Re: 6502 CPU

[nathan@cco.caltech.edu (Nathan Mates)]

In article <3mbd1l$av@naurouze.cert.fr>,
Jean-Francois Fabre <fabre@puigmal.cert.fr> wrote:
>Does any of you know about the undocumented instructions on the 6502 ?

   The 6502 does have some sort of undocumented instructions, but they
are not standardized. They can vary from one machine to the next, so
there is no acceptible list of undocumented instructions. If you have
a 6502 (not 65C02) based machine (Apple ][, ][+, unenhanced //e), you
could by trial and error determine their behavior on your machine, but
it probably wouldn't work on any other 6502 machine.

   To make things worse, the 65C02 (used in enhanced //e, //c, //c+,
and the GS in emulation mode) has no useful undocumented instructions.
Anything unlisted acts just like a NOP ($EA), so you can put all the
undocumented instructions you want in your program, but it won't do
anything useful.

   The Commodore 64 used a different processor (6510?), with
standardized undocumented opcodes; that's why programmers could
reliably use them.

-- 
<*> Nathan Mates http://www.ugcs.caltech.edu/~nathan/      <*>
# And he said to them, "To you has been given the secret of the
# Kingdom of God, but for those outside, everything comes in 
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