Date   : Mon, 23 Jan 1995 13:48:59 +0000 (GMT)
From   : Mark Cooke <ee2015@...>
Subject: Re: Scrolling Swapping

Hi Dave + everyone else,

> Chris Lam got back to me - he said he had the same problem with
> Revs.  Revs does not resync to the start of each frame using
> vsync - it just chains timer 1 interrupts - and you have to make
> the timings of that and the video hardware perfect.

Yep.  I caught that one on the list.  Well, I hope that my current design
methodology for my emulator will deal with this correctly.  It does mean a
bit of extra work keeping the VIA interrupts in sync though.  I currently
time the machine by using the opcode counters.  Hopefully slight errors
in timing won't cause a problem.

As a quick suggestion, could you attempt to ensure that the VIA ticks for
exactly n ticks each vertical sync.  It would require you to count how much
you decremented the timer by, and adjust for any discrepancy.  I think this
would partially solve your problem by causing the switching points to buzz
up and down around their correct positions.  Or, you could do what Chris
suggests, and do a special screen setup for Revs.

All the best.

Mark

+-----------------------------------------------------------------------+
| Mark Cooke                 |    Email : ee2015@...                    |
|----------------------------+------------------------------------------|
| bbc-emu-request@...           - Home of the BBC Emulator Mailing list |
|   URL :  http://kestrel.fen.bris.ac.uk/students/ee2015/Welcome.html   |
+-----------------------------------------------------------------------+