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6502 Technology} 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 {CPU}s, so it made sense to optimise 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)
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