MOS Technology 6502 Microprocessor Specification

1. Overview

The MOS Technology 6502 is an 8-bit microprocessor introduced in 1975. It is notable for its low cost, efficient instruction set, and widespread use in early personal computers and game consoles, including the Apple II, Commodore 64, Atari 2600, NES (Ricoh 2A03 variant), and BBC Micro.

2. General Characteristics

Feature	Description
Data width	8-bit
Address width	16-bit
Address space	64 KB (0x0000–0xFFFF)
Registers	A, X, Y, SP, PC, P
Endianness	Little-endian
Clock speed	~1–3 MHz (original NMOS)
Technology	NMOS

3. Programmer-Visible Registers

3.1 Accumulator (A)

8-bit register
Used for arithmetic, logic, and data transfer operations

3.2 Index Registers (X, Y)

8-bit registers
Used for indexing memory, counters, and offsets

3.3 Stack Pointer (SP)

8-bit register
Points to the current stack location
Stack resides in page $0100–$01FF

3.4 Program Counter (PC)

16-bit register
Holds address of next instruction

3.5 Processor Status Register (P)

Bit	Name	Description
7	N	Negative
6	V	Overflow
5	–	Unused (always set in pushes)
4	B	Break
3	D	Decimal Mode
2	I	Interrupt Disable
1	Z	Zero
0	C	Carry

4. Memory Map Conventions

Address Range	Usage
`$0000–$00FF`	Zero Page
`$0100–$01FF`	Hardware Stack
`$0200–$FFFF`	Program / Data / I/O

4.1 Interrupt Vectors

Vector	Address	Description
NMI	`$FFFA–$FFFB`	Non-maskable interrupt
RESET	`$FFFC–$FFFD`	Reset vector
IRQ/BRK	`$FFFE–$FFFF`	Interrupt request

5. Addressing Modes

Mode	Syntax	Description
Implied	`CLC`	Operand implied
Accumulator	`ASL A`	Operates on accumulator
Immediate	`LDA #$10`	Constant value
Zero Page	`LDA $20`	Zero-page address
Zero Page,X	`LDA $20,X`	Zero-page indexed
Zero Page,Y	`LDX $20,Y`	Zero-page indexed
Absolute	`LDA $1234`	Full 16-bit address
Absolute,X	`LDA $1234,X`	Indexed absolute
Absolute,Y	`LDA $1234,Y`	Indexed absolute
Indirect	`JMP ($1234)`	Pointer-based jump
Indexed Indirect	`LDA ($20,X)`	X-indexed pointer
Indirect Indexed	`LDA ($20),Y`	Y-indexed pointer
Relative	`BEQ label`	Branch offset

6. Instruction Set Summary

6.1 Load / Store

LDA, LDX, LDY
STA, STX, STY

6.2 Register Transfers

TAX, TAY, TXA, TYA, TSX, TXS

6.3 Stack Operations

PHA, PLA, PHP, PLP

6.4 Arithmetic

ADC – Add with Carry
SBC – Subtract with Carry

6.5 Logical

AND, ORA, EOR

6.6 Shifts and Rotates

ASL, LSR, ROL, ROR

6.7 Increment / Decrement

INC, INX, INY
DEC, DEX, DEY

6.8 Comparisons

CMP, CPX, CPY

6.9 Branching

BEQ, BNE, BMI, BPL, BCS, BCC, BVS, BVC

6.10 Jumps & Subroutines

JMP, JSR, RTS, RTI

6.11 Status Flag Control

CLC, SEC, CLI, SEI, CLV, CLD, SED

6.12 System Control

BRK, NOP

7. Cycle Timing (General)

Most instructions execute in 2–7 cycles
Additional cycles may be added for:
- Page boundary crossings
- Taken branches

8. Decimal Mode

When the D flag is set:

ADC and SBC operate using BCD arithmetic
Only valid on NMOS 6502 (behavior varies on CMOS derivatives)

9. Known Hardware Quirks

JMP (addr) indirect bug: page boundary wraparound
- Example: JMP ($10FF) reads from $10FF and $1000
No dedicated multiply or divide instructions
Stack is fixed to page $01xx

10. Variants and Derivatives

Variant	Notes
6502	Original NMOS
65C02	CMOS, fixes bugs, adds instructions
2A03	NES variant, decimal mode disabled
6510	Adds I/O port (Commodore 64)

11. Example Code

        LDX #$00
loop:   INX
        TXA
        STA $0200,X
        CPX #$10
        BNE loop
        BRK

Command Palette

github/legacy-circuit-mockups — references/6502.md