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From: jed@mb2c.UUCP
Newsgroups: net.micro.apple
Subject: Re: Apple //e communications problems
Message-ID: <150@mb2c.UUCP>
Date: Tue, 12-Jul-83 14:29:05 EDT
Article-I.D.: mb2c.150
Posted: Tue Jul 12 14:29:05 1983
Date-Received: Thu, 14-Jul-83 05:12:09 EDT
References: ecs.377
Lines: 23


The problem with losing characters when scrolling is indeed a problem
when using the built-in scrolling functions and certain software/hardware
combinations.  The problem has to do with the time it takes to scroll
the 1920 characters in memory (Apple-//e 80 col card).  Some of the
80-col cards for the ][ (which I should add cost a lot more too) use
a CRTC chip which uses an array of pointers to the lines on the screen, e.g.
to scroll the screen, all the program has to do is move all of the pointers
up by one and clear out an 80-byte memory area for the new line.  This is
obviously much faster than moving 1920 bytes.

SOFTERM has no magic trick for scrolling the screen faster, in fact it
scrolls slower.  What it does do however is never work on the scrolling
job long enough to lose a character at 9600 baud, assuming that the modem
or serial interface can buffer 1 character.  That is, by using instruction
timings, you can figure out about how many instructions you can execute in
1/960th second and you never do more than that without checking to see if
a character came in.  Since the Apple is not interrupt driven from the
factory, this is the only way to do it.  The characters which are brought
in during the scroll operation are buffered (SOFTERM uses 256 byte buffer)
and then processed when the operation is completed.

John Duncan (mb2c!jed)		Michigan Bell		(313) 424-0711