Date   : Wed, 14 Feb 1996 08:57:21 +0000 (GMT)
From   : Tom Seddon <T.W.Seddon@...>
Subject: Graphics speed hacks

Thanks for all the optimisation pointers. My emulator now runs at 150% 
beeb speed, even with both system VIA and user VIA timers running. I 
simply restricted the timer updates to once every 128 cycles and it's now 
much faster. I also did a 6502 instruction profile and optimised the most 
frequently used instructions. Who'd have thought that BIT was so often 
used :-)

My question concerns video problems (video being the next thing on my 
list to implement):

1. Do any of you know of any games that play with the CRTC registers 
halfway down the screen? Or can I just take the values from these at the 
beginning of each new vsync? I assume that most games just use the ULA, 
although some that might want a MAJOR mode change (eg from mode 4 to mode 
2--assuming that's possible in the first place) might do this.
(Actually that question goes for serious software as well, I just can't 
think of much serious software I'd want to run under the emulator ;-)

2. What happens when you select, using the video ULA, a mode with high 
6845 clock rate and 10 characters per line? Extrapolating from the values 
required for the standard modes, this would give you 80 pixels across and 
8 bits per pixel (what a concept!). I assume this is wrong--or is there 
more to the beeb than I thought? :-)--so what really does happen? (I 
don't have a real beeb to try this on).
Similarly, what happens with a low clock rate and 80 characters per line? 
640 pixels and 0-bit colour?!

Thanks,

--Tom

     .                     ** Tom Seddon **                     .
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