Date   : Mon, 20 Mar 1995 11:15:07 GMT
From   : David Alan Gilbert <gilbertd@...>
Subject: Re: Snapper

> Mike Borcherds wrote:
> 
> ;I've got a problem with snapper that I can't solve. It works fine
> ;on level 1, but on level 2 the "monsters" alternately move and then
> ;stand still with a period of about 1/2 second. On about level 4 
> ;they just stand still.
> 
> ;I know snapper relies on bit 7 of &FE40 toggling, but I _think_ I
> ;am emulating that correctly. Any other ideas?
> 
> 
> I had this prob too but I think it was with &FE60. I have to keep poking it
> with fairly random values to get snapper working continously. Not an ideal
> solution but it works okay.

I think your problem could be that bit 7 must be CLEARED when you write to
the timer which sets it.  Initially I thought that bit 7 just toggled - but
its not that simple; the bit is cleared on timer write and then toggles
on each timer underrun; so you can wait for bit 7 to go high after
clearing the timer.

(Note: All references to bit 7 in the above paragraph may well be inverted
:-) )

I don't think my emulator has any hacks in to get this to work.

Dave (gilbertd@...)