From: "Lone Tiger" <lonetiger AT mtu-net DOT ru>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Where to read signal for vertical retrace ?
Date: Mon, 28 Dec 1998 14:39:14 +0300
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Christian Hofrichter пишет в сообщении <368509BA DOT 2B73FBC1 AT gmx DOT de> ...
>To avoid flickering on my monitor when doing graphic operations in the
>vesa-modes, I want know from which port I have to read the signal for
>the vertical retrace.
>I want to know when the retrace starts and it would be nice if I knew
>when it ends.
>As this isn't a part of the vesa-interface (functions) this can cause
>problems on some graphic-cards which are not completely VGA-compatible,
>couldn't it ?
>So what can I do ?
>

There's a 3DA port on every VGA-compatible card. 3rd bit of it indicates
current state of
your video: 0 means retracing is performing, 1 means no retracing now. So I
can suggest
the following code to sync with retrace signal:

while( !(inportb( 0x3DA )&4) );
while( inportb( 0x3DA )&4 );

The first line is to assure that we're bound EXACTLY to the beginning of
retrace (you can get 1 on the very end of retrace process, so there'll be
distortions).