13 Facts About 6502


MOS Technology 6502 is an 8-bit microprocessor that was designed by a small team led by Chuck Peddle for MOS Technology.

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The design team had formerly worked at Motorola on the Motorola 6800 project; the 6502 is essentially a simplified, less expensive and faster version of that design.

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When it was introduced in 1975, the 6502 was the least expensive microprocessor on the market by a considerable margin.

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The 6502 simply removed this feature, in keeping with its design as an inexpensive controller being used for specific tasks and communicating with simple devices.

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The 6502 used a simpler system that handled comparisons by performing math on the accumulator and then examining result flags.

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MOS would introduce two microprocessors based on the same underlying design: the 6501 would plug into the same socket as the Motorola 6800, while the 6502 re-arranged the pinout to support an on-chip clock oscillator.

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Bill Mensch did the 6502; he was the designer of the 6820 Peripheral Interface Adapter at Motorola.

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The 6502 was next used in the Commodore PET and the Apple II, both released in 1977.

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The 6502 used in the NES was a second source version by Ricoh, a partial system on a chip, that lacked the binary-coded decimal mode but added 22 memory-mapped registers and on-die hardware for sound generation, joypad reading, and sprite list DMA.

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Internal logic runs at the same speed as the external clock rate, but despite the low clock speeds, the 6502's performance was competitive with other contemporary CPUs using significantly faster clocks.

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Chip uses the index and stack registers effectively with several addressing modes, including a fast "direct page" or "zero page" mode, similar to that found on the PDP-8, that accesses memory locations from addresses 0 to 255 with a single 8-bit address—code for the 6502 uses the zero page much as code for other processors would use registers.

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On some 6502-based microcomputers with an operating system, the operating system uses most of zero page, leaving only a handful of locations for the user.

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Not to be confused with SALLY, a custom 6502 designed for Atari nor with the similarly-named 65C02.

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