Date:	Tue, 13 May 1997 19:21:21 -0600
From:	spector AT EnchantedLearning DOT com
Subject:	Re: Non-standard video modes (New video mode?)
Newsgroups:	comp.os.msdos.djgpp
Message-ID:	<863568824.24177@dejanews.com>
Reply-To:	spector AT EnchantedLearning DOT com
Organization:	Enchanted Learning Software
References:	<33741974 DOT 4D77 AT EnchantedLearning DOT com> <DqrjdNAvcEdzEwfs AT talula DOT demon DOT co DOT uk>
Lines:	67
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Thanks for the information, Shawn.  I read the documentation you
suggested, as well as some others, and was able to put together a 512x384
256-color video mode similar to mode-X.

There's a catch though -- to get it to work, I need to store a '11' in
the two Clock Select (CS) bits in the VGA Miscellaneous Output Register. 
The meaning of this is completely omitted in all the documentation I've
been able to locate (2 books plus assorted files on the net).  These bits
control the VGA clock frequency (00 = 25-MHz, 01 = 28-MHz, 10 = external
clock, but 11 is not even mentioned as a possibility in the
documentation).

Does anyone know what this does?  (My guess is that maybe it doubles the
clock frequency to something like 50 MHz, but I don't know.)  Is this a
Super VGA extension?  And is this something that will work across the
gamut of SVGA video cards, or does it just happen to work on the one I
have (S3 Virge)?

Thanks in advance for any clues as to how this is working :-).	(Or,
better yet, does anybody know how to make a 512x384 8-bit mode with
standard VGA? I've tried, but I haven't yet been able to get my monitor
to stop rolling and sync correctly -- maybe VGA just doesn't support a
high enough dot frequency to do this.)

Mitchell
--
Mitchell Spector
Enchanted Learning Software
E-mail: spector AT EnchantedLearning DOT com
Web site: http://www.EnchantedLearning.com



In article <DqrjdNAvcEdzEwfs AT talula DOT demon DOT co DOT uk>,
  Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk> wrote:
>
> Mitchell Spector writes:
> >How does one go about programming support for non-standard video modes?
> >(I'd like 512x384 with 8-bit color, implemented so that it's useable by
> >Allegro.)  UniVBE is able to do this even when using video cards that
> >ostensibly don't support that resolution, and I'm curious how this can
> >be done.
>
> UniVBE does it by having lots of very chipset-specific code for each
> card that it supports: not a realistic option for most people!
>
> It is possible to get quite a lot of non-standard VGA modes by entering
> mode-X and tweaking the VGA register settings, but I've never heard of
> 512x384 being accessible in this way (certainly modes like 360x480,
> 376x282, and 400x600 are quite possible, even if they don't work on
> every monitor). To see how these are done, look at the modex.c file from
> Allegro. It enters standard mode 13h and then tweaks the appropriate
> registers, using some tables that list the changes required for each
> video mode.
>
> >Thanks in advance for any pointers to documentation or other
> >information.
>
> Get VGADOC3 from x2ftp.oulu.fi (I can't remember the exact path, it's in
> something like /pub/msdos/programming/docs).
>
> --
> Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
> Beauty is a French phonetic corruption of a short cloth neck ornament.

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