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
     
     
see also:
microprocessor mos technologies rockwell intel 8080 registers address bus 
accumulator index registers stack pointer addressing modes ram 
cpu bbc microcomputer apple ii commodore apple computer 
atari personal computers steve wozniak 6800 indirect jump 
broken 6510 nmos 8502 western design center 
65c02 65816 assembler macros pascal 
cross-assembler rti small-c