 3-39, 10-11
Long integer, defined, 10-7
Long real, defined, 10-7
OUTS instruction, A-2
Overflow Flag (OF), 2-7, 2-9, 2-44
P
Packed BCD, defined, 2-37
Packed decimal, defined, 10-7
Parity Flag (PF), 2-7, 2-9
PCB Relocation Register, 4-1, 4-3, 4-6
and math coprocessing, 10-2
Peripheral Control Block (PCB), 4-1
accessing, 4-4
M
Manuals, online, 1-5
Math coprocessing, 10-1
hardware support, 10-1
overview, 10-1
and DMA Unit, 10-3
and F-Bus operation, 4-5
base address, 4-6–4-7
Memory
addressing, 2-28–2-36
operands, 2-28
bus cycles, 4-4
reserved locations, 2-15
Memory devices‚ interfacing with, 3-6–3-7
Memory segments, 2-8
accessing, 2-5, 2-10, 2-11, 2-13
address
READY signals, 4-4
reserved locations, 4-6
wait states, 4-4
Peripheral control registers, 4-1, 4-6
Pointer, defined, 2-37
Poll register, 8-9, 8-19, 8-20
Poll Status register, 8-9, 8-19, 8-20, 8-21
base value, 2-10, 2-11, 2-12
Effective Address (EA), 2-13
Index-5
INDEX
Polling, 8-1, 8-9
POPA instruction, A-1
Refresh Base Address Register (RFBASE), 7-8
Refresh bus cycle‚ See Bus cycles
Refresh Clock Interval Register (RFTIME), 7-7,
7-8
Refresh Control Register (RFCON), 7-9, 7-10
Refresh Control Unit (RCU), 7-1–7-13
and bus hold protocol, 7-12–7-13
and Powerdown mode, 7-2
and Power-Save mode, 7-2, 7-7
block diagram, 7-1
Power consumption‚ reducing, 3-28
Power management, 5-10–5-14
Power management modes
and HALT bus cycles, 3-30
Powerdown mode, 7-2
Power-Save mode, 5-11–5-14, 7-2
and DRAM refresh rate, 5-13
and refresh interval, 7-7
control register, 5-12
bus latency, 7-7
entering, 5-11
exiting, 5-13
calculating refresh interval, 7-7
control registers, 7-7–7-10
initialization code, 7-10
operation, 7-2
initialization code, 5-13–5-14
Power-Save Register, 5-12
Priority Mask register, 8-17, 8-18, 8-28
Processor control instructions, 2-27
Processor Status Word (PSW), 2-1, 2-7, 2-41
bits defined, 2-7, 2-9
overview, 7-2–7-4
programming, 7-7–7-11
relationship to BIU, 7-1
Register operands, 2-27
Registers, 2-1
flag storage formats, 2-19
reset status, 2-7
control, 2-1
Program transfer instructions, 2-23–2-24
conditional transfers, 2-24, 2-26
interrupts, 2-26
data, 2-4, 2-5
general, 2-1, 2-4, 2-5
H & L group, 2-4
iteration control, 2-25
index, 2-5, 2-13, 2-34
unconditional transfers, 2-24
PUSH instruction, A-8
PUSHA instruction, A-1
P & I group, 2-4
pointer, 2-1, 2-5, 2-13
pointer and index, 2-4
segment, 2-1, 2-5, 2-11, 2-12
status, 2-1
Q
Relocation Register‚ See PCB Relocation Register
Reset
Queue status signals, 3-38
and bus hold protocol, 5-6
and clock synchronization, 5-6–5-10
cold, 5-7, 5-8
RC circuit for reset input, 5-7
warm, 5-7, 5-9
R
RCL instruction, A-10
RCR instruction, A-10
Read bus cycles‚ See Bus cycles
READY
ROL instruction, A-10
ROR instruction, A-10
and chip-selects, 6-15
and normally not-ready signal, 3-17–3-18
and normally ready signal, 3-16–3-17
and PCB accesses, 4-4
and wait states, 3-13–3-18
implementation approaches, 3-13
timing concerns, 3-17
S
SAL instruction, A-9
SAR instruction, A-9
SHL instruction, A-9
Short integer, defined, 10-7
Short real, defined, 10-7
SHR instruction, A-9
Real, defined, 10-7
Real-time clock, code example, 9-17–9-20
Refresh address, 7-4
Index-6
INDEX
SI register, 2-1, 2-5, 2-13, 2-22, 2-23, 2-30, 2-32,
2-34
Sign Flag (SF), 2-7, 2-9
Single-step trap (Type 1 exception), 2-43
Software
Timer Count Registers (TxCNT), 9-10
Timer Counter Unit (TCU), 9-1–9-23
application examples, 9-17–9-23
block diagram, 9-2
clock sources, 9-12
code example
configuring a digital one-shot, 9-17–9-23
configuring a real-time clock, 9-17–9-19
configuring a square-wave generator, 9-17–
9-22
counting sequence, 9-12–9-13
dual maxcount mode, 9-13–9-14
enabling and disabling counters, 9-15–9-16
frequency, maximum, 9-17
initializing, 9-11
80C187 floating-point routine, 10-16
80C187 initialization, 10-13–10-15
digital one-shot, 9-17–9-23
DMA initialization, 10-22–10-27
ICU initialization, 8-31
real-time clock, 9-17–9-19
square-wave generator, 9-17–9-22
TCU configurations, 9-17–9-23
timed DMA transfers, 10-22–10-27
data types, 2-37, 2-38
input synchronization, 9-17
interrupts, 9-16
dynamic code relocation, 2-13, 2-14
interrupts, 2-45
overview, 2-17
overview, 9-1–9-6
programming, 9-6–9-16
considerations, 9-16
See also Addressing modes, Instruction set
Square-wave generator, code example, 9-17–9-22
SRDY, See READY
SS register, 2-1, 2-5, 2-6, 2-13, 2-15, 2-30, 2-45
Stack frame pointers, A-2
Stack Pointer, 2-1, 2-5, 2-13, 2-15, 2-45
Stack segment, 2-5
pulsed output, 9-14–9-15
retriggering, 9-13–9-14
setup and hold times, 9-16
single maxcount mode, 9-13, 9-14–9-16
timer delay, 9-1
timing, 9-1
and BIU, 9-1
Stacks, 2-15
considerations, 9-16
START registers, CSU, 6-6, 6-7
STOP registers, CSU, 6-6, 6-8
String instructions, 2-22–2-23
and addressing modes, 2-34
and memory-mapped I/O ports, 2-36
operand locations, 2-13
TxOUT signal, 9-15
variable duty cycle output, 9-14–9-15
Timer Maxcount Compare Registers (TxCMPA,
TxCMPB), 9-11
Timers‚ See Timer Counter Unit (TCU)
Training, 1-7
operands, 2-36
Trap exceptions, 2-43
Strings
Trap Flag (TF), 2-7, 2-9, 2-43, 2-48
T-state
accessing, 2-13, 2-34
defined, 2-37
and bus cycles, 3-9
Synchronizing asynchronous inputs, B-1
and CLKOUT, 3-8
defined, 3-7
T
W
Wait states
Technical support, 1-6
Temporary real, defined, 10-7
Terminology
and bus cycles, 3-13
and bus ready inputs, 3-13
and chip-selects, 6-15–6-17
and DRAM controllers, 7-1
"above" vs "greater", 2-26
"below" vs "less", 2-26
device names, 1-2
Timer Control Registers (TxCON), 9-7, 9-8
Index-7
INDEX
and PCB accesses, 4-4
and READY input, 3-13
Word integer, defined, 10-7
World Wide Web, 1-6
Write bus cycle, 3-22
Z
Zero Flag (ZF), 2-7, 2-9, 2-23
Index-8
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