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From: eli@aspasia.gang.umass.edu (Eli Brandt)
Newsgroups: comp.arch
Subject: Re: Workstation Data Integrity
Message-ID: <18372@dime.cs.umass.edu>
Date: 14 Aug 90 18:51:00 GMT
Sender: news@dime.cs.umass.edu
Reply-To: eli@aspasia.CS.UMASS.EDU (Eli Brandt)
Organization: University of Massachusetts, Amherst
Lines: 38
References:

In article <1990Aug10.171744.9639@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>In article <2399@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.com (bill davidsen) writes:
>>| Most PCs (including the MACs I've seen) don't have or at least
>>| don't use parity.
>>  The IBM PC, AT, and PS/2 models use per-byte parity, as do all of the
>>clone machines built by other vendors. This provides adequate
>>protection... The term PC includes both business PCs, with minicomputer
>>features, and machines intended primarily for games and home use, which
>>are built as cheaply as possible...
>
>But, but, but... virtually all MSDOS software *explicitly ignores*
>parity errors.  A friend of mine, working for a clone builder, had
>an interesting story to tell.  They were horrified to discover that
>their parity circuit didn't work... after a good many of the machines
>were in the field and functioning fine!  It hadn't been caught in
>the factory because there is no way that software can test the IBMPC
>parity system, and it hadn't been caught by the customers because all
>the commercial software just ignored it.
>
>People who think their MSDOS "business PCs" are somehow "protected"
>against memory errors by the parity hardware are kidding themselves.
>-- 
>It is not possible to both understand  | Henry Spencer at U of Toronto Zoology
>and appreciate Intel CPUs. -D.Wolfskill|  henry@zoo.toronto.edu   utzoo!henry

Almost all PC hardware that I know of detects parity errors and handles them -
well, "handles" by crashing with a "Parity error" message.  Better than a
corrupted filesystem.  The one exception that I know of is that some laptops
leave off parity checking to save *weight*, of all things.  How much can 11% of
your DRAM weigh?  It's possible that some fly-by-night cloners leave off 
parity checking, but I've never heard of any machines that do this.

I can personally testify that PS/2's, at least, know about parity errors.  I
was playing around with the DRAM refresh rate and managed to get parity errors
quite definitively.  A parity error triggers an NMI which calls, I think, an
INT 1.  Not at all sure about that.  I don't know of any commercial software
that turns off parity checking (by trapping the interrupt, presumably).  Can
you name any?