Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mcnc!thorin!pooh!chen
From: chen@pooh.cs.unc.edu (Dave)
Newsgroups: comp.arch
Subject: Re: *big iron*
Message-ID: <9747@thorin.cs.unc.edu>
Date: 29 Sep 89 22:12:02 GMT
References: <22488@cup.portal.com> <280001@hpdml93.HP.COM> <9911@venera.isi.edu> <34298@lll-winken.LLNL.GOV> <2045@hudson.acc.virginia.edu>
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Reply-To: chen@pooh.cs.unc.edu (Dave)
Organization: University Of North Carolina, Chapel Hill
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In article <2045@hudson.acc.virginia.edu> gsh7w@astsun3 (Greg Scott Hennessy) writes:
>In article <34298@lll-winken.LLNL.GOV> brooks@maddog.llnl.gov (Eugene
>Brooks) writes: 
>#Thinking Machines' Data Vault is a fine example of the right way to
>#build an IO system these days. 
>#The system has 72 drives
>#implementing a 64 bit wide data path with one bit per drive. 
>
>What are the extra 8 drives used for? Parity?
>


They are there for SEC-DED, i.e. single error correction, double error
detection.  If one of the 64 drives goes bad, the data is can be completely
recovered simply by accessing every word in the vault.  When doing a
read the extra 8 bits allow you to tell which bit is wrong.  If two bits are
wrong it can be detected, but not corrected.  The method is described in
many computer architecture books, I think, and is used in most mainframe
memory systems.

Dave

_________________________David_T._Chen_(chen@cs.unc.edu)_______________________
It's funny, I hate the itching, but I don't mind the swelling.
		-- David Letterman