Xref: news2.mv.net comp.os.msdos.djgpp:8466
From: Elliott Oti <e DOT oti AT stud DOT warande DOT ruu DOT nl>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Allegro & MODE X help!! Please...
Date: Mon, 09 Sep 1996 20:17:47 -0700
Organization: Academic Computer Centre Utrecht, (ACCU)
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To: djgpp AT delorie DOT com
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Isjolly wrote:
> 
> Hi,
>      Does anybody know if it`s possible to use a full 1 MB or more of
> contiguos video mem. I`ve tried altering the restraints of Allegros 256k
> limit
> to 1MB but the machine just resets hmmm. Perhaps I`m just a thickster
> & I haven`t really grasped mode X`s abilties. Any help would be much
> appreciated.

I don't think it's possible with plain-VGA Mode-X.  The standard mode X's
resemble mode 13h in that they can access only the first 256Kb of video
memory; I don't think the VGA adapter can map more display memory
to the a0000 window, so even if you have 2Mb only the first 256Kb count.
The a0000 window is 256Kb long so to map more than 256Kb requires that
the memory be moved from the display memory in chunks, bank switching I
believe that's called, and VGA doesn't support that.
There is an allegro mode-X 640x400 but AFAIK it's a tweaked VESA mode.

Elliott