6502: The People's Processor

Overview

The MOS 6502, designed by Chuck Peddle and released in 1975, democratised computing. At $25 when competitors charged $300+, it made personal computers financially viable. Its elegant instruction set and zero-page addressing made assembly programming accessible to bedroom coders worldwide.

Fast facts

Clock speed: typically 1-2 MHz depending on system.
Data bus: 8-bit with 16-bit address bus (64KB addressable).
Registers: accumulator (A), two index registers (X, Y), stack pointer, status register.
Zero page: first 256 bytes ($00-$FF) accessed with single-byte addresses for speed.
Stack: fixed at $0100-$01FF, grows downward.

Architecture highlights

Instruction set: 56 official opcodes with multiple addressing modes each.
Addressing modes: immediate, absolute, zero-page, indexed, indirect, and combinations.
No multiplication/division: these must be implemented in software.
Little-endian: low byte stored first in memory.

Key addressing modes

Mode	Syntax	Example	Description
Immediate	#$nn	LDA #$40	Load literal value
Zero-page	$nn	LDA $40	Fast access to $00-$FF
Absolute	$nnnn	LDA $C000	Full 16-bit address
Indexed	$nnnn,X	LDA $C000,X	Address + X register
Indirect	($nnnn)	JMP ($FFFE)	Jump to address stored at location

Variants and descendants

6510: C64’s version with built-in I/O port.
6507: stripped-down version for Atari 2600 (13-bit address bus).
65C02: CMOS version with additional instructions.
65816: 16-bit successor used in Apple IIGS and SNES.

Cultural impact

The 6502 created the first generation of assembly programmers. Its constraints—limited registers, no hardware multiply—forced creative solutions that became hallmarks of 8-bit game design. The skills learnt on a 6502 transfer directly to understanding all computer architecture.

Eight bits that changed everything