MOS Technology 6502Architecture, Design, and Impact

Chris RancTravis Whitaker

Rochester Institute of Technolgoy

Spring 2016

MOS Technology 6502 Overview

I Released in 1975 in 40-pin DIP package.

I Competed directly with the Intel 8080and Zilog Z80.

I Lead to rapidly decreasing prices in themicroprocessor market.

I Widely popular in personal computerand system integration applications.

I MOS acquired by Commodore in 1976.

I Still used in embedded systems today.

History

I Chuck Peddle noticed Motorola customers were frustratedwith high 6800 unit cost.

I Proposed a lower area, lower power, lower cost 6800spin-off.

I New design leverages depletion-mode metal oxidesemiconductor transistors.

I Frustrated by Motorola management, Peddle joins MOSTechnology, who fund the 6502.

I 1975 saw a recession impact the silicon industry; 6502 salesflop as a result.

Beginnings of the Microcomputer

I To bolster sales, Peddle designsthe MDT-650 single-boardcomputer development platform.

I MDT-650 extremely popular withboth engineers and hobbyists.

I Apple, Commodore, Atari, BBCcapitalize on the emerginghobbyist/home computer market.

I Apple II, Commodore PET &VIC-20, BBC Micro, Atari 800 alldesigned around the 6502.

Specifications

I 3 8-bit General Purpose Registers

I 8-bit Stack Pointer

I 8-bit Status Register

I 16-bit Program Counter

I 1 Edge-Triggered Non-Maskable Interrupt

I 1 Maskable Level-Sensitive Interrupt

I External Memory Address/Data Bus

I RDY output for Hardware Step-Through

6502 Implementation

I 1-2 MHz typical clockfrequency.

I Two synchronizations percycle.

I Static PLA used forinstruction decoding.

I 2-stage concurrent fetchpipeline.

I 3,510 total transistors.

6502 Polysilicon Diffusion Layers

6502 Layer Overlay

6502 Visualization:http://www.visual6502.org/JSSim/index.html

Addressing Modes

I Absolute (ImmediateAddress)

I Branch-relative

I Zero Page

I Absolute/Zero-PageIndirection

Home Game Consoles Emerge

I Ted Dabney and Nolan Bushnell begin investigatingmicroprocessor-based video games.

I Unlike previous games based on dedicated discrete logic,microprocessor-based systems could play multiple games.

I Dabney and Bushnell realize that such a system wouldcreate it’s own proprietary software market.

I “Stella” prototype adapted to use the low-cost 6502 in1975.

I The Atari Video Computer System (later Atari 2600) wasreleased in 1977.

6502 in the Japanese Market

I After success in the arcade game market, Nintendo decidesto enter the personal computer market, work on the“Advanced Video System” begins.

I Management decides that the keyboard and terminal willdiscourage non-technophiles from purchasing the system;controllers are the only remaining interface in the finaldesign.

I Engineers design a system around the cost-effective 6502,essentially unknown in Japan at the time.

I Obscurity of the 6502 in the Japanese market led Nintendoto produce it’s own proprietary cross-developmentplatform.

I The “Famicom” is released to critical acclaim in 1983.

I The NES is released in the US in 1985.

I Nintento’s unified development platform and licensingmodel is still used in the console market today.