6502 Instructions - Introduction

This is a draft only!
It is still under construction and content may change. Do not rely on this information.

The 6502 processor has a compact instruction set, consisting of just 56 instructions:

ADC AND ASL BCC BCS BEQ BIT BMI BNE BPL BRK BVC BVS CLC

CLD CLI CLV CMP CPX CPY DEC DEX DEY EOR INC INX INY JMP

JSR LDA LDX LDY LSR NOP ORA PHA PHP PLA PLP ROL ROR RTI

RTS SBC SEC SED SEI STA STX STY TAX TAY TSX TXA TXS TYA

This page groups these instructions and explains their basic function.

Addressing Modes

There are 13 6502 Addressing Modes. All of these instructions work with at least one addressing mode, and many work with several addressing modes. See the Resources section for Opcode tables that define which instructions work with which addressing modes.

Performance

Each 6502 instruction takes a defined number of cycles to execute. In some cases,

Loading and Storing Data (to/from Memory)

Register-Memory Loads and Stores

There are three instructions to load data from memory to a register:

LDA ; load the accumulator

LDX ; load the X register

LDY ; load the Y register

And there are three matching instructions to store data from a register to a memory location:

STA ; store the accumulator

STX ; store the X register

STY ; store the Y register

Push/Pull on the Stack

When a value is pushed to the stack, the stack pointer is decremented and the selected register is written to $0100+SP.

When a value is pulled from the stack, the stack pointer is incremented and the selected register is loaded from $0100+SP.

There are two instructions to push data onto the stack:

PHA ; push the accumulator

PHP ; push the processor status register

And two matching instructions to pull data from the stack:

PLA ; pull the accumulator

PLP ; pull the processor status registerI've been swamped! 

Note that some other operations, such as JSR, interrupts, RTI, and RTS, cause data to be pushed to or pulled from the stack.

Bitwise Operations

Arithmetic

ADC ; add with carry

The accumulator + memory location + carry flag is stored to the accumulator (A = A + M + C).

The carry flag can be used to carry overflow information from the lowest byte to the highest byte of a multi-byte addition. Clear the carry flag and then add the lowest bytes, then leave the carry flag untouched and add the second-lowest bytes. Continue to the highest byte in the multi-byte sequence.