Re: [stella] "Illegal" opcodes

Subject: Re: [stella] "Illegal" opcodes
From: Julian Squires <tek@xxxxxxx>
Date: Sun, 18 Jun 2000 22:53:30 -0230
On Sat, Jun 17, 2000 at 05:17:49PM -0500, John K. Harvey wrote:
> Ok, since these can seemingly be useful on the Atari 2600 platform at the
> very least, does anyone know what they all are?  Perhaps a website
> dedicated to the 6502 has a complete list.  I'm assuming that the other 100
> or so illegal opcodes aren't all 'LAX'.
> This sounds like a great way to save cycles, and sounds like another one of
> those neat "programming tricks".  Perhaps some ROMs already use illegal
> opcodes, and that prevents their full working on certain emulators.  Has
> anyone ever encountered illegal opcodes in a disassembly project?

There is quite a lot of good documentation on these in the C64 world.
Particularly, was
one of my preferred documents.

I'm kind of surprised more people on the list haven't heard of the
undocumented instructions, especially given that I think myself and
others have mentioned them in postings before. (I don't post much
thanks to time constraints and my contempt for the binary filter which
doesn't like my PGP signatures)

A while ago, while working on a 6502 emulator, I wrote a 6502
assembler in perl, which supports the undocumented
instructions. (Also, a crude disassembler) Anyone interested? I didn't
release it because it seemed there was little to no demand, and DASM
has many more useful features. (this is truly barebones, although I am
willing to add features on request)

I was under the impression a number of commercial games used the
undocumented opcodes... ISTR pacman for the NES(?) or similar using
them, for example.

 |/|  Julian Squires <tek@xxxxxxx>

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