Date   : Fri, 02 Sep 1994 15:22:11 WET DST
From   : Bonfield James <jkb@...>
Subject: Re: UNDEFINED 6502 OPCODES

D. writes:
>I agree wholeheartedly. James and I have been discussing this most
>of the afternoon (since my last message here). He thinks I am talking
>about the 65C12, but I'm pretty convinced that even the humble old
>6502 (even if only in it's CMOS clothes) was guilty as charged.
>
>As I said to him, I've read an article somewhere documenting all
>of the 'undefined' codes, and warning that they didn't work on all
>CPUs - only certain models, or certain batches or something like that.

Note that the Rockwell CMOS 6502 (R65C02) does indeed have more opcodes -
documented ones. It also claims the the unused opcodes are all defined to be
NULLS. So that tidies that chip up.

Regarding the NMOS 6502, I expect this depends on the manufacturer. Certainly
the 6510 and 6502 (aka disk drive) in the C64 were claimed to have many extra
undocumented opcodes. I tried these on my 6502 and the ones I tried didn't
work (but Zalaga does by the way).

I suspect the real issue is which manufacturer Acorn used for their 6502
chips. Some batches of beebs would perhaps use on manufacturer from another.
Then again, maybe it is true that even the one manufacturer changed, but I
doubt that to be the case (too much hassle for no gain). Or maybe (seems
equally unlikely) Acorn switched to using 65C02 chips in some BBC machines
(eg. whilst they couldn't get hold of 6502s for some reason).

However, I feel that most games (all?) work on all BBC model B machines, with
the exception of the deliberately coded Master versions. Hence I'd be very
suprised if any game made assumptions of which batch of machine you happen to
have.

       James