Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!usc!apple!landon
From: landon@Apple.COM (Landon Dyer)
Newsgroups: comp.arch
Subject: Re: Workstation Data Integrity
Message-ID: <43884@apple.Apple.COM>
Date: 12 Aug 90 05:51:41 GMT
Organization: Apple Computer Inc., Cupertino, CA
Lines: 29


>Depends on the situation.  A parity error in a code page is harmless --
>just bring in a fresh copy from disk.

Assuming it's fresh.  Nearly all of the I/O systems I've seen on small
computers lack end-to-end parity or ECC.  For instance, SCSI data and commands
are subject to mangling by poor termination, bad connections, transients, and
firmware or hardware failure (e.g. an insane controller that wiggles a bus
line at random).

This (and to be fair, other real-world catasrophes including scrambled file
systems, flakey packet routers, media decay and buggy drivers) is what causes
some application writers to put checksum fields in their document formats.


    Q:	Let's get this straight.  The data on _disk_ is checksummed
	within an inch of its life.  The data in _memory_ is ECC'd
	and can't be harmed.  But going from disk to memory, the
	data is, ah, er ...

    A:	Let's see what the standard sez ... [flip, flip] ... "Naked in
	the breeze?"


-- 
Landon Dyer (landon@apple.com)  :::::::::::::::::::::::::::::::::::::::::::::::
:::::::::::::::::::::::::::::::::: making the merry-go-round SPIN FASTER
Apple Computer, Inc.            :: so that everyone has to HOLD ON TIGHTER
NOT THE VIEWS OF APPLE COMPUTER :: just to keep from being THROWN TO THE WOLVES