The 6502 has a relatively basic set of instructions, many having similar functions (e.g. memory access, arithmetic, etc.). The following sections list the complete set of 56 instructions in functional groups.
These instructions transfer a single byte between memory and one of the registers. Load operations set the negative (N) and zero (Z) flags depending on the value of transferred. Store operations do not affect the flag settings.
|LDX||Load X Register||N,Z|
|LDY||Load Y Register||N,Z|
|STX||Store X Register|
|STY||Store Y Register|
|TAX||Transfer accumulator to X||N,Z|
|TAY||Transfer accumulator to Y||N,Z|
|TXA||Transfer X to accumulator||N,Z|
|TYA||Transfer Y to accumulator||N,Z|
The 6502 microprocessor supports a 256 byte stack fixed between memory locations $0100 and $01FF. A special 8-bit register, S, is used to keep track of the next free byte of stack space. Pushing a byte on to the stack causes the value to be stored at the current free location (e.g. $0100,S) and then the stack pointer is post decremented. Pull operations reverse this procedure.
The stack register can only be accessed by transferring its value to or from the X register. Its value is automatically modified by push/pull instructions, subroutine calls and returns, interrupts and returns from interrupts.
|TSX||Transfer stack pointer to X||N,Z|
|TXS||Transfer X to stack pointer|
|PHA||Push accumulator on stack|
|PHP||Push processor status on stack|
|PLA||Pull accumulator from stack||N,Z|
|PLP||Pull processor status from stack||All|
The following instructions perform logical operations on the contents of the accumulator and another value held in memory. The BIT instruction performs a logical AND to test the presence of bits in the memory value to set the flags but does not keep the result.
The arithmetic operations perform addition and subtraction on the contents of the accumulator. The compare operations allow the comparison of the accumulator and X or Y with memory values.
|ADC||Add with Carry||N,V,Z,C|
|SBC||Subtract with Carry||N,V,Z,C|
|CPX||Compare X register||N,Z,C|
|CPY||Compare Y register||N,Z,C|
Increments & Decrements
|INC||Increment a memory location||N,Z|
|INX||Increment the X register||N,Z|
|INY||Increment the Y register||N,Z|
|DEC||Decrement a memory location||N,Z|
|DEX||Decrement the X register||N,Z|
|DEY||Decrement the Y register||N,Z|
Shift instructions cause the bits within either a memory location or the accumulator to be shifted by one bit position. The rotate instructions use the contents if the carry flag (C) to fill the vacant position generated by the shift and to catch the overflowing bit. The arithmetic and logical shifts shift in an appropriate 0 or 1 bit as appropriate but catch the overflow bit in the carry flag (C).
Jumps & Calls
The following instructions modify the program counter causing a break to normal sequential execution. The JSR instruction pushes the old PC onto the stack before changing it to the new location allowing a subsequent RTS to return execution to the instruction after the call.
Branch instructions break the normal sequential flow of execution by changing the program counter if a specified condition is met. All the conditions are based on examining a single bit within the processor status.
|BCC||Branch if carry flag clear|
|BCS||Branch if carry flag set|
|BEQ||Branch if zero flag set|
|BMI||Branch if negative flag set|
|BNE||Branch if zero flag clear|
|BPL||Branch if negative flag clear|
|BVC||Branch if overflow flag clear|
|BVS||Branch if overflow flag set|
Branch instructions use relative address to identify the target instruction if they are executed. As relative addresses are stored using a signed 8 bit byte the target instruction must be within 126 bytes before the branch or 128 bytes after the branch.
Status Flag Changes
The following instructions change the values of specific status flags.
|CLC||Clear carry flag||C|
|CLD||Clear decimal mode flag||D|
|CLI||Clear interrupt disable flag||I|
|CLV||Clear overflow flag||V|
|SEC||Set carry flag||C|
|SED||Set decimal mode flag||D|
|SEI||Set interrupt disable flag||I|
The remaining instructions perform useful but rarely used functions.
This page was last updated on 2nd January 2002