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From: dhinds@portia.Stanford.EDU (David Hinds)
Newsgroups: comp.arch
Subject: Re: Workstation Data Integrity
Message-ID: <1990Aug10.223619.6223@portia.Stanford.EDU>
Date: 10 Aug 90 22:36:19 GMT
References: <40694@mips.mips.COM> <2399@crdos1.crd.ge.COM> <1990Aug10.171744.9639@zoo.toronto.edu>
Organization: AIR, Stanford University
Lines: 28

In article <1990Aug10.171744.9639@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>>  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.
>
    Wait - what do you mean, the parity circuit didn't work?  That it
couldn't detect parity errors, or what?  On the IBM PC, and most clones,
I think, a parity error raises a non-maskable interrupt.  Under DOS, this
is not a recoverable error - i.e., a parity error hangs the system.  DOS
just prints some dumb message, and stops dead in its tracks.  I suppose
commercial software could patch the interrupt vector to try to recover
from the error, but no one bothers.  As far as I know, yes, there isn't a
way for software to tell if the parity system is working, but then
wouldn't that be a bit much to expect on a PC?

 -David Hinds
  dhinds@popserver.stanford.edu