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From: mdavis@seismo.css.gov@pro-sol.UCSD.EDU (Morgan Davis)
Newsgroups: comp.sys.apple
Subject: Re: screen memory question (oops!!)
Message-ID: <8612101434.AA05630@crash.UUCP>
Date: Wed, 10-Dec-86 02:35:09 EST
Article-I.D.: crash.8612101434.AA05630
Posted: Wed Dec 10 02:35:09 1986
Date-Received: Mon, 15-Dec-86 06:11:02 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 24

WHOA!  STOP!

Someone suggested using BSAVE's and BLOAD's to save and restore video memory.
That works fine for graphics images, but you CANNOT do such a thing with text
video RAM.  (well, you can, but it might really fry your system).

In an earlier message, I talked about "screen holes" in the video display
memory between $400 - $7ff in the Apple's main 40-column video bank.  Those
screen holes are reserved for peripheral card usage, and the disk operating
system will also make use of some of them.

By disturbing this RAM in any way, unpredictable results might occur.  The
only way you can safely save and load a screen image using a disk for storage
is to move only the display characters from video RAM into a safe buffer, and
then write it to disk (or ramdisk).  To read the image from disk, you must
load the file into a a safe buffer, and use a memory move to copy those bytes
back into the display memory (leaving the screen holes unscathed).

A word to the wise.

--Morgan Davis

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