Date   : Thu, 15 Feb 1996 11:19:03 GMT
From   : lamcw <lamcw@...>
Subject: Re: Graphics speed hacks

Tom Seddon wrote:

;1. Do any of you know of any games that play with the CRTC registers 
;halfway down the screen? 

The obvious ones are Elite and Revs. Most games are quite straightforward
and don't change the graphics registers during a frame.

But what good is a beeb emulator if it can't run these two games? ;)

Revs is very time critical. From my hacking it seems that the figure
of 19990 ticks (at 1MHz) is the duration of each TV frame. I managed
to get Revs to work without any fudging but not the Elite screen is
unstable :(



;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!). 

Isn't this Mode 0?


;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?!

Not sure about this. The high/low clock rate governs the horizontal 
resolution by a factor of 2.


Chris Lam,
MacBeebEm Keeper.