6502

Programming information on 6502 series machine code.

Overview

The 6502 is an 8 bit processor with a 16 bit address bus, giving a memory addressing range of 65536 bytes. It has one general purpose register, two indexing registers, a stack register, a status register and program counter.
The 6502 has no direct input/output capabilities and hence hardware must be memory mapped.
All addresses are comprised of two bytes and are arranged little endian.
There are three interrupt types including non-maskable, maskable and software. An interrupt bit in the status register controls maskable interrupts. On receipt of an interrupt the processor will push the registers on the stack and vector through the following addresses:

• FFFAH/FFFBH Non Maskable Interrupt (NMI)
• FFFCH/FFFDH Processor Reset
• FFFEH/FFFFH Maskable Interrupt
• FFFEH/FFFFH Software Interrupt (Break)

Instruction Set

The 6502 supports 56 instruction types in the following categories:

Arithmetic/Logical

ADC
SBC
AND
ASL
LSR
ROL
ROR
CMP
CPX
CPY
DEC
EOR
ORA
BIT

Register Move/Increment/Decrement

LDA
LDX
LDY
STA
STX
STY
TAX
TSX
TXA
TXS
DEC
DEX
DEY
INC
INX
INY

Jumps/Branches

JMP
BRK
JSR
RTI
RTS
BCC
BCS
BEQ
BMI
BNE
BPL
BVC
BVS

Stack

PHP
PLP
PHA
PLA

Flags

CLC
SEC
CLI
SEI
CLV
CLD
SED

Other

NOP

Memory Addressing

The 6502 has the following addressing modes:
Implied
Immediate
Zero Page
Absolute
Zero Page Indexed
Absolute Indexed
Zero Page Pre Indirect
Zero Page Post Indirect

Miscellanous Routines

Some simple tasks are very common in 6502 programming.
Examples are in pseudo assembly language, with labels where appropriate.

Multi Byte Addition

CLC
LDA a_lo_byte
ADC b_lo_byte
STA c_lo_byte
LDA a_hi_byte
ADC b_hi_byte
STA c_hi_byte

Two Byte Multiplication

LDA #0
STA lo_result
STA hi_result
LDX #9
CLC
.next_bit
ROR hi_result
ROR lo_result
DEX
BEQ exit
LSR operand_1
BCC next_bit
CLC
LDA hi_result
ADC operand_2
STA hi_result
JMP next_bit
.exit

Block Move

The two addresses are stored in zero page. The Y register contains the size of the block to move minus one.

.next_byte
LDA (address_from),Y
STA (address_to),Y
DEY
BNE next_byte

Internet resources

Because of its enduring popularity as the core of the Commodore 64 and as an embedded systems controller, there are a number of Web sites and pages about the 6502 architecture. Here are just a couple of them:

• 6502.org has documents, code resources and a forum about the microprocessor.
• A detailed analysis (C64 oriented) of instruction timings and undocumented opcodes.