Date   : Thu, 15 Feb 1996 13:16:00 +0000 (GMT)
From   : Tom Seddon <T.W.Seddon@...>
Subject: Re: Graphics speed hacks

On Thu, 15 Feb 1996, lamcw wrote:

> 
> 
> 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.

Don't these just play with the ULA registers though? I'm going to cater 
for changes here in mid-frame, it's just that it would make things easier 
if I could assume that 6845 register changes don't take effect until the 
next vsync. Modes 4/5, 0/1/2 have the same 6845 configuration, just 
different ULA settings. This is what I was meaning; all games switch 
between modes 4/5 and modes 1/2 (or mode 0 if it's Watford Quest Paint) 
WITHOUT changing the CRTC registers (since it doesn't need to, the mode 
it's switching to has the same CRTC settings). I've never seen a game that 
switches from modes 1 or 2 to modes 4 or 5 halfway down the screen. 
Perhaps it's impossible.

> ;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?

Mode 0 is 80 characters per line, high 6845 clock rate.
Mode 1 is 40 characters per line, high 6845 clock rate.
Mode 2 is 20 characters per line, high 6845 clock rate.
10 characters per line, high 6845 clock rate... any offers?!

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

I understand how the ULA bits govern which mode is being displayed; I 
would just like to know what I should do when the ULA bits are set to 
strange values (black screen? display mess? what?). My real BBC is at 
home, which is why I have to ask.

--Tom

     .                     ** Tom Seddon **                     .
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