This book is a guide to the 6502 Assembly language. This book will teach the different memory addressing modes and instructions of the 8-bit 6502 processor.
You might want to learn 6502 assembly language programming if you want to do Atari 2600/8-bit family/5200/7800 Programming, Commodore PET/VIC/64/128 Programming, Acorn 8 Bit Programming, Apple I/II Programming, NES Programming or Super NES Programming.
Syntax
Syntax will vary between assemblers - this book will use the following syntax throughout:
| Syntax | Base | Example |
|---|---|---|
%00001111 | Binary | LDA #%0001 |
$FA | Hexadecimal | LDA #$0E |
123 | Decimal | LDA #100 |
Registers
| Register | Size (bits) | Purpose |
|---|---|---|
| Accumulator (A) | 8 | Used to perform calculations on data. Instructions can operate directly on the accumulator instead of spending CPU cycles to access memory |
| X register (X) | 8 | Used as an index in some addressing modes |
| Y register (Y) | 8 | Used as an index in some addressing modes |
| Program Counter (PC) | 16 | Points to the address of the next instruction to be executed |
| Stack Pointer (SP) | 8 | Stores the stack index into which the next stack element will be inserted. The address of this position is $0100 + SP. SP is initially set to $FD |
| Status (SR) | 8 | Each bit represents a status flag. Flags indicate the state of the CPU, or information about the result of the previous instruction. See the table below for a description of each flag |
| Bit | Symbol | Name | Description |
|---|---|---|---|
| 7 | N | Negative |
Compare: Set if the register's value is less than the input value |
| 6 | V | Overflow |
Arithmetic: Set if a signed overflow occurred during addition or subtraction, i.e. the sign of the result differs from the sign of both the input and the accumulator |
| 5 | - | (Unused) | Always set |
| 4 | B[1] | Break | Set if an interrupt request has been triggered by a BRK instruction |
| 3 | D | Decimal | Decimal mode: mathematical instructions will treat the inputs and outputs as decimal numbers. E.g. $09 + $01 = $10 |
| 2 | I | Interrupt Disable | Disables interrupts while set |
| 1 | Z | Zero |
Compare: Set if the register's value is equal to the input value |
| 0 | C | Carry | Carry/Borrow flag used in math and rotate operations
Arithmetic: Set if an unsigned overflow occurred during addition or subtraction, i.e. the result is less than the initial value |
Memory layout
16-bit values are stored in memory in little-endian, so the least significant byte is stored before the most significant. E.g. if address $0000 contains $FF and address $0001 contains $00, reading a two-byte value from $0000 will result in $00FF.
Signed integers are in two's complement and can represent values from -128 (%11111111) to +127 (%01111111). Bit 7 is set if the integer is negative.
The 6502's program counter is 16 bits wide, so up to 2^16 (65536) bytes of memory are addressable. Certain regions of memory are reserved for particular purposes:
| Region | Contents | Description |
|---|---|---|
$0000 - $00FF |
Zero page | The first page of memory, which is faster to access than other pages. Instructions can specify addresses within the zero page with a single byte as opposed to two, so instructions that use the zero page instead of any other page require one less CPU cycle to execute |
$0100 - $01FF |
Stack | Last-in first-out data structure. Grows backwards from $01FF to $0100.Used by some transfer, stack, and subroutine instructions |
$0200 - $FFFF |
General-purpose | Memory that can be used for whatever purpose needed. Devices that use the 6502 processor may choose to reserve sub-regions for other purposes, such as memory-mapped I/O |
Memory Addressing Modes
Each instruction uses one of thirteen memory addressing modes, which determines the operand of the instruction. An example is provided for each.
Accumulator: A
The Accumulator is implied as the operand, so no address needs to be specified.
Example
Using the ASL (Arithmetic Shift Left) instruction with no operands, the Accumulator is always the value being shifted left.
ASL
Implied: i
The operand is implied, so it does not need to be specified.
Example
The operands being implied here are X, the source of the transfer, and A, the destination of the transfer.
TXA
Immediate: #
The operand is used directly to perform the computation.
Example
The value $22 is loaded into the Accumulator.
LDA #$22
Absolute: a
A full 16-bit address is specified and the byte at that address is used to perform the computation.
Example
The value at address $D010 is loaded into the X register.
LDX $D010
Zero Page: zp
A single byte specifies an address in the first page of memory ($00xx), also known as the zero page, and the byte at that address is used to perform the computation.
Example
The value at address $0002 is loaded into the Y register.
LDY $02
Relative: r
The offset specified is added to the current address stored in the Program Counter (PC). Offsets can range from -128 to +127.
Example
The offset $2D is added to the address in the Program Counter (say $C100). The destination of the branch (if taken) will be $C12D.
BPL $2D
Absolute Indirect: (a)
The little-endian two-byte value stored at the specified address is used to perform the computation. Only used by the JMP instruction.
Example
The addresses $A001 and $A002 are read, returning $FF and $00 respectively. The address $00FF is then jumped to.
JMP ($A001)
Absolute Indexed with X: a,x
The value in X is added to the specified address for a sum address. The value at the sum address is used to perform the computation.
Example
The value $02 in X is added to $C001 for a sum of $C003. The value $5A at address $C003 is used to perform the add with carry (ADC) operation.
ADC $C001,X
Absolute Indexed with Y: a,y
The value in Y is added to the specified address for a sum address. The value at the sum address is used to perform the computation.
Example
The value $03 in Y is added to $F001 for a sum of $F004. The value $EF at address $F004 is incremented (INC) and $F0 is written back to $F004.
INC $F001,Y
Zero Page Indexed with X: zp,x
The value in X is added to the specified zero page address for a sum address. The value at the sum address is used to perform the computation.
Example
The value $02 in X is added to $01 for a sum of $03. The value $A5 at address $0003 is loaded into the Accumulator.
LDA $01,X
Zero Page Indexed with Y: zp,y
The value in Y is added to the specified zero page address for a sum address. The value at the sum address is used to perform the computation.
Example
The value $03 in Y is added to $01 for a sum of $04. The value $E3 at address $0004 is loaded into the Accumulator.
LDA $01,Y
Zero Page Indexed Indirect: (zp,x)
The value in X is added to the specified zero page address for a sum address. The little-endian address stored at the two-byte pair of sum address (LSB) and sum address plus one (MSB) is loaded and the value at that address is used to perform the computation.
Example
The value $02 in X is added to $15 for a sum of $17. The address $D010 at addresses $0017 and $0018 will be where the value $0F in the Accumulator is stored.
STA ($15,X)
Zero Page Indirect Indexed with Y: (zp),y
The value in Y is added to the address at the little-endian address stored at the two-byte pair of the specified address (LSB) and the specified address plus one (MSB). The value at the sum address is used to perform the computation. Indeed addressing mode actually repeats exactly the Accumulator register's digits.
Example
The value $03 in Y is added to the address $C235 at addresses $002A and $002B for a sum of $C238. The Accumulator is then exclusive ORed with the value $2F at $C238.
EOR ($2A),Y
Instructions
These are the instructions for the 6502 processor including an ASCII visual, a list of affected flags, and a table of opcodes for acceptable addressing modes.
Load and Store
Load Accumulator with Memory: LDA |
Load Index X with Memory: LDX |
Load Index Y with Memory: LDY | ||||||||||||||||||||||||||||||||||||||||||
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M -> A Flags: N, Z |
M -> X Flags: N, Z |
M -> Y Flags: N, Z | ||||||||||||||||||||||||||||||||||||||||||
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Store Accumulator in Memory: STA |
Store Index X in Memory: STX |
Store Index Y in Memory: STY | ||||||||||||||||||||||||||||||||||||||||||
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A -> M Flags: none |
X -> M Flags: none |
Y -> M Flags: none | ||||||||||||||||||||||||||||||||||||||||||
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Arithmetic
Add Memory to Accumulator with Carry: ADC |
Subtract Memory from Accumulator with Borrow: SBC | ||||||||||||||||||||||||||||||||||||
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A + M + C -> A Flags: N, V, Z, C |
A - M - ~C -> A Flags: N, V, Z, C | ||||||||||||||||||||||||||||||||||||
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Increment and Decrement
Increment Memory by One: INC |
Increment Index X by One: INX |
Increment Index Y by One: INY | ||||||||||||||||||
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M + 1 -> M Flags: N, Z |
X + 1 -> X Flags: N, Z |
Y + 1 -> Y Flags: N, Z | ||||||||||||||||||
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| ||||||||||||||||||
Decrement Memory by One: DEC |
Decrement Index X by One: DEX |
Decrement Index Y by One: DEY | ||||||||||||||||||
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M - 1 -> M Flags: N, Z |
X - 1 -> X Flags: N, Z |
Y - 1 -> Y Flags: N, Z | ||||||||||||||||||
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Shift and Rotate
Arithmetic Shift Left One Bit: ASL |
Logical Shift Right One Bit: LSR | ||||||||||||||||||||||||
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C <- 7 6 5 4 3 2 1 0 <- 0 Flags: N, Z, C |
0 -> 7 6 5 4 3 2 1 0 -> C Flags: N, Z, C | ||||||||||||||||||||||||
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Rotate Left One Bit: ROL |
Rotate Right One Bit: ROR | ||||||||||||||||||||||||
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C <- 7 6 5 4 3 2 1 0 <- C Flags: N, Z, C |
C -> 7 6 5 4 3 2 1 0 -> C Flags: N, Z, C | ||||||||||||||||||||||||
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Logic
AND Memory with Accumulator: AND |
OR Memory with Accumulator: ORA | ||||||||||||||||||||||||||||||||||||
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A & M -> A Flags: N, Z |
A | M -> A Flags: N, Z | ||||||||||||||||||||||||||||||||||||
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Exclusive-OR Memory with Accumulator: EOR
A ^ M -> A
Flags: N, Z
| Addressing Mode | Opcode |
|---|---|
| a | 4D |
| a,x | 5D |
| a,y | 59 |
| # | 49 |
| zp | 45 |
| (zp,x) | 41 |
| zp,x | 55 |
| (zp),y | 51 |
Compare and Test Bit
The Negative (N), Zero (Z), and Carry (C) status flags are used for conditional (branch) instructions.
All Compare instructions affect flags in the same way:
| Condition | N | Z | C |
|---|---|---|---|
| Register < Memory | 1 | 0 | 0 |
| Register = Memory | 0 | 1 | 1 |
| Register > Memory | 0 | 0 | 1 |
Compare Memory and Accumulator: CMP |
Compare Memory and Index X: CPX |
Compare Memory with Index Y: CPY | ||||||||||||||||||||||||||||||||||
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A - M Flags: N, Z, C |
X - M Flags: N, Z, C |
Y - M Flags: N, Z, C | ||||||||||||||||||||||||||||||||||
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Test Bits in Memory with Accumulator: BIT
A & M
Flags: N = M7, V = M6, Z
| Addressing Mode | Opcode |
|---|---|
| a | 2C |
| # | 89 |
| zp | 24 |
Branch
Branch on Carry Clear: BCC |
Branch on Carry Set: BCS | ||||||||
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Branch if C = 0 Flags: none |
Branch if C = 1 Flags: none | ||||||||
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Branch on Result not Zero: BNE |
Branch on Result Zero: BEQ | ||||||||
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Branch if Z = 0 Flags: none |
Branch if Z = 1 Flags: none | ||||||||
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Branch on Result Plus: BPL |
Branch on Result Minus: BMI | ||||||||
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Branch if N = 0 Flags: none |
Branch if N = 1 Flags: none | ||||||||
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Branch on Overflow Clear: BVC |
Branch on Overflow Set: BVS | ||||||||
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Branch if V = 0 Flags: none |
Branch if V = 1 Flags: none | ||||||||
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Transfer
Transfer Accumulator to Index X: TAX |
Transfer Index X to Accumulator: TXA | ||||||||
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A -> X Flags: N, Z |
X -> A Flags: N, Z | ||||||||
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Transfer Accumulator to Index Y: TAY |
Transfer Index Y to Accumulator: TYA | ||||||||
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A -> Y Flags: N, Z |
Y -> A Flags: N, Z | ||||||||
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Transfer Stack Pointer to Index X: TSX |
Transfer Index X to Stack Pointer: TXS | ||||||||
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S -> X Flags: N, Z |
X -> S Flags: none | ||||||||
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Stack
Push Accumulator on Stack: PHA |
Pull Accumulator from Stack: PLA | ||||||||
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A -> S Flags: none |
S -> A Flags: N, Z | ||||||||
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Push Processor Status on Stack: PHP |
Pull Processor Status from Stack: PLP | ||||||||
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P -> S Flags: none |
S -> P Flags: all | ||||||||
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The processor status is stored as a single byte with the following flags bits from high to low: NV--DIZC.
Subroutines and Jump
Jump to New Location: JMP
Jump to new location by changing the value of the program counter.
Warning: When used with the absolute indirect addressing mode, a hardware bug can result in unexpected behavior when the specified address is $xxFF.
E.g. JMP $(11FF) will read the low byte from $11FF and the high byte from $1100, instead of reading the high byte from $1200 as one would expect. This is due to an overflow in the lower byte of the indirect address not being carried into the upper byte.
Flags: none
| Addressing Mode | Opcode |
|---|---|
| a | 4C |
| (a) | 6C |
Jump to New Location Saving Return Address: JSR
Jumps to a subroutine
The address before the next instruction (PC - 1) is pushed onto the stack: first the upper byte followed by the lower byte. As the stack grows backwards, the return address is therefore stored as a little-endian number in memory.
PC is set to the target address.
Flags: none
| Addressing Mode | Opcode |
|---|---|
| a | 20 |
Return from Subroutine: RTS
Return from a subroutine to the point where it called with JSR.
The return address is popped from the stack (low byte first, then high byte).
The return address is incremented and stored in PC.
Flags: none
| Addressing Mode | Opcode |
|---|---|
| i | 60 |
Return from Interrupt: RTI
Return from an interrupt.
SR is popped from the stack.
PC is popped from the stack.
Flags: all
| Addressing Mode | Opcode |
|---|---|
| i | 40 |
Set and Clear
Clear Carry Flag: CLC |
Set Carry Flag: SEC | ||||||||
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0 -> C Flags: C = 0 |
1 -> C Flags: C = 1 | ||||||||
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Clear Decimal Mode: CLD |
Set Decimal Mode: SED | ||||||||
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0 -> D Flags: D = 0 |
1 -> D Flags: D = 1 | ||||||||
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Clear Interrupt Disable Status: CLI |
Set Interrupt Disable Status: SEI | ||||||||
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0 -> I Flags: I = 0 |
1 -> I Flags: I = 1 | ||||||||
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Clear Overflow Flag: CLV |
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0 -> V Flags: V = 0 |
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Miscellaneous
Break: BRK
Force an Interrupt
Flags: B = 1, I = 1
| Addressing Mode | Opcode |
|---|---|
| i | 00 |
No Operation: NOP
No Operation
Flags: none
| Addressing Mode | Opcode |
|---|---|
| i | EA |
Instruction table
| High nibble | Low nibble | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 0A | 0B | 0C | 0D | 0E | 0F | |
| 00 | BRK i | ORA (zp,x) | ORA zp | ASL zp | PHP i | ORA # | ASL A | ORA a | ASL a | |||||||
| 10 | BPL r | ORA (zp),y | ORA zp,x | ASL zp,x | CLC i | ORA a,y | ORA a,x | ASL a,x | ||||||||
| 20 | JSR a | AND (zp,x) | BIT zp | AND zp | ROL zp | PLP i | AND # | ROL A | BIT a | AND a | ROL a | |||||
| 30 | BMI r | AND (zp),y | AND zp,x | ROL zp,x | SEC i | AND a,y | AND a,x | ROL a,x | ||||||||
| 40 | RTI i | EOR (zp,x) | EOR zp | LSR zp | PHA i | EOR # | LSR A | JMP a | EOR a | LSR a | ||||||
| 50 | BVC r | EOR (zp),y | EOR zp,x | LSR zp,x | CLI i | EOR a,y | EOR a,x | LSR a,x | ||||||||
| 60 | RTS i | ADC (zp,x) | ADC zp | ROR zp | PLA i | ADC # | ROR A | JMP (a) | ADC a | ROR a | ||||||
| 70 | BVS r | ADC (zp),y | ADC zp,x | ROR zp,x | SEI i | ADC a,y | ADC a,x | ROR a,x | ||||||||
| 80 | STA (zp,x) | STY zp | STA zp | STX zp | DEY i | TXA i | STY a | STA a | STX a | |||||||
| 90 | BCC r | STA (zp),y | STY zp,x | STA zp,x | STX zp,y | TYA i | STA a,y | TXS i | STA a,x | |||||||
| A0 | LDY # | LDA (zp,x) | LDX # | LDY zp | LDA zp | LDX zp | TAY i | LDA # | TAX i | LDY a | LDA a | LDX a | ||||
| B0 | BCS r | LDA (zp),y | LDY zp,x | LDA zp,x | LDX zp,y | CLV i | LDA a,y | TSX i | LDY a,x | LDA a,x | LDX a,y | |||||
| C0 | CPY # | CMP (zp,x) | CPY zp | CMP zp | DEC zp | INY i | CMP # | DEX i | CPY a | CMP a | DEC a | |||||
| D0 | BNE r | CMP (zp),y | CMP zp,x | DEC zp,x | CLD i | CMP a,y | CMP a,x | DEC a,x | ||||||||
| E0 | CPX # | SBC (zp,x) | CPX zp | SBC zp | INC zp | INX i | SBC # | NOP i | CPX a | SBC a | INC a | |||||
| F0 | BEQ r | SBC (zp),y | SBC zp,x | INC zp,x | SED i | SBC a,y | SBC a,x | INC a,x | ||||||||
References
- ↑ , The B flag does not represent an actual CPU register
Further reading
- Owad, Tom, "Apple I Replica Creation", Syngress, 2005. ISBN 193183640X
- 6502.org the 6502 microprocessor resource, particularly the Tutorials and Primers page.
- NES Programming: The Nintendo Entertainment System uses a version of the 6502
- Super NES Programming: the Super NES uses the 65c816, a descendant of the 6502
- History of Apple Inc.: early Apple computers all used some version of the 6502
- History of Computers/The Rise of the Microcomputer
- X86 Disassembly/Disassemblers and Decompilers#Disassembly of 8 bit CPU code mentions some 6502 disassemblers
- Computer Programming/Hello world#Accumulator .2B index register machine: MOS Technology 6502.2C CBM KERNEL.2C MOS assembler syntax