6502

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The 6502 CPU

6502 info taken from Wikipedia.org

The MOS Technology 6502 is an 8-bit microprocessor designed by Chuck Peddle for MOS Technology in 1975. When it was introduced it was the least expensive full featured CPU on the market by far, at about 1/6th the price, or less, of competing designs from larger companies such as Motorola and Intel. It was nevertheless faster than most of them, and, along with the Zilog Z80, sparked off a series of computer projects that would eventually result in the home computer revolution of the 1980s. The 6502 design was originally second-sourced by Rockwell and Synertek and later licensed to a number of companies; it is still made for embedded systems.

The 6502 was used, among others, for Commodore's 8-bit machines.

Originally the CPC was destined to be designed around the 6502 processor. But when Amstrad approached Locomotive Software to develop a Basic for it with a very tight deadline, Locomotive PLC, who already had a Z80 Basic in the works, urged and convinced Amstrad to switch to the Z80.


Registers

Register Size Description Notes
A (Accumulator) 8-bit Main register for arithmetic, logic, and data transfer Most operations use this register
X (Index Register X) 8-bit Used for indexing memory and loop counters Can be used for addressing modes like Indexed Indirect, Zero Page Indexed, and Absolute Indexed
Y (Index Register Y) 8-bit Used for indexing memory and loop counters Often used in Absolute and Zero Page Indexed addressing
P (Processor Status) 8-bit
  • bit7 - N - Negative Flag
  • bit6 - V - Overflow Flag
  • bit5 - Unused (always set to 1)
  • bit4 - B - Break Command
  • bit3 - D - Decimal Mode Flag
  • bit2 - I - Interrupt Disable Flag
  • bit1 - Z - Zero Flag
  • bit0 - C - Carry Flag
 The status register is modified by arithmetic and logic operations, as well as interrupts
S (Stack Pointer) 8-bit Points to the current location in the stack Stack is located in page 1 ($0100-$01FF), 8-bit S is offset to this base
PC (Program Counter) 16-bit Points to the next instruction to be executed Automatically increments as instructions are executed


6502 Instruction set

Standard instructions

Mnemonic Operation Addressing Modes Flags Description
No arg A #$nn $nnnn $nnnn,X $nnnn,Y ($nnnn) $nn $nn,X $nn,Y ($nn,X) ($nn),Y $nnnn N V - B D I Z C
ADC A + M + C → A, C 69 6D 7D 79 65 75 61 71 N V - - - - Z C Add Memory to Accumulator with Carry
AND A ∧ M → A 29 2D 3D 39 25 35 21 31 N - - - - - Z - "AND" Memory with Accumulator
ASL C ← /M7...M0/ ← 0 0A 0E 1E 06 16 N - - - - - Z C Arithmetic Shift Left
BCC Branch on C = 0 90 - - - - - - - - Branch on Carry Clear
BCS Branch on C = 1 B0 - - - - - - - - Branch on Carry Set
BEQ Branch on Z = 1 F0 - - - - - - - - Branch on Result Zero
BIT A ∧ M, M7 → N, M6 → V 2C 24 N V - - - - Z - Test Bits in Memory with Accumulator
BMI Branch on N = 1 30 - - - - - - - - Branch on Result Minus
BNE Branch on Z = 0 D0 - - - - - - - - Branch on Result Not Zero
BPL Branch on N = 0 10 - - - - - - - - Branch on Result Plus
BRK PC + 2↓, [FFFE] → PCL, [FFFF] → PCH 00 - - - - - 1 - - Break Command
BVC Branch on V = 0 50 - - - - - - - - Branch on Overflow Clear
BVS Branch on V = 1 70 - - - - - - - - Branch on Overflow Set
CLC 0 → C 18 - - - - - - - 0 Clear Carry Flag
CLD 0 → D D8 - - - - 0 - - - Clear Decimal Mode
CLI 0 → I 58 - - - - - 0 - - Clear Interrupt Disable
CLV 0 → V B8 - 0 - - - - - - Clear Overflow Flag
CMP A - M C9 CD DD D9 C5 D5 C1 D1 N - - - - - Z C Compare Memory and Accumulator
CPX X - M E0 EC E4 N - - - - - Z C Compare Index Register X To Memory
CPY Y - M C0 CC C4 N - - - - - Z C Compare Index Register Y To Memory
DEC M - 1 → M CE DE C6 D6 N - - - - - Z - Decrement Memory By One
DEX X - 1 → X CA N - - - - - Z - Decrement Index Register X By One
DEY Y - 1 → Y 88 N - - - - - Z - Decrement Index Register Y By One
EOR A ⊻ M → A 49 4D 5D 59 45 55 41 51 N - - - - - Z - "Exclusive OR" Memory with Accumulator
INC M + 1 → M EE FE E6 F6 N - - - - - Z - Increment Memory By One
INX X + 1 → X E8 N - - - - - Z - Increment Index Register X By One
INY Y + 1 → Y C8 N - - - - - Z - Increment Index Register Y By One
JMP [PC + 1] → PCL, [PC + 2] → PCH 4C 6C - - - - - - - - JMP Indirect
JSR PC + 2↓, [PC + 1] → PCL, [PC + 2] → PCH 20 - - - - - - - - Jump To Subroutine
LDA M → A A9 AD BD B9 A5 B5 A1 B1 N - - - - - Z - Load Accumulator with Memory
LDX M → X A2 AE BE A6 B6 N - - - - - Z - Load Index Register X From Memory
LDY M → Y A0 AC BC A4 B4 N - - - - - Z - Load Index Register Y From Memory
LSR 0 → /M7...M0/ → C 4A 4E 5E 46 56 0 - - - - - Z C Logical Shift Right
NOP No operation EA - - - - - - - - No Operation
ORA A ∨ M → A 09 0D 1D 19 05 15 01 11 N - - - - - Z - "OR" Memory with Accumulator
PHA A↓ 48 - - - - - - - - Push Accumulator On Stack
PHP P↓ 08 - - - - - - - - Push Processor Status On Stack
PLA A↑ 68 N - - - - - Z - Pull Accumulator From Stack
PLP P↑ 28 N V - - D I Z C Pull Processor Status From Stack
ROL C ← /M7...M0/ ← C 2A 2E 3E 26 36 N - - - - - Z C Rotate Left
ROR C → /M7...M0/ → C 6A 6E 7E 66 76 N - - - - - Z C Rotate Right
RTI P↑ PC↑ 40 N V - - D I Z C Return From Interrupt
RTS PC↑, PC + 1 → PC 60 - - - - - - - - Return From Subroutine
SBC A - M - ~C → A E9 ED FD F9 E5 F5 E1 F1 N V - - - - Z C Subtract Memory from Accumulator with Borrow
SEC 1 → C 38 - - - - - - - 1 Set Carry Flag
SED 1 → D F8 - - - - 1 - - - Set Decimal Mode
SEI 1 → I 78 - - - - - 1 - - Set Interrupt Disable
STA A → M 8D 9D 99 85 95 81 91 - - - - - - - - Store Accumulator in Memory
STX X → M 8E 86 96 - - - - - - - - Store Index Register X In Memory
STY Y → M 8C 84 94 - - - - - - - - Store Index Register Y In Memory
TAX A → X AA N - - - - - Z - Transfer Accumulator To Index X
TAY A → Y A8 N - - - - - Z - Transfer Accumulator To Index Y
TSX S → X BA N - - - - - Z - Transfer Stack Pointer To Index X
TXA X → A 8A N - - - - - Z - Transfer Index X To Accumulator
TXS X → S 9A - - - - - - - - Transfer Index X To Stack Pointer
TYA Y → A 98 N - - - - - Z - Transfer Index Y To Accumulator

Illegal instructions

Mnemonic Operation Addressing Mode Flags Description
No arg #$nn $nnnn $nnnn,X $nnnn,Y $nn $nn,X $nn,Y ($nn,X) ($nn),Y N V - B D I Z C
ANC A ∧ M → A, N → C 0B, 2B N - - - - - Z C "AND" Memory with Accumulator then Move Negative Flag to Carry Flag
ARR (A ∧ M) / 2 → A 6B N V - - - - Z C "AND" Accumulator then Rotate Right
ASR (A ∧ M) / 2 → A 4B 0 - - - - - Z C "AND" then Logical Shift Right
DCP M - 1 → M, A - M CF DF DB C7 D7 C3 D3 N - - - - - Z C Decrement Memory By One then Compare with Accumulator
ISC M + 1 → M, A - M → A EF FF FB E7 F7 E3 F3 N V - - - - Z C Increment Memory By One then SBC then Subtract Memory from Accumulator with Borrow
JAM Stop execution 02, 12, 22, 32, 42, 52, 62, 72, 92, B2, D2, F2 - - - - - - - - Halt the CPU
LAS M ∧ S → A, X, S BB N - - - - - Z - "AND" Memory with Stack Pointer
LAX M → A, X AB AF BF A7 B7 A3 B3 N - - - - - Z - Load Accumulator and Index Register X From Memory
NOP No operation 1A, 3A, 5A, 7A, DA, FA 80, 82, 89, C2, E2 0C 1C, 3C, 5C, 7C, DC, FC 04, 44, 64 14, 34, 54, 74, D4, F4 - - - - - - - - No Operation
RLA C ← /M7...M0/ ← C, A ∧ M → A 2F 3F 3B 27 37 23 33 N - - - - - Z C Rotate Left then "AND" with Accumulator
RRA C → /M7...M0/ → C, A + M + C → A 6F 7F 7B 67 77 63 73 N V - - - - Z C Rotate Right and Add Memory to Accumulator
SAX A ∧ X → M 8F 87 97 83 - - - - - - - - Store Accumulator "AND" Index Register X in Memory
SBC A - M - ~C → A EB N V - - - - Z C Subtract Memory from Accumulator with Borrow
SBX (A ∧ X) - M → X CB N - - - - - Z C Subtract Memory from Accumulator "AND" Index Register X
SHA A ∧ X ∧ V → M 9F 93 - - - - - - - - Store Accumulator "AND" Index Register X "AND" Value
SHS A ∧ X → S, S ∧ (H + 1) → M 9B - - - - - - - - Transfer Accumulator "AND" Index Register X to Stack Pointer then Store Stack Pointer "AND" Hi-Byte In Memory
SHX X ∧ (H + 1) → M 9E - - - - - - - - Store Index Register X "AND" Value
SHY Y ∧ (H + 1) → M 9C - - - - - - - - Store Index Register Y "AND" Value
SLO M * 2 → M, A ∨ M → A 0F 1F 1B 07 17 03 13 N - - - - - Z C Arithmetic Shift Left then "OR" Memory with Accumulator
SRE M / 2 → M, A ⊻ M → A 4F 5F 5B 47 57 43 53 N - - - - - Z C Logical Shift Right then "Exclusive OR" Memory with Accumulator
XAA (A ∨ V) ∧ X ∧ M → A 8B N - - - - - Z - Non-deterministic Operation of Accumulator, Index Register X, Memory and Bus Contents


Official opcode matrix

Addressing modes: A - accumulator, # - immediate, zpg - zero page, abs - absolute, ind - indirect, X - indexed by X register, Y - indexed by Y register, rel - relative
High nibble Low nibble
0 1 2 4 5 6 8 9 A C D E
0 BRK ORA (ind,X) ORA zpg ASL zpg PHP ORA # ASL A ORA abs ASL abs
1 BPL rel ORA (ind),Y ORA zpg,X ASL zpg,X CLC ORA abs,Y ORA abs,X ASL abs,X
2 JSR abs AND (ind,X) BIT zpg AND zpg ROL zpg PLP AND # ROL A BIT abs AND abs ROL abs
3 BMI rel AND (ind),Y AND zpg,X ROL zpg,X SEC AND abs,Y AND abs,X ROL abs,X
4 RTI EOR (ind,X) EOR zpg LSR zpg PHA EOR # LSR A JMP abs EOR abs LSR abs
5 BVC rel EOR (ind),Y EOR zpg,X LSR zpg,X CLI EOR abs,Y EOR abs,X LSR abs,X
6 RTS ADC (ind,X) ADC zpg ROR zpg PLA ADC # ROR A JMP (ind) ADC abs ROR abs
7 BVS rel ADC (ind),Y ADC zpg,X ROR zpg,X SEI ADC abs,Y ADC abs,X ROR abs,X
8 STA (ind,X) STY zpg STA zpg STX zpg DEY TXA STY abs STA abs STX abs
9 BCC rel STA (ind),Y STY zpg,X STA zpg,X STX zpg,Y TYA STA abs,Y TXS STA abs,X
A LDY # LDA (ind,X) LDX # LDY zpg LDA zpg LDX zpg TAY LDA # TAX LDY abs LDA abs LDX abs
B BCS rel LDA (ind),Y LDY zpg,X LDA zpg,X LDX zpg,Y CLV LDA abs,Y TSX LDY abs,X LDA abs,X LDX abs,Y
C CPY # CMP (ind,X) CPY zpg CMP zpg DEC zpg INY CMP # DEX CPY abs CMP abs DEC abs
D BNE rel CMP (ind),Y CMP zpg,X DEC zpg,X CLD CMP abs,Y CMP abs,X DEC abs,X
E CPX # SBC (ind,X) CPX zpg SBC zpg INC zpg INX SBC # NOP CPX abs SBC abs INC abs
F BEQ rel SBC (ind),Y SBC zpg,X INC zpg,X SED SBC abs,Y SBC abs,X INC abs,X
Blank opcodes (e.g., F2) and all opcodes whose low nibbles are 3, 7, B and F are undefined in the official 6502 instruction set.


Opcode matrix including illegal opcodes

High nibble Low nibble
0 1 2 3 4 5 6 7 8 9 A B C D E F
0 BRK ORA (ind,X) JAM SLO (ind,X) NOP zpg ORA zpg ASL zpg SLO zpg PHP ORA # ASL A ANC # NOP abs ORA abs ASL abs SLO abs
1 BPL rel ORA (ind),Y JAM SLO (ind),Y NOP zpg,X ORA zpg,X ASL zpg,X SLO zpg,X CLC ORA abs,Y NOP SLO abs,Y NOP abs,X ORA abs,X ASL abs,X SLO abs,X
2 JSR abs AND (ind,X) JAM RLA (ind,X) BIT zpg AND zpg ROL zpg RLA zpg PLP AND # ROL A ANC # BIT abs AND abs ROL abs RLA abs
3 BMI rel AND (ind),Y JAM RLA (ind),Y NOP zpg,X AND zpg,X ROL zpg,X RLA zpg,X SEC AND abs,Y NOP RLA abs,Y NOP abs,X AND abs,X ROL abs,X RLA abs,X
4 RTI EOR (ind,X) JAM SRE (ind,X) NOP zpg EOR zpg LSR zpg SRE zpg PHA EOR # LSR A ALR # JMP abs EOR abs LSR abs SRE abs
5 BVC rel EOR (ind),Y JAM SRE (ind),Y NOP zpg,X EOR zpg,X LSR zpg,X SRE zpg,X CLI EOR abs,Y NOP SRE abs,Y NOP abs,X EOR abs,X LSR abs,X SRE abs,X
6 RTS ADC (ind,X) JAM RRA (ind,X) NOP zpg ADC zpg ROR zpg RRA zpg PLA ADC # ROR A ARR # JMP (ind) ADC abs ROR abs RRA abs
7 BVS rel ADC (ind),Y JAM RRA (ind),Y NOP zpg,X ADC zpg,X ROR zpg,X RRA zpg,X SEI ADC abs,Y NOP RRA abs,Y NOP abs,X ADC abs,X ROR abs,X RRA abs,X
8 NOP # STA (ind,X) NOP # SAX (ind,X) STY zpg STA zpg STX zpg SAX zpg DEY NOP # TXA ANE # STY abs STA abs STX abs SAX abs
9 BCC rel STA (ind),Y JAM SHA (ind),Y STY zpg,X STA zpg,X STX zpg,Y SAX zpg,Y TYA STA abs,Y TXS TAS abs,Y SHY abs,X STA abs,X SHX abs,Y SHA abs,Y
A LDY # LDA (ind,X) LDX # LAX (ind,X) LDY zpg LDA zpg LDX zpg LAX zpg TAY LDA # TAX LXA # LDY abs LDA abs LDX abs LAX abs
B BCS rel LDA (ind),Y JAM LAX (ind),Y LDY zpg,X LDA zpg,X LDX zpg,Y LAX zpg,Y CLV LDA abs,Y TSX LAS abs,Y LDY abs,X LDA abs,X LDX abs,Y LAX abs,Y
C CPY # CMP (ind,X) NOP # DCP (ind,X) CPY zpg CMP zpg DEC zpg DCP zpg INY CMP # DEX AXS # CPY abs CMP abs DEC abs DCP abs
D BNE rel CMP (ind),Y JAM DCP (ind),Y NOP zpg,X CMP zpg,X DEC zpg,X DCP zpg,X CLD CMP abs,Y NOP DCP abs,Y NOP abs,X CMP abs,X DEC abs,X DCP abs,X
E CPX # SBC (ind,X) NOP # ISC (ind,X) CPX zpg SBC zpg INC zpg ISC zpg INX SBC # NOP SBC # CPX abs SBC abs INC abs ISC abs
F BEQ rel SBC (ind),Y JAM ISC (ind),Y NOP zpg,X SBC zpg,X INC zpg,X ISC zpg,X SED SBC abs,Y NOP ISC abs,Y NOP abs,X SBC abs,X INC abs,X ISC abs,X


Block Diagram

Mcs6502-block-diagram.svg


Links

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