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From: dennis@rlgvax.UUCP (Dennis.Bednar)
Newsgroups: comp.sys.ibm.pc
Subject: Re: terminal programs
Message-ID: <702@rlgvax.UUCP>
Date: Fri, 13-Nov-87 01:48:51 EST
Article-I.D.: rlgvax.702
Posted: Fri Nov 13 01:48:51 1987
Date-Received: Sun, 15-Nov-87 05:58:17 EST
References: <3449@uwmcsd1.UUCP> <15000049@silver>
Organization: Computer Consoles Inc, Reston VA
Lines: 25

In article <15000049@silver>, creps@silver.bacs.indiana.edu writes:
> 
> guess that you mean what is usually called "snow." Snow is caused when
> there is a conflict in accessing the video controller. It can be programmed
> around, in that you can tell your program not to try to access the
> video while it's busy. Evidently, the terminal program you used didn't do

The trick is to write to the video memory only when the hardware is
doing a horizontal retrace or vertical retrace.  Definition:
A horiz retrace means that the video beam is sweeping back from
the right side to the left side, preparing to paint the next odd
or even row across your screen (left to right).  Vertical retrace
is when the video beam has reached the bottom right corner, and
must go back to the top left corner of the screen.  In both cases
the video beam is off, not producing any color or information.
When the video beam is painting a row left to right across a row,
the memory is said to be "busy" (being fetched from by the 6845
video controller chip) - this is precisely is the time you want
to avoid writing to the shared video memory.

-- 
FullName:	Dennis Bednar
UUCP:		{uunet|sundc}!rlgvax!dennis
USMail:		CCI; 11490 Commerce Park Dr.; Reston VA 22091
Telephone:	+1 703 648 3300