Date   : Sat, 11 Dec 2004 16:07:55 -0000
From   : tom@...
Subject: Re: Video ULA shenanigans

Whilst on the subject of odd tricks, another one is to go into mode 5 
(or "mode 8") but set up the CRTC registers as if in mode 7. This 
seems to put the circuitry in a slightly different mode, where it 
reads and displays bytes as if in teletext. (I think the Master 
Advanced Reference Manual mentions this in passing somewhere.)

You end up with each cell containing the bitmap from the appropriate 
teletext address, copied 8 times (one for each scanline of the 
teletext cell). Set it up in mode 2 appropriately, with suitable 
tweaking of R9, and you can get a square 80x25 display with chunky 
addressing (if not actual chunky pixels).

I actually used this for something, too -- a scrolling message thingy 
that I wrote when I was at school. By using mode 5 you could have 
multiple colours without messing around with the funny teletext 
control codes. (The next trick was to increase R1 so you could run it 
on several side-by-side beebs and (with the right timing) it would 
look like a continuous message...)

I was going to write a page for the technical section of the model-b 
website:

http://modelb.bbcmicro.com/tech-chunky.html

but I never really got very far, as you will tell when you reach the 
last sentence. It's not linked to from the technical section contents 
for this reason. (The cycle counts in the table also list a label as 
having a cost of 6 cycles -- this is the cost of the JSR.)

Model-b does have approximate support for the freaky ULA modes, in 
that it displays something kind-of-like, but doesn't handle this mode 
7 addressing style. Currently the addressing style is hardcoded into 
the relevant teletext/bitmap sections of the video system, and is 
thus inferred from teh ULA teletext bit rather than from the CRTC 
address. I may fix this in a later version.

--Tom

On 10 Dec 2004 at 10:52, Richard_Talbot-Watkins@... wrote:

> 
> 
> 
> 
> Tom Walker <tommowalker@...> wrote on 09/12/2004 15:36:10:
> 
> > > so what does a real Beeb do when given:
> > > 80 characters per line, low clock rate
> >
> > This generates a mode 0 like display, but with every
> > other character column completely white.
> 
> How odd.  I guess it's just a strange side effect of the coding of the ULA.
> 
> > > 10 characters per line, high clock rate
> >
> > This is like mode 2, but first pixel in each byte is
> > duplicated - the second pixel is not displayed at all.
> 
> As for this one, this basically *is* an 8bpp 80 pixel wide mode.  Can't
> think of any use for it at all unfortunately.
> 
> > Another potential mode is high clock rate, teletext -
> > I haven't tried this though.
> 
> This one I hadn't considered.  Has anyone ever got any results out of this?
> An 80-column teletext mode would be great, but as possible as it seems in
> my head, I bet there's gotchas...
> 
> Rich
> 
> 
> 
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