Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!apple!mattd
From: mattd@Apple.COM (Matt Deatherage)
Newsgroups: comp.sys.apple
Subject: Re: IIgs Software Reboot
Message-ID: <37346@apple.Apple.COM>
Date: 18 Dec 89 00:49:40 GMT
References: <3388@sage.cc.purdue.edu> <0ZWjDCW00WAB00WG8F@andrew.cmu.edu> <3389@sage.cc.purdue.edu>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 50

In article <3389@sage.cc.purdue.edu> aj0@sage.cc.purdue.edu (Eric Mulholland) writes:
>In article bh1e+@ANDREW.CMU.EDU (Brendan Gallagher Hoar) writes:
>>Apple changed the boot block on system 5.0 to allow the loading of
>>sparse PRODOS files.
>
The original message posted this as a word of "warning".  Not only should
nobody need to be "warned", but merely "informed" (yes, I'm still being
optimistic), but in fact this change took place for System 4.0.  It was
changed again for 5.0 because an annoying developer pointed out that the code
in the 4.0 boot block didn't correctly zero out the last half of a sparse
block.  We eventually got sick of this developer whining and hired him, and
today he's known as Dave Lyons.  (GET TO KNOW HIM!)

>    I was curious to what the change to the boot block is and now read
>that it's for sparse file handling.  When would the ProDOS file need to
>be sparsed?  The only reason I can come up with why is the quit code for
>ProDOS 8.  But IF that code was improved to be more "user friendly" than
>this enter pathname stuff, than the extra space causing the sparseness
>would be used up, for a good cause.
>    Seriously, why would a prodos file need to be sparsed?
>     ____
> Y_,_|[]|   Eric Mulholland
>{|_|_|__|   aj0@sage.cc.purdue.edu
>//oo--OO    ...!pur-ee!sage.cc!aj0

We'll ignore arguments for or against the existence of sparse files in the
ProDOS file system ("why would a prodos file need to be sparsed") and look
at the problem when the essential boot file PRODOS is sparse.

Before this change, there was no telling if the system could boot that way
or not.  The boot block read block zero into a space where there should have
been 512 zeroes, and that could cause a problem.

This was discovered in the testing of 4.0 (the initial release of GS/OS).
The ProDOS FST automatically makes a file sparse if it could be, and we were
interested to discover that using GS/OS to make a file-by-file copy of System
Disk 3.1 produced a sparse PRODOS boot file.  In other words, copying a disk
file-by-file left you with a system software disk you couldn't boot.  Since
it was a hole in the system that might cause problems some day, it was
changed.

-- 
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