#02 MOS Technology 6502 Processor: Architecture, History, and Computers

1. Architecture and Design

The MOS Technology 6502 is an 8-bit microprocessor with a 16-bit address bus, built using 8μm NMOS technology with 4528 transistors (3510 enhancement and 1018 depletion load). The die size is 3.9 × 4.3 mm (16.6 mm²), and it requires a single +5V power supply.

Pinout diagram of the MOS 6502 microprocessor showing all 40 pins, including address and data buses and control signals

1.1 Internal Organization

The 6502 has a minimalist register set consisting of only 6 registers:

8-bit registers:

• A (Accumulator) – the main arithmetic register
• X and Y – index registers
• P (Status Register) – processor flags register
• S (Stack Pointer) – 8-bit stack pointer (fixed on page $01)

16-bit register:

• PC (Program Counter) – program counter

Unlike the Z80, the 6502 has no register pairs – all 16-bit operations must be performed through separate instructions for the low and high byte.

Little-endian Architecture

The 6502 is a little-endian processor, meaning 16-bit values are stored with the low byte first. For example, address $1234 is stored in memory as $34 $12.

Zero Page

One of the 6502’s key features is the Zero Page – the first 256 bytes of memory ($0000–$00FF) act like pseudo-registers. Instructions referencing the Zero Page:

• Use only one address byte instead of two
• Execute faster
• Take up less code space

The stack is permanently located on page $01 ($0100–$01FF).

1.2 Addressing Modes

The 6502 offers 13 addressing modes, including:

1.3 Instruction Set

The 6502 has 56 basic instructions (compared to the Z80’s 158). With different addressing modes, this totals 151 opcodes. Categories include:

• Load/Store – LDA, LDX, LDY, STA, STX, STY
• Transfer – TAX, TAY, TXA, TYA, TSX, TXS
• Arithmetic – ADC, SBC, CMP, CPX, CPY
• Logic – AND, ORA, EOR, BIT
• Shifts – ASL, LSR, ROL, ROR
• Conditional Branches – BCC, BCS, BEQ, BNE, BMI, BPL, BVC, BVS
• Control – JMP, JSR, RTS, BRK, RTI, NOP

2. History of Creation

2.1 Founding of MOS Technology

The 6502 story began in 1973, when Chuck Peddle was working at Motorola on the 6800 processor. Peddle saw market demand for a much cheaper processor than the ~$300 solutions of the time.

When Motorola’s management rejected his proposal, he left the company. In August 1974, he and seven other Motorola engineers – Harry Bawcom, Wil Mathys, Rod Orgill, Ray Hirt, Mike Janes, Terry Holdt, and Bill Mensch – joined MOS Technology in Valley Forge, Pennsylvania.

Chuck Peddle and his team displaying a large layout of the MOS 6502 microprocessor / team6502.org

2.2 Design Team

MOS Technology was a small semiconductor firm founded by John Paivinen, a former GE colleague of Peddle. The company specialized in calculator chips and had its own mask lab and production line.

The “Motorola Eight” team included:

• Chuck Peddle – project leader, instruction set architecture
• Bill Mensch – circuit designer, transistor-level implementation
• Rod Orgill & Wil Mathys – instruction set design
• Harry Bawcom, Ray Hirt, Mike Janes, Terry Holdt – various design tasks

2.3 Design Process

The 6502 was largely designed by hand on paper. Without advanced simulation tools, most verification was done mentally or with basic aids.

Key innovations included:

• Use of depletion-load NMOS instead of traditional enhancement-load
• Simplified architecture with minimal registers
• Optimization for low manufacturing cost while maintaining performance

3. Evolution and Variants

3.1 Basic Versions:

• 6502 – 1 MHz
• 6502A – 2 MHz
• 6502B – 3 MHz
• 6502C – 4 MHz

3.2 Later Extensions:

• 6510 (1982) – used in Commodore 64, with built-in 6-bit I/O port

• 65C02 (1981) – CMOS version (Western Design Center):
  – Lower power consumption
  – Extra instructions
  – Improved addressing modes
  – Better noise immunity

• 65816 (1984) – 16-bit extension, backward-compatible with 6502, used in Apple IIGS and Super Nintendo

4. Computers Using the 6502

4.1 American Pioneers:

• Apple I (1976) – Steve Wozniak’s first computer, using a $25 6502 at 1 MHz
• Apple II (1977) – hugely successful home computer with color graphics and built-in BASIC
• Commodore PET (1977) – Commodore’s first 6502-based computer
• Atari 400/800 (1979) – advanced graphics/sound, using 6502B ~1.79 MHz
• VIC-20 (1981) – Commodore’s first million-selling home computer
• Commodore 64 (1982) – best-selling home computer ever, using the 6510 at 1 MHz

Apple II, By All About Apple museum official web site – http://www.allaboutapple.com/, CC BY-SA 3.0

Atari 800, By Evan-Amos – Own work, Public Domain

4.2 Game Consoles:

• Atari 2600 (1977) – used 6507 (a simplified 28-pin 6502) at 1.19 MHz
• Nintendo Famicom/NES (1983/1985) – used Ricoh 2A03, a 6502 clone without decimal mode

Atari 2600, By Evan-Amos – Own work, Public Domain

Nintendo Famicon, By Evan-Amos – Own work, Public Domain

4.3 British Systems:

• BBC Micro (1981) – educational computer with 6502A 2 MHz, widely used in schools
• Acorn Atom (1980) – entry-level 6502 machine
• Acorn Electron (1983) – budget alternative to BBC Micro

BBC Micro, By BBC_Micro.jpeg: Stuart Bradyderivative work: Ubcule (talk), Public Domain

4.4 Other European Implementations:

• Less popular in Eastern Bloc than the Z80, but used in some niche systems and as coprocessors

5. DIY Computers and Hobbyist Projects

5.1 Ben Eater 6502 Computer:

• 65C02 at 1 MHz
• 32K RAM, 32K ROM with demo programs
• 16×2 LCD display
• Serial interface (bit-banging or 65C51 ACIA)
• Supported with step-by-step YouTube tutorials and kits

5.3 Other DIY Projects:

• Dirk Grappendorf’s 6502 Home Computer – blend of C64 and Epson HX-20 features
• mike42/6502-computer – PCB project, 65C02 at 1.8432 MHz with SD card support
• Fast PDIP 6502 Computer – running up to 32.768 MHz
• Sven Krasser’s 6502 Computer – hybrid design using a real 6502 with FPGA support

6. Modern Applications

6.1 Embedded Systems:

The 65C02 is still produced by Western Design Center and used in:

• Industrial control systems
• Medical devices (e.g., pacemakers)
• Consumer appliances (e.g., microwaves)
• Industrial automation

Bill Mensch claims 6502 derivatives are found in over 5 billion devices worldwide.

6.2 Educational Significance:

The 6502 remains a perfect learning tool for:

• Computer architecture basics
• Assembly language programming
• Microprocessor fundamentals
• Embedded systems design

Its simplicity makes it ideal for newcomers to low-level computing.