Date   : Fri, 02 Sep 1994 14:49:00 BST
From   : tim@... (Tim Morley)
Subject: Re: UNDEFINED 6502 OPCODES

D.LODGE@... writes ("Re: UNDEFINED 6502 OPCODES"):
> >umm, you could be right. when trying to run zalaga on my beeb emulator, it
> >bombs out with an invalid opcode - &EC i believe. i'm not saying my emulator
> >is without fault (heaven forbid) but it IS suspicious because other games
> >are running flawlessly.
> 
> The reason is that on Zalaga, Orlando (the programmer) decided to trap the
> unknown instruction (? - I've been programming ARM for too long :) vector and
> use it to redefine some opcodes which I believe are on the 65c02 - if the
> emulator can't emulate this vector trapping then Zalaga won't run...

Well a 6502 isn't quite that clever, Unfortunatley I think we are
going to have to some detective work to find out what the
'undocumented' instructions do. I seem to remember that there are
things in there that do actually something other then just a NOP. I
beleiev that there are patterns in the way that the instructions are
layed out, and some of the undocumented instructions are the correct
things for the place in the patern!

Tim