I ran my own blog from November 2006 till October 2014. All posts are still online, but I don't have time to update it anymore. Please note that all images and media files have been removed when the backup was moved to a new host in early 2016. Enjoy!
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.
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 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.
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 homecomputer 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 homecomputer and video game market, it still remains a quite popular design that can still be found as the core of many microcontroller chips today.