Date   : Mon, 02 Mar 2009 13:33:13 +0100
From   : kortink@... (John Kortink)
Subject: Fwd: New Screen Modes with ARM7 Co-Processor?

On Mon, 2 Mar 2009 11:57:13 +0000 (GMT), tommowalker@...
wrote:

>
>> >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!

I know the difference. I think we are talking at cross
purposes here.

The object of the exercise is to double the vertical
resolution of existing modes. Not to halve the horizontal
resolution in the process.

Hence 'interlace sync + video' mode could do the trick,
but would require 40K linearly addressable memory. Which
brings me back to my earlier message.

>> 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.

There is no 'address switching trick'. All the CRTC starts
doing is skip every other line in memory and skip one when
starting the odd frame. But we still need 40K. See above.

>> 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.

On the contrary. You do need it. Only when you're in
'interlace sync' mode you don't. Which is exactly my
point.

>> 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.

That would be interesting. I'll have a try.

>> >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).

That's nice, but not what we were talking about.


John Kortink

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