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  6502
       
           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 
          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)