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From: mw22#@ANDREW.CMU.EDU (Michael Alan Wertheim)
Newsgroups: comp.sys.apple
Subject: Re: screen memory question
Message-ID: 
Date: Mon, 15-Dec-86 10:47:34 EST
Article-I.D.: andrew.MS.V3.18.mw22.80021110.clarion.ibm032.214.0
Posted: Mon Dec 15 10:47:34 1986
Date-Received: Tue, 16-Dec-86 21:07:53 EST
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   There seems to be a lot of confusion regarding 40 and 80 column text.

   The original Apple II had three display modes -- 40 column text, lo res
graphics, and hi res graphics.  Each of these modes has two pages.  Text and
lo res page 1 occupy the memory from $400 to $7ff, and text and lo res page 2
occupy $800 to $bff.  Applesoft programs normally reside in the page 2
memory.  If you load a program that has some strings, rem statements, or data
statements near the beginning, and flip the page 2 softswitch ($C055 I
think), you will see the strings and rem/data statements along with a lot of
other garbage, which is the tokenized form of your program.  Also note that
you can BSAVE a screen from page 1 or page 2, but you can BLOAD the image
only back to page 2.

   An 80-column card in effect doubles each of the display modes by using
page 1 from main memory and the corresponding page 1 from auxiliary memory.
The 80-column text screen uses $400 to $7ff of main memory and $400 to $7ff
of auxiliary memory.  Page 2 is not used.  Likewise, there is also an
80-column double lo res that uses the same memory and that can easily be
accessed from Applesoft.  This was covered in a past issue of Open Apple.  If
anyone's interested, I'll post a short summary of how to do this.

   Double hi res uses the memory from $2000 to $3fff in both main and
auxiliary memory.  I've heard that there is a double hi res page two that
uses the memory from $4000 to $6fff in both main and auxiliary memory.


Mike Wertheim
(Carnegie Mellon University)