From: ludvig <ludvig AT club-internet DOT fr>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: dirty stuff 2
Date: Sun, 24 Aug 1997 04:06:44 +0200
Organization: Grolier Interactive Europe
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References: <33FB734F DOT 7E2F AT wanadoo DOT fr>
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To: djgpp AT delorie DOT com
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Alan Poppleton wrote:
> 
> Hello everyone,
> 
> Thanks for the responses I have recieved concerning my previous question
> about 'dirty stuff'.  What is the best method for updating only the
> portions of the screen that have changed without having the screen
> flicker?  If for example I am in 640x480x256, would i use two 480 arrays
> corresponding to the minimum and maximum position to update for every
> line? Or is there a better way that would allow much more precise
> updating?  If so, could someone please explain to me the full procedure.
> 
> Thanks a lot,
>         Tony
Try to find a VGA RetraceRoutine.
It might help you, it waits for the screens updating to be finished or
it waits just until it starts(so you can peacefully write to the screen
after the retrace).
I found this on the net another day, maybe it will help you:

  mov dx,3dah                
@wait1:
  in al,dx                    // Screen is updated
  test al,08h
  jnz @wait1
@wait2:
  in al,dx                    // Screen sterts to re-update
  test al,08h
  jz @wait2

After this routine the screen starts to be drawn=you can start to flush
from top->bottom. If you want to know exactly were the retrace is(line
etc) I don't know but if you find something, I would be interrested...

Ludvig  <ludvig AT club-internet DOT fr>