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From: ludemann@ubc-cs.UUCP (Peter Ludemann)
Newsgroups: net.micro.pc
Subject: Re: Addition help with Video Retrace Code
Message-ID: <1208@ubc-cs.UUCP>
Date: Fri, 9-Aug-85 01:17:04 EDT
Article-I.D.: ubc-cs.1208
Posted: Fri Aug  9 01:17:04 1985
Date-Received: Tue, 13-Aug-85 01:46:16 EDT
References: <1884@druxq.UUCP>
Reply-To: ludemann@ubc-cs.UUCP (Peter Ludemann)
Organization: UBC Department of Computer Science, Vancouver, B.C., Canada
Lines: 24
Keywords: IBM-PC colour monitor snow
Summary: An easier way to avoid snow.

In article <1884@druxq.UUCP> bradley@druxq.UUCP (DavidsonBC) writes:
>This is a different way to control the monitor to remove the fuzz or
>snow.  The snow is caused by the video chip and the CPU trying to
>access the same section of memory at the same time ...
>
>	The basic way to avoid the snow is to avoid contension for the
>	video buffer, the most reliable way to do this is to disable
>	the video while writting to the video buffer.

There's a much easier way.  One of the bits in a port for the video
controller tells you whether or not the monitor is doing retrace.
Just loop busily while the bit says retrace isn't happening and
then write your bit of the screen.  I managed to write about
100 characters without any snow but I suspect more are possible.

I don't have the code handy, but if anyone's interested, I'll look
it up and post it.  But if you've got the IBM Tech Reference
handy, it shouldn't be hard to figure out.

One thing: my own IBM-"compatible" doens't have the snow problem
and leaves the port looking as if retrace never happens.
The scren writing program ran at least twice as fast as
on the IBM-PC.  And both were much faster than using BIOS.
     ubc-vision!ubc-cs!ludemann.UUCP