6502

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: 16-bit register: 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: The stack is permanently located on page $01 ($0100–$01FF). 1.2 Addressing Modes The 6502 offers 13 addressing modes, including: Mode Example Operation Immediate LDA #$EA A ← $EA Absolute LDA $0314 A ← M($0314) Absolute,X LDA $0314,X A ← M($0314+X) Zero Page LDA $02 A ← M($02) Zero Page,X LDA $02,X A ← M($02+X) (Zero Page,X) LDA ($02,X) A ← M(PTR($02+X)) (Zero Page),Y LDA ($02),Y A ← M(PTR($02)+Y) 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: 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: 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: 3. Evolution and Variants 3.1 Basic Versions: 3.2 Later Extensions: 4. Computers Using the 6502 4.1 American Pioneers: 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, By Evan-Amos – Own work, Public Domain Nintendo Famicon, By Evan-Amos – Own work, Public Domain 4.3 British Systems: BBC Micro, By BBC_Micro.jpeg: Stuart Bradyderivative work: Ubcule (talk), Public Domain 4.4 Other European Implementations: 5. DIY Computers and Hobbyist Projects 5.1 Ben Eater 6502 Computer: 5.3 Other DIY Projects: 6. Modern Applications 6.1 Embedded Systems: The 65C02 is still produced by Western Design Center and used in: 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: Its simplicity makes it ideal for newcomers to low-level computing.