Date   : Mon, 02 Mar 2009 12:50:58 +0100
From   : kortink@... (John Kortink)
Subject: Fwd: New Screen Modes with ARM7 Co-Processor?

On Mon, 2 Mar 2009 11:01:43 +0000 (GMT), tommowalker@...
wrote:

>
>> 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).

Not really. That's interlaced display of a single framebuffer.
To double the vertical resolution would require linear access
to 40K of memory, which we don't have. With two framebuffers,
(switched between every frame) that drops to 20K, which we do
have.

>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.

That would require switching CRTC start address mid-screen. I
doubt that that would work on the real CRTC (it will probably
only be copied into a counter every frame).

>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.

There's probably no software workaround for that, that's true.

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

To increase screen resolution ? I doubt it.


John Kortink

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