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From: witters@fluke.UUCP (John Witters)
Newsgroups: net.micro
Subject: Re: net.digital: Is parity *really* worth it?
Message-ID: <694@vax1.fluke.UUCP>
Date: Thu, 2-Aug-84 11:36:10 EDT
Article-I.D.: vax1.694
Posted: Thu Aug  2 11:36:10 1984
Date-Received: Sat, 4-Aug-84 02:23:02 EDT
References: <678@sbcs.UUCP>
Organization: John Fluke Mfg. Co., Everett, WA
Lines: 38

I feel the primary advantage of parity is to let a user know when the memory
has suddenly gone "flakey".  In this case, I think it is better to crash the
system rather than let it run and corrupt data (i.e. disk file directorys).
The parity detection prevents a user from operating a system until the problem
is corrected.  I agree that memory chips these days are extremely reliable.
However, this doesn't console the poor user unlucky enough to have a chip go
bad that wipes out all data since the last backup.

Another advantage is that parity will prevent a system from exhibiting bizzare
and un-reproducable behavior due to a bad memory.  Parity will crash the system
before this behavior occurs, and will immediately indicate where the problem
lies.

The primary disadvantage of parity is soft errors.  This causes complaints from
users that the system normally works O.K., but reports a parity error roughly
once a month (or three months or six months).  This problem can be solved with
more hardware.  The approach is to treat the error like a page fault in a
virtual memory system:  branch to the error routine, re-read the memory cell to
find out if it is a hard error, then restart the instruction that caused the
error.  If it was a hard error, then crash the system.  This requires a
processor with instructions that can be halted and re-started, and hardware to
record where memory errors occur in, addition to the parity detection
circuitry.  Of course, all this extra hardware will reduce reliability.  Most
people prefer to deal with a few extra user complaints than add this kind of
hardware.  If the system already uses virtual memory, it may not take much
extra hardware to use this scheme.

The decision to use parity depends on the system.  It doesn't make much sense
for a home computer that has a total of eight memory chips.  It makes much more
sense for a workstation that has a few hundred chips.


						John Witters
						John Fluke Mfg. Co. Inc.
						P.O.B. C9090 M/S 243F
						Everett, Washington  98206

						(206) 356-5274