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From:	empty AT sans DOT vuw DOT ac DOT nz (Malcolm Taylor)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Video question
Date:	31 Jul 1995 23:30:31 GMT
Organization:	SANS, Student Access Network System
Lines:	30
References:	<DCL5Iu DOT 3M8 AT agora DOT rdrop DOT com>
Nntp-Posting-Host:	mu.sans.vuw.ac.nz
To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Dj-Gateway:	from newsgroup comp.os.msdos.djgpp

Steve Havelka (shavelk AT agora DOT rdrop DOT com) wrote:
: Hi.  I have written a few graphics programs (mode 13h) for DJGPP,
: and they access video memory at 0xd0000000.  If I try to run
: them under a DPMI server, though, they GPF when they try to
: access the video memory.  Is there another way to access video
: memory so the programs don't crap out under DPMI?

Several ways. The above way is only possible on the 1.12m4 and earlier 
version of djgpp in the nodpmi mode. The VRAM page fault handler is 
specific to go32, so if you use dpmi you don't get these buffers. Djgpp 
v2.0 is DPMI only, so you'll run into this problem if (when) you upgrade.
The video memory is actually at 0xa0000 in physical memory. You can 
access this 64k segment using the sys/farptr.h routines or by using 
assembly code directly. The sad point here is that you have to handle the 
page changing yourself, making graphics routines harder.
The way I got around this problem was to write a BitBlt routine that 
Blted an area of memory to screen, and then work entirely (almost, I have 
written a few specific routines to write to the screen) in memory 
buffers which can then be Blted to the screen. This is actually quite 
fast in some situations (esp. when working with a slow video card).
There is a hack available (I think) to do the page fault handling under 
DPMI, but it isn't guaranteed to work.

Hope this helps.

Malcolm

: --
: Steve
: Life is pain.  -- Siddhartha

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