Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 Fluke 1/4/84; site fluke.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!houxm!houxz!vax135!cornell!uw-beaver!microsoft!fluke!witters 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