Mail Archives: djgpp/1995/05/25/04:23:33

Date: Thu, 25 May 95 03:04:14 -0400
From: Wonkoo Kim <>
To: "Junaid A. Walker" <junaid AT dino DOT eng DOT monash DOT edu DOT AU>,
Charles Sandmann <sandmann AT praline DOT no DOT NeoSoft DOT com>
Reply-To: Wonkoo Kim <>
Cc: mat AT ardi DOT com, DJGPP Mailing List <djgpp AT sun DOT soe DOT clarkson DOT edu>
Subject: Re: Compiling gdb, dpmi DS:VRAM hack.

> Date: Thu, 25 May 95 12:27:27 +1000
> From: "Junaid A. Walker" <junaid AT dino DOT eng DOT monash DOT edu DOT AU>

> /* Well here it is; how to access video ram at 0xa0000 without using
>    a segment overide (ie using the current process's DS). Works well
>    under MS Windows 3.1 in a full screen DOS box. Try running this app
>    in the background (DOS settings 'Runs in Background' crossed).Then
>    switch away from this app (CTRL-ESC) and run any other program.
>    Windows will remap the video ram at 0xa0000 to a temporary system
>    ram buffer while this app runs minimized; hence this app will
>    run faster when minimized. The screen will be correctly restored 
>    when we switch back to this app.
>       This program works by removing the segment limit on DS and
>    using 32-bit wrap around in the linear address space to access
>    memory at linear address 0xa0000. In fact we now have access to
>    all memory including DOS conventional memory. This circumvents
>    all memory protection (except unmapped memory), so best to save
>    the original DS for normal use, and use the new unlimited DS for
>    the duration that video memory access is needed.
> 	Compile with 'gcc prog.c -lpc' and then 'go32 prog'. You
>    should see some blue lines more down the screen for a while, after 
>    that it will display the 16-bit speed of your video card.
> 	Al-Junaid Walker 22/5/95.
> */

I tested on P90/16MB, ATI Graphics Ultra+/2MB,
in a DOS box under OS/2 Warp:
initial ds segment limit=0005FFFF


Current DS linear base address=00400000

Remapped Video RAM process virtual address=FFCA0000

New DS linear address limit=FFFFFFFF


[I selected another window after a key, to put into background.]

1000 blits took 2911ms. Blit rate=10992KBytes/s

No crash.

BTW, in full-screen DOS box, upper part of screen was drawing down
something like staircases, but middle and bottom parts were flashing 
with many blue stripes.  Was it normal?

// Wonkoo Kim

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