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From: CSvax:Pucc-H:ags@pur-ee.UUCP
Newsgroups: net.micro,net.micro.apple
Subject: Re: apple OS
Message-ID: <298@pucc-h>
Date: Thu, 29-Sep-83 02:31:30 EDT
Article-I.D.: pucc-h.298
Posted: Thu Sep 29 02:31:30 1983
Date-Received: Sat, 24-Sep-83 02:20:43 EDT
References: <3564@duke.UUCP>
Organization: Purdue University Computing Center
Lines: 62


  Ned Danieley has raised some interesting questions concerning the way Apple
Pascal remembers volume names.  The system is reasonably intelligent, but it
is not foolproof.  I will paraphrase:

Q. Suppose I have two volumes on line (FOO: and BAR:).  While my program is
   running, I remove BAR: and insert JUNK:.  If my program now tries to reset
   JUNK:TEMP, will it succeed?

A. If TEMP exists on the volume JUNK:, the system will find it.  It will try
   all the drives, if it has to.  If JUNK: is on line, but there is no file
   TEMP, your program (assuming it does its own checking) will get an ioresult
   of 10, which means "No such file on specified volume."

Q. If JUNK: is not on line, can my program correctly determine the reason for
   failure?

A. If the system fails to find JUNK: in any of the drives, your program will 
   get an ioresult of 9, meaning "No device -- volume is not on line."  You
   may then take appropriate action (such as pausing to remind the user to
   "Put in JUNK:").

Q. Suppose the file I want to open is BAR:TEMP, and that I have just removed
   BAR: and put in JUNK:.  The system still has the directory for BAR: in
   memory.  Won't this cause trouble?

A. You're right.  In this case the system does not double-check.  On the other
   hand, if you ask for #5:TEMP (assuming BAR: was last seen in drive #5), then
   the system does re-read the directory before proceeding.  (What can I say?
   I have been using Apple Pascal for two years and this has never happened --
   even though when I first started I had only one drive and I had to put in a
   new disk every 3.8 seconds.)  Probably the best defensive measure is to do
   a unitclear on each drive before you start opening files.  I now have three
   drives and I don't often swap disks anymore except to make backups, so I'm
   not particularly worried.  This is the only thing resembling a serious
   problem that I have ever found in the system (I know of two minor bugs in
   addition to this).  I only wish that VSOS 2.1 (the operating system for the
   Control Data Cyber 205) were as stable and bug-free as Apple Pascal 1.1.
   Incidentally, Apple has announced that version 1.2 will be available in
   the fourth quarter of 1983, with full support for all features of the
   //e, including 128K bank-switched memory and double-density graphics, as
   well as hard disk support.

Q. What if I put two volumes named FOO: on line with different directories?

A. The manual specifically warns against doing th.  An important special case
   is making backups -- you put the original and the backup (both named FOO:)
   on line and specify the copy in terms of unit numbers rather than volume
   names.  The system makes the copy and then reminds you that you have two
   volumes with the same name.  It keeps reminding you, if necessary, on
   every command until it notices that you have removed one.  The manual
   also points out that you should not replace a disk by another having
   the same name, for obvious reasons.  Since my backup disks all live
   across the room from the computer, and nothing else has the same name,
   I don't worry about this restriction.

This has gotten too long.

					Dave Seaman
					Purdue University User Services
					Pucc-H:ags