On Thu, 6 Jun 2002, Sean 'Captain Napalm' Conner wrote:
> I've actually heard that some of the undocumented 6502 opcodes appeared to
> do two operations, but I've never programmed a 6502 (a 6809 yes, and I found
> out that encoding a TRF 8b,16b would set the high byte of the 16b register
> to $FF---I never did see what the undocumented opcodes did though; I tended
> to stay away from the undocumented stuff as a programmer).
The "undocumented" opcodes in the 6502 do indeed trigger two operations as
the revelation of the simplified decoder whereby no instructions have
their lower two bits set to 1 reveals.
It was a matter of experimentation to issue one such instruction and see
what effect it had on the accumulator and registers. I know some
instructions would, for instance, do an OR operation and then an ADD, or
maybe a LOAD and then a logical operation.
I have a document somewhere that describes these operations, and it is
abound on the web.
Here's one such description (rather lengthy):
http://www.s-direktnet.de/homepages/k_nadj/opcodes.html
...and another (you have to download a zip file):
http://www.programmersheaven.com/zone5/cat440/15555.htm
Go to Google for more.
Sellam Ismail Vintage Computer Festival
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Received on Thu Jun 06 2002 - 20:11:49 BST