Date   : Mon, 12 Dec 1994 14:07:30 +0000
From   : lamcw <lamcw@...>
Subject: Re: Snapper/Monsters/Rocket raid runs!

Dave Gilbert wrote:

;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.

Nice work, what speeds are you getting? what's a p90?

;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.

Yep, I found this out too. It's best to leave a enabled interrupt 
'floating' around for a while.


Chris Lam.