Date   : Mon, 02 Mar 2009 11:01:43 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: Fwd: New Screen Modes with ARM7 Co-Processor?

> One somewhat unexploited trick is the fact that the 6845 sends out odd 
> and even frames from the same memory source, which halves the potential 
> vertical resolution. Switching back and forth between two framebuffers 
> on every frame is a solution, but there's no hardware support, so this
> has to be done by software, on interrupt.

There _is_ hardware support in the 6845 for this - try switching on interlace 
sync+video (all the vertical registers will need reprogramming though - 
see the datasheet). On the model B you still need to keep everything in 32k,
which limits what modes you can use. On the Master (and B+?) you can switch
from normal to shadow halfway down the screen and get eg full 640x512, but
this needs careful timing.

If you don't use the hardware support then you've got a 50/50 chance of getting
the frames the wrong way round. There was a demo in 8BS that did this, but
needed user intervention if it started up wrong.

And there are far more unexploited tricks in the 6845 than that...

Tom