6502

The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured CPU on the market by a considerable margin, costing less than one-sixth the price 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 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.
See more at Wikipedia.org...

<hardware> An eight-bit microprocessor designed by MOS Technologies around 1975 and made by Rockwell.
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 (stack was preset from address 256 to 511). It used these index and stack registers effectively, with more addressing modes, including a fast zero-page mode that accessed memory locations from address 0 to 255 with an 8-bit address (it didn't have to fetch a second byte for the address).
Back 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.
The 6502 was used in the BBC Microcomputer, Apple II, Commodore, Apple Computer and Atari personal computers. Steve Wozniak described it as the first chip you could get for less than a hundred dollars (actually a quarter of the 6800 price).
The 6502's indirect jump instruction, JMP (xxxx), was broken. If the address was hexadecimal xxFF, the processor would not access the address stored in xxFF and xxFF + 1, but rather xxFF and xx00. The 6510 did not fix this bug, nor was it fixed in any of the other NMOS versions of the 6502 such as the 8502. Bill Mensch at Western Design Center was probably the first to fix it, in the 65C02.
The 6502 also had undocumented instructions.
The 65816 is an expanded version of the 6502.
There is a 6502 assembler by Doug Jones jones@cs.uiowa.edu which supports macros and conditional features and can be used for linkage editing of object files. It requires Pascal.
See also cross-assembler, RTI, Small-C.
(2001-01-02)