The 6502 is an 8-bit processor designed by MOS Technology in 1975, based on the design of the Motorola 6800. When it was introduced it was the least expensive full featured CPU on the market by far, at about 1/6th the price, or less, of competing designs from larger companies such as Motorola and Intel. It was nevertheless faster than most of them, and, along with the Zilog Z80, sparked off a series of computer projects that would eventually result in the home computer revolution of the 1980s. The 6502 design was originally second-sourced by Rockwell and Synertek and later licensed to a number of companies; it is still made for embedded systems.
Unlike the Intel 8080 and its kind, the 6502 had very few registers. It was an 8-bit processor with 16-bit address bus. Inside was one 8-bit data register (accumulator), two 8-bit index registers and an 8-bit stack pointer. When the 6502 was introduced, RAM was actually faster than CPUs, so it made sense to optimize for RAM access rather than increase the number of registers on a chip.
6502 processors were used in a variety of home computers of the early 80s, for example in:
References:
Documentation archive at 6502.org
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References: Synertec Datasheets at 6502.org