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From: cprice@mips.COM (Charlie Price)
Newsgroups: comp.arch
Subject: Re: Workstation Data Integrity
Message-ID: <41161@mips.mips.COM>
Date: 28 Aug 90 23:13:06 GMT
References: <1990Aug3.204358.330@portia.Stanford.EDU> <40694@mips.mips.COM> <2399@crdos1.crd.ge.COM> <1990Aug10.171744.9639@zoo.toronto.edu> <2421@crdos1.crd.ge.COM> <1990Aug18.210132.25203@sco.COM> <2434@crdos1.crd.ge.COM> <6797.26d6edce@vax1.tcd.ie>
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Reply-To: cprice@mips.COM (Charlie Price)
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In article <6797.26d6edce@vax1.tcd.ie> rwallace@vax1.tcd.ie writes:
>
>If the operating system just told you about it when there was a parity error
 ...
>The error is probably in either an unused area of
>memory (e.g. at the DOS prompt) or in a section of code that isn't going to be
>executed on this session with the program (e.g. error handling code). (I'm
>talking only about transient errors here: the boot time memory check will get
>other kinds). So ignoring the parity error will probably have no effect. If
>it's in a section of code that will be executed, the machine will just crash
>which is what would have happened anyway.
 [some stuff deleted]
>The point is that ignoring a parity error is a pretty safe thing to do; there's
>very little chance of getting a misleading answer.

I sense some confusion.
A boot-time memory check might detect a permanent error,
and this seems to be what you are talking about,
but this isn't what parity is mostly for.

Mostly, parity is to detect transient errors caused by
alpha particles (or some such).
The memory chip doesn't have a permanent problem,
it just forgot the value of a bit.
Parity is computed and written for every store operation.
For every data item fetched (data or instruction) the parity of
the data bits is computed and compared to the parity bit(s) that
were also fetched from memory.  If the computed and stored bits
are not the same, you have a parity error.
A (parity) error in a memory location is only detected at the
time it is fetched, so you are probably going to want the right data.
so your argument about it being unimportant is basically invalid.
-- 
Charlie Price    cprice@mips.mips.com        (408) 720-1700
MIPS Computer Systems / 928 Arques Ave. / Sunnyvale, CA   94086-23650