Date   : Mon, 02 Mar 2009 11:57:13 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: Fwd: New Screen Modes with ARM7 Co-Processor?

> >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.

That's interlace sync mode. Interlace sync + video does work as I've described.
Look in the datasheet!

> 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.

Hence the address switching trick.

> That would require switching CRTC start address mid-screen.

Which is entirely possible and has been done before! (see Uridium)

But you don't need it. The main/shadow select bit isn't in the CRTC, it's
in a latch at &FE34, and it is possible to switch it mid-frame.

> I doubt that that would work on the real CRTC (it will probably
> only be copied into a counter every frame).

I've tried it and it does.

> >And there are far more unexploited tricks in the 6845 than that...
> 
> To increase screen resolution ? I doubt it.

No (except overscan). But there are tricks for other stuff (eg scrolling,
screen splitting, etc).

Tom