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From: davido@lucifer.UUCP (David Ornstein)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Writing directly to screen memory
Message-ID: <7@lucifer.UUCP>
Date: Tue, 4-Nov-86 22:16:09 EST
Article-I.D.: lucifer.7
Posted: Tue Nov  4 22:16:09 1986
Date-Received: Mon, 10-Nov-86 21:35:41 EST
References: <442@uwmacc.UUCP>
Distribution: net
Organization: The Ninth Plane, Newton, MA.
Lines: 19
Summary: Checking CRT controller is to avoid snow

In article <442@uwmacc.UUCP>, curtis@uwmacc.UUCP writes:
> I have been perusing through some assembler code written who knows where,
> and I noticed that when a particular routine writes to screen memory,
> it first reads the video controller scan status and makes sure it is 
> low, then after it goes low it waits until it goes high before writing
> to the screen buffer in memory.  The associated comment remarks that
> it must go high before it is safe to write directly to screen buffer
> memory.

The code here is checking to make sure that the CRT is doing a retrace so that
no snow will be created on the screen during update.  If you write directly
to screen memory on the IBM CGA (Color Graphics Adaptor) during a non-retrace 
period, you get snow.

david

{seismo!ihnp4}!mirror!lucifer!davido

(David Ornstein @ Lucifer @ The Ninth Plane - voice: 617-655-9191)