Date   : Wed, 14 Feb 1996 22:18:20 +0000 (GMT)
From   : James Fidell <james@...>
Subject: Re: Graphics speed hacks

Tom Seddon wrote:

> 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 :-)

It's surprising how a few instructions are used far more than nearly all
the others.  I don't think I have my statistics around any more, but I've
left the instruction counting code in v0.3.

> 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?

Yes.  Elite.  Revs.  Many of the title screens from the Ultimate games
use effects created by manipulating the CRTC all over the place.

> Or can I just take the values from these at the beginning of each new
> vsync?

You really shouldn't.

> 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 ;-)

I don't think that all that many games play with the CRTC once they're
running, but some of the good ones definitely do.

> 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?!

I'll have to try this on mine and find out.  I'd guess that both end up
looking like a mess, though.

James.

-- 
 "Yield to temptation --             | Work: james@...          
  it may not pass your way again"    | Play: james@...                 
                                     | http://www.OiT.co.uk/~james/
        - Lazarus Long               |              James Fidell