Date   : Fri, 05 Jan 1996 09:06:39 +0000 (GMT)
From   : jkb@...
Subject: Re: Timings for 65c[01]2 NOP instructions

James Fidell wrote:

>Which, I am afraid, just leads to more questions...
>
>I thought that all the unused 65c02 opcodes were supposed to be NOPs ?
>Was that only the 65c12 ?

I thought so to (although I don't know whether they were defined to be 1 byte
NOPS - probably). I thought it was basically only the boring 6502 that had the
undefined behaviour.

I remember seeing some game protection that basically had two bits of code
interleaved by cunning use of undefined opcodes. I verified that the undefined
opcodes on the 6502 do indeed follow the expected multi-byte lengths (gleaned
from looking at the table and grouping them together). My test program was
simply something like:

LDX #&02
UNDEFINED CODE
DEX
DEX
STX &70
RTS

Of course it may do all sorts of strange things to &CA and &CACA :-)

       James
--
James Bonfield (jkb@...              )   Tel: 01223 402499   Fax: 01223 412282
Medical Research Council - Laboratory of Molecular Biology,
Hills Road, Cambridge, CB2 2QH, England.