Date   : Mon, 12 Dec 1994 13:43:50 GMT
From   : David Alan Gilbert <gilbertd@...>
Subject: Snapper/Monsters/Rocket raid runs!

Well last night I got the 8271 emulation to the point where
DNFS and Replica were prepared to load files reliably and
after a bit of hacking about and fixing I got
Snapper, Monsters, Rocket Raid, Snooker and one or two
other games to work.  They are playable on my P90 - but
speed is a serious problem - although I have some ideas.

BCD arithmetic now works fully (now that I have got the
sense of the carry the correct way round - I had it
wrong in subtract BCD and the oxygen value in Monsters
was doing very strange things!).
Mode 7 emulation is complete.

Rocket Raid uses a wierd method of waiting for the start of screen - it
goes something like

.loop
   LDA #2
   BIT &FE4D (system VIA interrupt flag register)
   BEQ loop

Bit 2 is 'start of frame interrupt on CA1 (?)' - the problem
is that it does this with interrupts are enabled.  You have
to be careful in the way you code interrupt entry etc. otherwise
the interrupt routine always gets to the VIA before your BIT.

Dave