# 6502 Assembly

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:
Numerical representations :
| Syntax | Base | Example |
| %00001111 | Binary | LDA #%0001 |
| $FA | Hexadecimal | LDA #$0E |
| 123 | Decimal | LDA #100 |

## Registers

Registers :
| Register | Size (bits) | Purpose |
| Accumulator (A) | 8 | Used to perform calculations on data. Most 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 (S) | 8 | Stores the stack index into which the next stack element will be written. The address of this position is $0100 + SP. SP is initially set to $FD. TSX and TXS are the only instructions that let you directly modify S. |
| Status (P) | 8 | Each bit represents a status flag. Flags indicate the state of the CPU, or information about the result of the previous instruction. PHP and PLP can save/restore P from the stack. Various instructions can directly set or clear bits in P: SEC, CLC, SEI, CLI, SED, CLD, CLV. See the table below for a description of each flag. |
Status Flags :
| Bit | Symbol | Name | Description |
| 7 | N | Negative | Compare: Set if the register's value is less than the input value Otherwise: Set if the result was negative, i.e. bit 7 of the result was set. |
| 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. BIT: Set to bit 6 of the input. Other: The original 6502 has an external pin called "SO" (Set Overflow) that hardware can use to make the V flag set. The purpose is to react to hardware events quicker than an IRQ. Most common 6502 compatible platforms do not have anything that uses this feature, or do not use it. |
| 5 | - | (Unused) | Always set |
| 4 | B | 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 "Binary Coded Decimal" (BCD) numbers. E.g. $09 + $01 = $10 NOTE: The 6502-compatible CPU in the NES does not implement decimal mode, so decimal mode will not change the behavior on that CPU only, but you can still clear/set the flag. |
| 2 | I | Interrupt Disable | Disables IRQ interrupts while set. NMIs and RESETs are not affected. |
| 1 | Z | Zero | Compare: Set if the register's value is equal to the input value BIT: Set if the result of logically ANDing the accumulator with the input results in 0. Otherwise: Set if result was zero. NOTE: Compare (CMP, CPX, CPY) instructions work by subtracting, but not keeping the result. Therefore Z will be set if the value is 0, and therefore that's why BEQ tests the Z flag, and that's why you do not have to CMP #0 before BEQ. |
| 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 (or equal to the initial value, if the carry flag was set going in) Compare: Set if register's value is greater than or equal to the input value Shifting: Set to the value of the eliminated bit of the input, i.e. bit 7 when shifting left, or bit 0 when shifting right |

## 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 (%10000000) 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:
Memory regions :
| 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.

### 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.

### Immediate: #

The operand is used directly to perform the computation.
Example
The value $22 is loaded into the Accumulator.

### Absolute: a

A full 16-bit address is specified and the byte at that address is used to perform the computation.
Example
The value $24 at address $D010 is loaded into the X register.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

## 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 |
| M -> A Flags: N, Z | M -> X Flags: N, Z | M -> Y Flags: N, Z |
| Addressing ModeOpcode aAD a,xBD a,yB9 #A9 zpA5 (zp,x)A1 zp,xB5 (zp),yB1 | Addressing ModeOpcode aAE a,yBE #A2 zpA6 zp,yB6 | Addressing ModeOpcode aAC a,xBC #A0 zpA4 zp,xB4 |
| Store Accumulator in Memory: STA | Store Index X in Memory: STX | Store Index Y in Memory: STY |
| A -> M Flags: none | X -> M Flags: none | Y -> M Flags: none |
| Addressing ModeOpcode a8D a,x9D a,y99 zp85 (zp,x)81 zp,x95 (zp),y91 | Addressing ModeOpcode a8E zp86 zp,y96 | Addressing ModeOpcode a8C zp84 zp,x94 |

### Arithmetic

| Add Memory to Accumulator with Carry: ADC | Subtract Memory from Accumulator with Borrow: SBC |
| A + M + C -> A Flags: N, V, Z, C | A - M - ~C -> A Flags: N, V, Z, C |
| Addressing ModeOpcode a6D a,x7D a,y79 #69 zp65 (zp,x)61 zp,x75 (zp),y71 | Addressing ModeOpcode aED a,xFD a,yF9 #E9 zpE5 (zp,x)E1 zp,xF5 (zp),yF1 |

### Increment and Decrement

| Increment Memory by One: INC | Increment Index X by One: INX | Increment Index Y by One: INY |
| M + 1 -> M Flags: N, Z | X + 1 -> X Flags: N, Z | Y + 1 -> Y Flags: N, Z |
| Addressing ModeOpcode aEE a,xFE zpE6 zp,xF6 | Addressing ModeOpcode iE8 | Addressing ModeOpcode iC8 |
| Decrement Memory by One: DEC | Decrement Index X by One: DEX | Decrement Index Y by One: DEY |
| M - 1 -> M Flags: N, Z | X - 1 -> X Flags: N, Z | Y - 1 -> Y Flags: N, Z |
| Addressing ModeOpcode aCE a,xDE zpC6 zp,xD6 | Addressing ModeOpcode iCA | Addressing ModeOpcode i88 |

### Shift and Rotate

| Arithmetic Shift Left One Bit: ASL | Logical Shift Right One Bit: LSR |
| 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 |
| Addressing ModeOpcode a0E a,x1E A0A zp06 zp,x16 | Addressing ModeOpcode a4E a,x5E A4A zp46 zp,x56 |
| Rotate Left One Bit: ROL | Rotate Right One Bit: ROR |
| 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 |
| Addressing ModeOpcode a2E a,x3E A2A zp26 zp,x36 | Addressing ModeOpcode a6E a,x7E A6A zp66 zp,x76 |

### Logic

| AND Memory with Accumulator: AND | OR Memory with Accumulator: ORA | Exclusive-OR Memory with Accumulator: EOR |
| A & M -> A Flags: N, Z | A | M -> A Flags: N, Z | A ^ M -> A Flags: N, Z |
| Addressing ModeOpcode a2D a,x3D a,y39 #29 zp25 (zp,x)21 zp,x35 (zp),y31 | Addressing ModeOpcode a0D a,x1D a,y19 #09 zp05 (zp,x)01 zp,x15 (zp),y11 | Addressing ModeOpcode a4D a,x5D a,y59 #49 zp45 (zp,x)41 zp,x55 (zp),y51 |

### 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 |
| A - M Flags: N, Z, C | X - M Flags: N, Z, C | Y - M Flags: N, Z, C |
| Addressing ModeOpcode aCD a,xDD a,yD9 #C9 zpC5 (zp,x)C1 zp,xD5 (zp),yD1 | Addressing ModeOpcode aEC #E0 zpE4 | Addressing ModeOpcode aCC #C0 zpC4 |
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 |
| Branch if C = 0 Flags: none | Branch if C = 1 Flags: none |
| Addressing ModeOpcode r90 | Addressing ModeOpcode rB0 |
| Branch on Result not Zero: BNE | Branch on Result Zero: BEQ |
| Branch if Z = 0 Flags: none | Branch if Z = 1 Flags: none |
| Addressing ModeOpcode rD0 | Addressing ModeOpcode rF0 |
| Branch on Result Plus: BPL | Branch on Result Minus: BMI |
| Branch if N = 0 Flags: none | Branch if N = 1 Flags: none |
| Addressing ModeOpcode r10 | Addressing ModeOpcode r30 |
| Branch on Overflow Clear: BVC | Branch on Overflow Set: BVS |
| Branch if V = 0 Flags: none | Branch if V = 1 Flags: none |
| Addressing ModeOpcode r50 | Addressing ModeOpcode r70 |

### Transfer

| Transfer Accumulator to Index X: TAX | Transfer Index X to Accumulator: TXA |
| A -> X Flags: N, Z | X -> A Flags: N, Z |
| Addressing ModeOpcode iAA | Addressing ModeOpcode i8A |
| Transfer Accumulator to Index Y: TAY | Transfer Index Y to Accumulator: TYA |
| A -> Y Flags: N, Z | Y -> A Flags: N, Z |
| Addressing ModeOpcode iA8 | Addressing ModeOpcode i98 |
| Transfer Stack Pointer to Index X: TSX | Transfer Index X to Stack Pointer: TXS |
| S -> X Flags: N, Z | X -> S Flags: none |
| Addressing ModeOpcode iBA | Addressing ModeOpcode i9A |

### Stack

| Push Accumulator on Stack: PHA | Pull Accumulator from Stack: PLA |
| A -> S Flags: none | S -> A Flags: N, Z |
| Addressing ModeOpcode i48 | Addressing ModeOpcode i68 |
| Push Processor Status on Stack: PHP | Pull Processor Status from Stack: PLP |
| P -> S Flags: none | S -> P Flags: all |
| Addressing ModeOpcode i08 | Addressing ModeOpcode i28 |
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.
P 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 |
| 0 -> C Flags: C = 0 | 1 -> C Flags: C = 1 |
| Addressing ModeOpcode i18 | Addressing ModeOpcode i38 |
| Clear Decimal Mode: CLD | Set Decimal Mode: SED |
| 0 -> D Flags: D = 0 | 1 -> D Flags: D = 1 |
| Addressing ModeOpcode iD8 | Addressing ModeOpcode iF8 |
| Clear Interrupt Disable Status: CLI | Set Interrupt Disable Status: SEI |
| 0 -> I Flags: I = 0 | 1 -> I Flags: I = 1 |
| Addressing ModeOpcode i58 | Addressing ModeOpcode i78 |
| Clear Overflow Flag: CLV | |
| 0 -> V Flags: V = 0 | |
| Addressing ModeOpcode iB8 |

### 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

Instruction table :
| High nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low nibble | Low 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 | BIT # | 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

## Related Wikibooks

* NES Programming: The Nintendo Entertainment System uses a version of the 6502
* 65c02 Assembly, a descendant of the 6502, used in many home computers, video game consoles, and embedded systems.
* 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

## Further reading

* MOS Technology 6502, wikipedia.org
* Owad, Tom, "Apple I Replica Creation", Syngress, 2005. ISBN 193183640X
* 6502.org the 6502 microprocessor resource, particularly the Tutorials and Primers page.
* 6502 Instruction Set, masswerk.at
* 6502 Family CPU Reference by Michael Steil, pagetable.com
* NMOS 6502 Opcodes by John Pickens et al., 6502.org
* Wikiversity: Learning 6502 assembly, wikiversity.org