From:	Ron Grunwald <RGRUNWAL AT wasp DOT cs DOT cowan DOT edu DOT au>
Organization:	Edith Cowan University
To:	ccsnad AT bath DOT ac DOT uk (Neil A Drage)
Date:	Mon, 10 Jul 1995 17:10:24 GMT+800
Subject:	Re: DPMI and graphics
Reply-To:	r DOT grunwald AT cowan DOT edu DOT au
Cc:	djgpp AT sun DOT soe DOT clarkson DOT edu

Neil Drage wrote:

> Anyway. If you are compiling using DJGPP with DPMI is
> it still possible to write directly to the screen, using
> address 0xD0000000 I mean?
> 

There is an alternate way to read/write from display memory directly 
under DPMI. This requires creating an LDT descriptor first though. 
Eg.

Use function 0000h of the DPMI interface API to allocate a 
descriptor. In INTEL syntax:

DPMI_INTERFACE  equ  31h

mov      ax,0000h
mov      cx,1          ;allocate one descriptor
int      DPMI_INTERFACE
jc       error
mov      gs,ax          ;get segment selector
jmp      fill_desc

error:
.
.
.

Once you've created a descriptor you have to initialize it for the 
segment that you're trying to access.

mov      ax,0007h          ;Set segment base address
mov      bx,gs          ;get segment selector

;Assuming your display segment starts at A000:0000h
mov      cx,000Ah          ;cx:dx hold the 32-bit linear base addr. 
xor      dx,dx            ;of the segment

int      DPMI_INTERFACE

Next, specify the size of the segment. I'm assuming 128Kb for an 
super-VGA card.
mov      ax,0008h          ;Set segment limit
mov      cx,0002h          ;cx:dx hold the 32-bit segment limit
xor      dx,dx
int      DPMI_INTERFACE


To read/write directly from display memory all you have to do is to 
place the segment selector into a segment register, and specify the 
desired offset within the display segment. Eg.

mov      gs,display_selector
mov      esi,000001F2h
mov      al,gs:esi          ;read a byte from display memory


Of course what is really handy with this sort of programming is the 
DPMI interface programming specification. I'm not sure whether you've 
got this, but unfortunately I forgot where I retrieved my copy from.


Regards, Ron.

 ********************************************************************
 | Author.............. Ron Grunwald                                | 
 | Internet............ r DOT grunwald AT cowan DOT edu DOT au                     |
 | Phone............... (09)273 8027 or (09)273 8468                |
 |------------------------------------------------------------------|
 | Department.......... Computer Operations and Systems Management  |
 | Division/Faculty.... Information Technology                      | 
 | Institute........... Edith Cowan University, Churchlands         | 
 | Location............ Perth, Western Australia                    |
 ********************************************************************
 "I don't have any solution but I certainly admire the problem!"

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