6502
An eight -bit {microprocessor } designed by {MOS
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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