6502 Assembly Language Subroutines
For information on translations and book distributors outside of the U.S.A. please write OSBORNE/. McGraw-Hill at the above address. 6502 ASSEMBLY LANGUAGE
Advanced 6502 Assembly Language Programming on the Apple //e
A9 42. LDA #$42. ;Load Accumulator. Immediate. 85 42. STA $42. ;Store Accumulator. Zero Page. 75 42. ADC $42X. ;Add with Carry. Zero Page Indexed by X.
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Programming the 65816
Including the 6502, 65C02 and 65802
Distributed and published under
COPYRIGHT LICENSE AND PUBLISHING AGREEMENT
withAuthors David Eyes and Ron Lichty
EFFECTIVE APRIL 28, 1992
Copyright © 2007 by The Western Design Center, Inc.2166 E. Brown Rd. Mesa, AZ 85213
480-962-4545 (p) 480-835-6442 (f)
www.westerndesigncenter.comThe Western Design Center
2Table of Contents
1)Chapter One..........................................................................................................12
Basic Assembly Language Programming Concepts..................................................................................12
Binary Numbers....................................................................................................................................................12
Grouping Bits into Bytes.......................................................................................................................................13
Hexadecimal Representation of Binary................................................................................................................14
The ACSII Character Set.....................................................................................................................................15
Boolean Logic........................................................................................................................................................16
Logical And........................................................................................................................................................16
Logical Or..........................................................................................................................................................17
Logical Exclusive Or...........................................................................................................................................17
Logical Complement...........................................................................................................................................17
Signed Numbers....................................................................................................................................................18
Storing Numbers in Decimal Form.......................................................................................................................19
Computer Arithmetic............................................................................................................................................20
Microprocessor Programming..............................................................................................................................20
Machine Language..............................................................................................................................................20
Assembly Language............................................................................................................................................22
Writing in Assembly Language............................................................................................................................22
Basic Programming Concepts...............................................................................................................................23
Selection Between Paths......................................................................................................................................24
2)Chapter Two..........................................................................................................26
Architecture of the 6502............................................................................................................................26
Microprocessor Architecture................................................................................................................................26
The 6502 Registers................................................................................................................................................26
The Accumulator................................................................................................................................................27
The X and Y Index Registers...............................................................................................................................29
The Status Register.............................................................................................................................................29
The Stack Pointer................................................................................................................................................31
The Program Counter..........................................................................................................................................33
Addressing Modes.................................................................................................................................................33
The 6502 System Design........................................................................................................................................38
Memory Order of Multiple-Byte Values..............................................................................................................39
Memory-Mapped Input/Output............................................................................................................................39
NMOS Process......................................................................................................................................................40
Bugs and Quirks...................................................................................................................................................40
3)Chapter Three.......................................................................................................41
Architecture of the 65C02.........................................................................................................................41
The 65C02 Architecture........................................................................................................................................41
Addressing Modes.................................................................................................................................................41
CMOS Process......................................................................................................................................................42
Bugs and Quirks...................................................................................................................................................42
4)Chapter Four.........................................................................................................44
Sixteen-Bit Architecture The 65816 and the 65802...................................................................................44
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3Power-On Status: 6502 Emulation Mode..................................................................................................45
The Full-Featured 65x Processor: The 65816 in Native Mode................................................................45
The Program Bank Register.................................................................................................................................47
The Data Bank Register........................................................................................................................................48
The Direct Page Register......................................................................................................................................48
The Stack Pointer..................................................................................................................................................48
Accumulator and Index Registers........................................................................................................................48
Switching Registers Between Eight and Sixteen Bits............................................................................................50
The Status Register...............................................................................................................................................50
6502/65C02 Addressing Modes on the 65816........................................................................................................51
New 65816 Addressing Modes..............................................................................................................................52
The 65802 Native Mode.............................................................................................................................55
Emulation Mode....................................................................................................................................................58
Emulation Mode Registers....................................................................................................................................60
Switching Between 6502 Emulation and Native Modes............................................................................61
Switching from Emulation to Native Mode..........................................................................................................61
Switching from Native to Emulation Mode..........................................................................................................61
65802/65816 Bugs and Quirks...................................................................................................................62
5)Chapter Five..........................................................................................................64
SEP, REP, and Other Details....................................................................................................................64
The Assembler Used in This Book........................................................................................................................66
Address Notation...................................................................................................................................................68
6)Chapter Six............................................................................................................69
First Examples: Moving Data....................................................................................................................69
Loading and Storing Registers.............................................................................................................................71
Effect of Load and Store Operations on Status Flags............................................................................................73
Moving Data Using the Stack..............................................................................................................................73
Pushing the Basic 65x Registers..........................................................................................................................76
Pulling the Basic 65x Registers...........................................................................................................................76
Pushing and Pulling the 65816's Additional Registers..........................................................................................78
Pushing Effective Addresses................................................................................................................................79
Other Attributes of Push and Pull........................................................................................................................79
Moving Data Between Registers...........................................................................................................................79
Storing Zero to Memory.......................................................................................................................................86
Block Moves..........................................................................................................................................................87
7)Chapter Seven.......................................................................................................89
SimpleAddressing Modes..........................................................................................................................89
Immediate Addressing..........................................................................................................................................90
Absolute Addressing.............................................................................................................................................92
Direct Page Addressing.........................................................................................................................................94
Absolute Indexed with X and Absolute Indexed with Y Addressing...................................................................98
Direct Page Indexed with X and Direct Page Indexed with Y Addressing........................................................101
Accumulator Addressing....................................................................................................................................103
Implied Addressing.............................................................................................................................................103
Direct Page Indirect Addressing.........................................................................................................................104
Absolute Long Addressing..................................................................................................................................105
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4Absolute Long Indexed with X Addressing........................................................................................................108
Direct Page Indirect Long...................................................................................................................................109
Block Move..........................................................................................................................................................110
8)Chapter Eight......................................................................................................111
The Flow of Control.................................................................................................................................111
Jump Instructions...............................................................................................................................................112
Conditional Branching........................................................................................................................................114
Branching Based on the Zero Flag.....................................................................................................................115
Branching Based on the Carry Flag...................................................................................................................117
Branching Based on the Negative Flag..............................................................................................................118
Branching Based on the Overflow Flag..............................................................................................................119
Limitations of Conditional Branches..................................................................................................................119
Unconditional Branching....................................................................................................................................119
9)Chapter Nine.......................................................................................................122
Built-In Arithmetic Functions.................................................................................................................122
Increment and Decrement..................................................................................................................................123
Addition and Subtraction: Unsigned Arithmetic..............................................................................................127
Signed Arithmetic...............................................................................................................................................134
Signed Comparisons............................................................................................................................................136
Decimal Mode.....................................................................................................................................................137
10)Chapter Ten......................................................................................................139
Logic and Bit Manipulation Operations.................................................................................................139
Logic Functions...................................................................................................................................................139
Logical AND....................................................................................................................................................140
Logical OR.......................................................................................................................................................142
Logical Exclusive-Or........................................................................................................................................143
Bit Manipulation.................................................................................................................................................145
Shifts and Rotates...............................................................................................................................................146
11)Chapter Eleven.................................................................................................154
The Complex Addressing Modes.............................................................................................................154
Relocating the Direct Page..................................................................................................................................155
Assembler Addressing Mode Assumptions.........................................................................................................156
Direct Page Indirect Indexed With Y Addressing..............................................................................................158
Direct Page Indexing Indirect Addressing.........................................................................................................161
Absolute Indexed Indirect Addressing...............................................................................................................163
Direct Page Indirect Long Indexed with Y Addressing......................................................................................165
Stack Relative Addressing..................................................................................................................................166
Stack Relative Indirect Indexed Addressing......................................................................................................168
Push Effective Instructions.................................................................................................................................169
12)Chapter Twelve.................................................................................................174
The Basic Building Block:.......................................................................................................................174
The Subroutine....................................................................................................................................................174
The Jump-To-Subroutine Instruction................................................................................................................175
The Return-from-Subroutine Instruction..........................................................................................................175
JRS Using Absolute Indexed Indirect Addressing.............................................................................................177
The Long Jump to Subroutine............................................................................................................................178
Return from Subroutine Long............................................................................................................................178
Branch to Subroutine..........................................................................................................................................179
Coding a Subroutine: How and When................................................................................................................180
6502 Eight-Bit Negation - A Library Example..................................................................................................180
65C02, 65802, and 65816 Eight-Bit Negation....................................................................................................181
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56502 Sixteen-Bit Negation................................................................................................................................181
65802 and 65816 Sixteen-Bit Negation..............................................................................................................181
Parameter Passing..............................................................................................................................................182
13)Chapter Thirteen..............................................................................................192
Interrupts and System Control Instructions...........................................................................................192
Processing Interrupts.........................................................................................................................................197
Interrupt Response Time...................................................................................................................................200
Status Register Control Instruction....................................................................................................................201
No Operation Instructions..................................................................................................................................202
14)Chapter Fourteen.............................................................................................204
Selected Code Samples.............................................................................................................................204
6502 Multiplication...........................................................................................................................................205
65C02 Multiplication........................................................................................................................................205
65802 and 65816 Multiplication........................................................................................................................205
6502 Division...................................................................................................................................................209
65C02 Division.................................................................................................................................................211
65802/65816 Division.......................................................................................................................................211
Calling an Arbitrary 6502 Routine.....................................................................................................................213
Testing Processor Type.......................................................................................................................................219
Compiler-Generated 65816 Code for a RecursiveProgram...............................................................................220
The Same Example Hand-Coded in Assembly Language...................................................................................224
The Sieve of Eratosthenes Benchmark...............................................................................................................226
15)Chapter Fifteen................................................................................................230
DEGUG16 - A 65816 Programming Tool...............................................................................................230
16)Chapter Sixteen................................................................................................276
Design and Debugging.............................................................................................................................276
Debugging Checklist...........................................................................................................................................276
Decimal Flag....................................................................................................................................................276
Adjusting Carry Prior to Add / Subtract.............................................................................................................276
65x Left-to-Right Syntax...................................................................................................................................276
65x Branches....................................................................................................................................................277
6502 Jump Bug.................................................................................................................................................277
Interrupt-Handling Code....................................................................................................................................277
65802/65816: Emulation Versus Native Mode...................................................................................................277
65802/65816: Eight-Bit Versus Sixteen-Bit Registers........................................................................................278
65802/65816: The Direct Page..........................................................................................................................278
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665802/65816: Stack Overruns Program or Data.................................................................................................278
65802/65816: JSR/JSL and RTS/RTL................................................................................................................278
65802/65816: MVN/MVP.................................................................................................................................278
Return Address.................................................................................................................................................279
Inconsistent Assembler Syntax..........................................................................................................................279
Generic Bugs: They Can Happen Anywhere......................................................................................................279
Uninitialized Variables......................................................................................................................................279
Missing Code....................................................................................................................................................279
Failure to Increment the Index in a Loop............................................................................................................279
Failure to Clean Up Stack..................................................................................................................................280
Immediate Data Versus Memory Location.........................................................................................................280
Initializing the Stack Pointer from a Subroutine.................................................................................................280
Top-Down Design and Structured Programming..............................................................................................280
17)Chapter Seventeen............................................................................................283
The Addressing Modes............................................................................................................................283
Absolute Addressing.........................................................................................................................................288
Absolute Indexed, X Addressing.......................................................................................................................289
Absolute Indexed, Y Addressing.......................................................................................................................290
Absolute Indexed Indirect Addressing...............................................................................................................291
Absolute Indirect Addressing............................................................................................................................292
Absolute Indirect Long Addressing...................................................................................................................293
JMP [addr]........................................................................................................................................................293
Absolute Long Addressing................................................................................................................................294
Absolute Long Indexed, X Addressing..............................................................................................................295
Accumulator Addressing...................................................................................................................................296
Block Move Addressing....................................................................................................................................297
Direct Page Addressing.....................................................................................................................................298
Direct Page Indexed, X Addressing...................................................................................................................299
Direct Page Indexed, Y Addressing...................................................................................................................300
Direct Page Indexed Indirect, X Addressing.......................................................................................................301
Direct Page Indirect Addressing........................................................................................................................302
Direct Page Indirect Long Addressing...............................................................................................................303
Direct Page Indirect Indexed, Y Addressing.......................................................................................................304
Direct Page Indirect Long Indexed, Y Addressing..............................................................................................305
Immediate Addressing.......................................................................................................................................306
Implied Addressing...........................................................................................................................................307
Program Counter Relative Addressing...............................................................................................................308
Program Counter Relative Long Address...........................................................................................................309
Stack (Absolute) Addressing.............................................................................................................................310
Stack (Direct Page Indirect) Addressing............................................................................................................312
Stack (Interrupt) Addressing..............................................................................................................................314
Stack (Program Counter Relative) Addressing...................................................................................................316
Stack (Pull) Addressing.....................................................................................................................................317
Stack (Push) Addressing....................................................................................................................................319
Stack (RTI) Addressing.....................................................................................................................................321
Stack (RTL) Addressing....................................................................................................................................322
Stack (RTS) Addressing....................................................................................................................................323
Stack Relative Addressing.................................................................................................................................324
Stack Relative Indirect Indexed, Y Addressing..................................................................................................325
18)Chapter Eighteen..............................................................................................326
The Instruction Sets.................................................................................................................................326
Add With Carry..................................................................................................................................................327
And Accumulator with Memory.........................................................................................................................328
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7Shift Memory or Accumulator Left....................................................................................................................329
Branch if Carry Clear.........................................................................................................................................330
Branch if Carry Set.............................................................................................................................................331
Branch if Equal...................................................................................................................................................332
Test Memory Bits against Accumulator.............................................................................................................333
Branch if Minus..................................................................................................................................................334
Branch if Not Equal............................................................................................................................................335
Branch if Plus......................................................................................................................................................336
Branch Always....................................................................................................................................................337
Software Break...................................................................................................................................................338
Branch Always Long...........................................................................................................................................340
Branch if Overflow Clear...................................................................................................................................341
Branch if Overflow Set.......................................................................................................................................342
Clear Carry Flag.................................................................................................................................................343
Clear Decimal Mode Flag...................................................................................................................................344
Clear Interrupt Disable Flag..............................................................................................................................345
Clear Overflow Flag............................................................................................................................................346
Compare Accumulator with Memory.................................................................................................................347
Co-Processor Enable...........................................................................................................................................349
Compare Index Register X with Memory...........................................................................................................350
Compare Index Register Y with Memory CPY..................................................................................................351
Decrement Index Register X...............................................................................................................................353
Decrement Index Register Y...............................................................................................................................354
Exclusive-OR Accumulator with Memory.........................................................................................................355
Increment Index Register X................................................................................................................................358
Increment Index Register Y................................................................................................................................359
Jump to Subroutine Long (Inter-Bank).............................................................................................................361
Jump to Subroutine.............................................................................................................................................362
Load Accumulator from Memory.......................................................................................................................363
Load Index Register X from Memory................................................................................................................364
Load Index Register Y from Memory................................................................................................................365
Logical Shift Memory or Accumulator Right.....................................................................................................366
Block Move Next.................................................................................................................................................367
Block Move Previous...........................................................................................................................................368
No Operation.......................................................................................................................................................369
OR Accumulator with Memory..........................................................................................................................370
Push Effective Absolute Address........................................................................................................................372
Push Effective Indirect Address.........................................................................................................................373
Push Effective PC Relative Indirect Address.....................................................................................................374
Push Accumulator...............................................................................................................................................375
Push Data Bank Register....................................................................................................................................376
Push Direct Page Register...................................................................................................................................377
Push Program Bank Register.............................................................................................................................378
Push Processor Status Register...........................................................................................................................379
Push Index Register............................................................................................................................................380
Push Index Register............................................................................................................................................381
Pull Accumulator................................................................................................................................................382
Pull Data Bank Register......................................................................................................................................383
Pull Direct Page Register....................................................................................................................................384
Pull Status Flags..................................................................................................................................................385
Pull Index Register X from Stack.......................................................................................................................386
Pull Index Register Y from Stack.......................................................................................................................387
Reset Status Bits..................................................................................................................................................388
Rotate Memory or Accumulator Left.................................................................................................................389
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8Rotate Memory or Accumulator Right...............................................................................................................390
Return from Interrupt........................................................................................................................................391
Return from Subroutine Long............................................................................................................................393
Return from Subroutine.....................................................................................................................................394
Subtract with Borrow from Accumulator..........................................................................................................395
Set Carry Flag.....................................................................................................................................................397
Set Decimal Mode Flag.......................................................................................................................................398
Set Interrupt Disable Flag..................................................................................................................................399
Set Status Bits.....................................................................................................................................................400
Store Accumulator to Memory...........................................................................................................................401
Stop the Processor...............................................................................................................................................402
Store Index Register X to Memory.....................................................................................................................403
Store Index Register Y to Memory.....................................................................................................................404
Store Zero to Memory.........................................................................................................................................405
Transfer Accumulator to Index Register X........................................................................................................406
Transfer Accumulator to Index Register Y........................................................................................................407
Transfer 16-Bit Accumulator to Direct Page Register.......................................................................................408
Transfer Accumulator to Stack Pointer.............................................................................................................409
Transfer Direct Page Register to 16-Bit Accumulator.......................................................................................410
Test and Reset Memory Bits Against Accumulator............................................................................................411
Test and Set Memory Bits Against Accumulator...............................................................................................412
Transfer Stack Pointer to 16-Bit Accumulator...................................................................................................413
Transfer Stack Pointer to Index Register X.......................................................................................................414
Transfer Index Register X to Accumulator........................................................................................................415
Transfer Index Register X to Stack Pointer.......................................................................................................416
Transfer Index Register X to Y..........................................................................................................................417
Transfer Index Register Y to Accumulator........................................................................................................418
Transfer Index register Y to X............................................................................................................................419
Wait for Interrupt...............................................................................................................................................420
Reserved for Future Expansion..........................................................................................................................421
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