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From: sdh@joevax.UUCP (Retief of the CDT)
Newsgroups: comp.sys.apple
Subject: Re: // screen memory question
Message-ID: <368@joevax.UUCP>
Date: Thu, 4-Dec-86 10:15:46 EST
Article-I.D.: joevax.368
Posted: Thu Dec  4 10:15:46 1986
Date-Received: Sat, 6-Dec-86 21:44:24 EST
References: <18800002@uiucme>
Organization: Bell Communications Research Inc., Morristown, NJ
Lines: 43

> How do you create windows on the //e (or //c) in 80 column mode?  What I would
> like to do is save the area that I want to write the window to, plop the
> window there and when I am done with the window, replace the window with
> the original stuff.  I have no trouble doing this on my pc because I know 
> where screen memory starts.  I can't find reference to this in any of the
> manuals that I have.
> 
> Any help will be appreciated...
> Matt

Don't know how much this will help, but here goes.

In 40 column mode, the page 1 one of the text screen starts at $400, and
ends at $7ff, with page 2 following it from $800 to $bff.  If memory serves
me correctly, the dual pages are combined to produce one 80 column screen.
So the first 40 characters on one the top line go from $400 to $427, and the
second 40 go from $800 to $827.

This makes good sense because programs that were written in 40 column mode and
access the screen directly will still operate normally.  The only problem I
have is that Applesoft programs generally start at $800, and I know from
experience that it takes a bit of twiddling to get them to start elsewhere.

How to test this:  without anything important  in memory, turn on 80 column
mode and start changing bytes of screen memory.  If the second half of the
screen does not start at $800, most likely it is being taken out of one of
the banks of extended memory.

The //e reference manual should give memory maps of all screen memory.

One other caveat:
There are several bytes that go undisplayed on the text screen.  These are
used by DOS to remember the position of the R/W head.  If you change these
bytes, drive will not be able to find what its looking for the next time
around and will have to re-align itself (the icky grinding noise).  I don't
know where these bytes are.  I think they are the last few bytes.  Its been
so long, I forget.  It makes sense though.  One text screen is 1024 bytes.
Divide that by 40 characters and you should get the number of lines of
text on a screen: 25.6, not quite, but close.

Good luck
Steve Hawley
bellcore!sdh