Date   : Thu, 18 Jan 1996 00:12:19 +0000 (GMT)
From   : James Fidell <james@...>
Subject: Re: Illegal opcodes

> > Has anyone figured out Orlando's code from the Zalaga loader - as
far as I
> > can tell the loader *code* jumps to the start of an ascii message
which is
> > an author credit/copyright message. (Most of the start compromises
> > non-standard opcodes.) Presumably somewhere in this mess we jump to some
> > more sensibly code, but it's difficult to tell. It also looks like there 
> > might be a tune-player in there of sorts, but that's hard to tell also.
>
> I once had a BASIC disassembler which I modified to do all the illegal
> opcodes. Don't ask me to find it, it's long gone, but a new one shouldn't
> take you more than a few hours to knock out.

Xbeeb will display op-codes as it executes them.  Xbeeb v0.3 has all
the illegal 6502 opcodes and the R65C02 and R65C12 opcodes too.  If I
can just get a couple more things done, I should be able to release it
very soon.

> I had a cmos 65c02 in my beeb because the standard one used to overheat.
> The _only_ program that was affected by this was Zalaga, which used to 
> leave trails behind all the moving sprites. I assume he was using the 
> illegal opcode to store zeros on the screen. Git.

A strong possibility.  He only actually used three undocumented opcodes
though -- from my changes log they must be SAX, ASR and SLO.

James.

-- 
 "Yield to temptation --             | Work: Somewhere at Netcom UK (soon)
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        - Lazarus Long               |              James Fidell