From: lonniem AT mail DOT utexas DOT edu
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Synchronising graphics with the screen
Date: Tue, 10 Dec 1996 22:27:50 GMT
Organization: The University of Texas at Austin, Austin, Texas
Lines: 30
Message-ID: <58kh54$jgq@geraldo.cc.utexas.edu>
References: <32AD6A7A DOT 41C67EA6 AT ic DOT ac DOT uk>
NNTP-Posting-Host: slip-4-12.ots.utexas.edu
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Apparently-To: djgpp AT delorie DOT com

Richard Hopkins <r DOT hopkins AT ic DOT ac DOT uk> wrote:

>I am useing DJGPP, and have written several graphicsy type programs.
>Unfortunately the plotting commands are completely out of synch with the
>monitor. Consequently the image looks crap. Please help.
>Is there some internal register or variable which can be monitored using
>inline assembly? An interrupt?
>thank you
>Hoppy

Yes you can wait for vertical retrace.  This is the time during which
the monitors electron beam is returning to the top left hand corner
from the bottom right hand corner and it gives you quite a bit of time
to do many screen oriented things such as blitting a double buffer or
various other things.  Here's some code:

asm ("movw $0x03DA, %%dx       \n\t"  // 03DAh is the port
        "wait_for_retrace:                 \n\t"
        "inb %%dx, %%al                   \n\t"  // in status register
        "testb $0x08, %%al               \n\t"  // test if retracing
        "jnz wait_for_retrace             \n\t"  // if no do it again

        ... do what ever you want after this.

Hope this helps


Lonnie Mcullough
lonniem AT mail DOT utexas DOT edu