THAT SPARKED THE COMPUTER REVOLUTION OF THE 1980s
BY Jos Kirps
The MOS 6502 is an 8-bit microprocessor with a 16-bit
address bus, designed by Chuck Peddle in 1975. Along with the
Zilog Z80 it sparked a series of computer projects that would
eventually result in the home computer revolution of the 1980s
simplified version of the 6502, was used in the Atari 2600 videogame
console. The 8502 was a 2 Mhz version of the 6502 which was used
in the Commodore 128. Millions of computer systems with MOS 6502
processors shipped during the 1980's.
Despite the relatively low clock speed of 1 Mhz,
the 6502's performance was actually competitive with other CPUs
using higher clock speeds in the late 1970's and early 1980's
(the Zilog Z80 for example). It has only very few registers -
one 8-bit accumulator register (A), two 8-bit index registers
(X and Y), an 8-bit processor status register (P), an 8-bit stack
pointer (S), and a 16-bit program counter (PC) and a quite simple
instruction set. The 16 bit address but allowed to allocate up
to 64 kb of memory.
One of the first computers to use the 6502 were
the Apple I (1976), the Apple II, and the Commodore PET, the Atari
home computers and the BBC Micro. The famous Commodore 64 used
a MOS 6510, which was a successor of the 6502 with a digital I/O
port and a three-state bus. The 6507, a simplified version of
the 6502, was used in the Atari 2600 video game console. The 8502
was a 2 Mhz version of the 6502 which was used in the Commodore
128. Millions of computer systems with MOS 6502 processors shipped
during the 1980's.
The MOS 6502 had been very popular among assembly
language programmers (mostly because if it's simplistic design),
and even 31 years later it is today used to teach assembly language
and computer architecture by many universities.
Several companies produced 16 bit derivatives of
the 6502, for example the Western Design Center 65C816 (still
widely used today) or the (not fully compatible) Mitsubishi 65816.
A planned Synertek SY6516 was never released. 32-bit derivatives
include the Western Design Center W65T32 Terbium, a 6502 compatible
chip with a 32-bit address bus, a 16-bit data bus, and a variable
length instruction set.
The MOS 6502 clearly dominated the 8 bit home computer
and videogame world, but then Apple, Commodore and Atari all switched
to the Motorola 68K architecture with their next generation 16
bit computers (the Macintosh, the Amiga and the ST). Although
the 6502 architecture faded in the home computer and video game
market, it still remains a quite popular design that can still
be found as the core of many micro controller chips today.