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Path: utzoo!linus!philabs!cmcl2!seismo!hao!ames!barry
From: barry@ames.UUCP (Kenn Barry)
Newsgroups: net.micro.apple
Subject: Re: Banging noise on Apple //e,c disk drive...
Message-ID: <982@ames.UUCP>
Date: Mon, 13-May-85 02:52:19 EDT
Article-I.D.: ames.982
Posted: Mon May 13 02:52:19 1985
Date-Received: Thu, 16-May-85 03:17:50 EDT
References: <573@spp2.UUCP>
Distribution: net
Organization: NASA-Ames Research Center, Mtn. View, CA
Lines: 37

>I am using the Beagle Bros. program GPLE.  When I BLOAD the program,
>the disk sometimes bangs.  It doesn't always do it, just sometimes.
>There doesn't seem to be a pattern.  What could cause this?
 
	DOS keeps pointers that inform it about the disk currently being
accessed (slot, volume, drive) in a few unused bytes of the main text
screen page ($400-$7FF). If the file being loaded overwrites these locations,
DOS temporarily loses track of where things are. One of the things that
happens is that the read/write head recalibrates (goes back to track
0, the only track on the disk it can locate absolutely; other tracks
are found by offset from the current track), which is the banging noise
you hear - the head bangs against the stopping point at track 0. This
is also why the noise occurs when you boot a disk - DOS has no previous
information on the location of the read/write head, and must recalibrate.

>Also, I once did a BSAVE on screen memory, and when I BLOADed it back
>using the same drive it was saved with, it doesn't rattle.  When
>I use another drive it rattles.  Does anybody know what is going on?

	Yup. When you reload the file from the same drive, the bytes
DOS uses in screen memory are overwritten with the *same values* they
had when the file was saved, which are also the same values as the ones
being overwritten when you reload the file from the same slot, drive,
volume. If you switch drives, the "drive" pointer gets changed from "1"
to "2" (or 2 to 1) by the loading of the file, causing the recalibration.
	What to do about it? Ignore it. It does no harm. The only time
I know of where it's a problem is with some hard disk systems. Programs
that overwrite these locations may not load properly from a hard disk,
because DOS will lose track of the correct volume number permanently.
If this happens, resave the problem file with the appropriate volume
number in place.

-  From the Crow's Nest  -                      Kenn Barry
                                                NASA-Ames Research Center
                                                Moffett Field, CA
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