Date   : Mon, 05 Sep 1994 10:39:03 +0000
From   : lamcw <lamcw@...>
Subject: UNDEFINED 6502 OPCODES - It's true....

hi everyone,

just a quick note about my findings about invalid opcodes. i consulted
the 64doc note on the 6502... it was very useful. i implemented 3 or 4
of these 'invalid' opcodes and hey presto... zalaga started to work.
anyone interested in what they were i will post later (i can't remember
from memory what they were precisely).

yes, my guess is that they were implemented to confuse hackers... it would
have knock me out 10 years ago if i had come across stuff like this on my beeb.
these opcodes i included do not do anything which can't be done in 2 or 3 
valid opcodes, maybe it was done for speed but hey, whats a few cycles between
friends?

also got planetoids to work, snapper does but the ghosts freeze when i eat a
pill and revs, wow, there are 2 interrupts to change the screen mode on the
fly. however the bottom 3rd of the screen was fine and i managed to crawl
around the track in 1st gear! occasionally the the video display picked up the
mode 4 setting and the screen went into wierd mode 4 ... great fun!!!

my concern now is the speed of the thing... in mode 2, it's about 25%
real beeb speed. it's a trade off between smoother graphics or greater speed.

anyway i'm giving the emulator a rest for a few days cos i've got my phd viva
to revise for on wednesday. i know which i'd sooner be working on...


chris lam.