Date   : Fri, 02 Dec 2016 09:05:38 -0000
From   : rgcolem.myorg@... (Rob Coleman)
Subject: Updated BeebWiki articles

On 02/12/2016 07:37, Nicola Giacobbe wrote:
> By the way and asking to all the experts out there: is there some way to
> reduce the vertical resolution of a MODE while keeping it screen-height?
> I mean, I have found many references to reducing the number of rows on a
> graphics mode but the final result is that I end using only a part of
> the available screen area. Something akin to a 256x128 MODE 5 could be
> very useful.
> Any ideas?
I don't think a full screen 256x128 mode 5 is possible due to the way that 
the CRTC automatically increments the row counter.

I haven't tried this but it is possible to display the first scan line in a 
row over and over again. You set the CRTC to display one scan line per 
character in R9 and then use the vertical rupture technique to change the 
screen start address on every scan line.

The problem with this is that you're only using 1/8th of the available 
screen memory so you quickly run out unless you drastically reduce the 
vertical resolution. A 256 x 32 pixel MODE 1 would still take up 16K. On a 
Master, using shadow RAM and all of the screen memory (40KB), you could get 
up to 256 x 80.  You could use even more main memory to increase the 
resolution further but it would make hardware scrolling difficult (if not 
impossible - I haven't thought through the details).

There's a brief discussion about this technique (and a link to a demo that 
implements it) on the RetroSoftware boards:
http://www.retrosoftware.co.uk/forum/viewtopic.php?f=3&t=122