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From: jak@interlan.Interlan.COM (Jeff Koehler)
Newsgroups: comp.sys.next,comp.arch
Subject: Re: parity is for farmers?
Summary: you gotta need it to appreciate it!
Message-ID: <1991Jun5.155332.485@interlan.Interlan.COM>
Date: 5 Jun 91 15:53:32 GMT
References: <1991May21.232331.24888@cs.umn.edu> <1991May22.234515.24685@milton.u.washington.edu> <1991May23.203950.20953@zoo.toronto.edu> <1991May25.062358.13694@m.cs.uiuc.edu> <1991Jun4.143404.17535@sobeco.com> <1991Jun5.005019.23499@ux1.cso.uiuc.edu>
Reply-To: jak@interlan.interlan.com
Organization: Racal InterLan, Inc., Boxborough, MA (1-800-LAN-TALK)
Lines: 53


	Here's my two cents on parity RAM -- anyone that wants to 'use it'
	for purposes other than parity, or believes they can get around it,
	most likely has never had any bad RAMs!

	I have had parity errors on two of 'my' machines, both 386-AT style
	boxes -- one at home, one at work.  In both cases, I found the
	parity errors to be real.  The errors broke down like this:

		1)	motherboard 256kx1 'original', in the lower 640k.

		2)	motherboard 'add-on' SIMMs, 4Mx9's, which also
			turned out to be in the lower 640k. (I removed
			the original 256kx1's).

	I feel lucky becuase the errors actually occured in memory below
	the 1-MByte limit, where a quick hack-job memory tester I wrote
	could easily find them.  Sure enough, the errors were real.
	
	Case 2) only happened when walking a '1' across zeros, where 
	the '1' was in bit 0 and the error occured in bit '16' (I am
	pointing this out to illustrate the quirky nature of bad RAMs).

	Needless to say, I was unconvinced the first few times I experienced
	a system halt (by the BIOS), but as it occurred more and more often,
	I thought it was time to do something.

	How would I have found this without parity?  Well, my memory tester
	would find it, but not the simple power-up sequence tests in the
	BIOS ROMs (in fact, I had to disable the NMI for parity errors in
	order to get my program to be able to print the errored location to
	the screen).  But how long would strange things have been happening
	if there was no parity?  How many hours would I have wasted with
	errant programs, etc.?  Sound absurd?  Wait 'till *YOUR* memory
	goes bad!  I certainly have a few 1-2-3 spreadsheets with some '|'
	characters that changed to a '{' because I told the BIOS's NMI 
	handler to 'keep going' rather than reboot.

	The machines we use today at home may easily have more memory than
	the original CRAY-1S I used to write diagnostics for back in college.
	Technicians there pointed out that the ECC memory would log an error
	every few days when under heavy use -- and believe me, If there was
	a PC with ECC instead of parity, I would be waiting in line ...
	but just like the person who will drive around for years with no spare
	tire in the trunk, there will be people trying to subvert the purpose
	of those 'wasted bits' in the ECC memory.
							Jak

((((((((((((((((((((((((((((((((((|))))))))))))))))))))))))))))))))))))))))
(( Sr Hardware Engineer       ))    ((  Jeff Koehler                     ))
(( Racal InterLan, Inc.       ))    ((  jak@interlan.com                 ))
(( Boxboro, MA  508-263-9929  ))    ((  imagine code with this many '('s!))
((((((((((((((((((((((((((((((((((|))))))))))))))))))))))))))))))))))))))))