Article View: comp.sys.ibm.pc
Article #14390Re: Writing directly to screen memory
From: David Ornstein
Date: Wed, 05 Nov 1986 03:16
Date: Wed, 05 Nov 1986 03:16
19 lines
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1480 bytes
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)
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Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: arc1-ca.pugleaf.net!utzoo!mnetor!seismo!lll-crg!rutgers!mit-eddie!husc6!necntc!mirror!lucifer!davido 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