6502

hardware An eight-bit microprocessor designed by MOSTechnologies 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 addressbus. 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 personalcomputers. 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 [email protected]

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)