22 Facts About MOS 6502

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

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

MOS 6502 was designed by many of the same engineers that had designed the Motorola 6800 microprocessor family.

MOS 6502 contributed in many areas, including the design of the 6850 ACIA.

MOS 6502 initially approached Mostek CEO L J Sevin, but he declined.

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MOS 6502 designed and fabricated custom ICs for customers and had developed a line of calculator chips.

MOS 6502 delivered on the promise, the new line was ready by June 1975.

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

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

At MOS 6502 Technology, the "layout" was a very manual process done with color pencils and vellum paper.

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.

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

When MOS 6502 Technology arrived at Wescon, they found that exhibitors were not permitted to sell anything on the show floor.

In March 1976, the now independent MOS 6502 Technology was running out of money and had to settle the case.

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

The MOS 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.

MOS 6502 is a little-endian 8-bit processor with a 16-bit address bus.

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

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 MOS 6502 uses the zero page much as code for other processors would use registers.

MOS 6502 is capable of performing addition and subtraction in binary or binary-coded decimal.

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

MOS 6502 had several bugs and quirks, which had to be accounted for when programming it:.