Date   : Wed, 02 Nov 1994 11:00:46 +0000
From   : lamcw <lamcw@...>
Subject: Re: Reading BBC disks

david barnett wrote:

;Its sole merit is that some games would
;work which accessed the controller directly (usually for copy security)
;would work.  Related to this is the issue of emulating illegal opcode
;effects on the 6502 itself.  

i think the issue of 6502 invalid opcodes is probably more important a
problem than the emulation of the 8271. i've come across some games which
use say, a dozen, invalid opcodes to 'decode' itself before running. the
snag here is that you're never really 100% sure you implemented one invalid
opcode properly before proceeding to the next. i wouldn't be surprised if many
of the later games (late '80s) use most of them in an attempt to scare off
hackers.


chris lam,
aston univ,
u.k.