Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!eos!ames!mailrus!cornell!calvin!johns From: johns@calvin.EE.CORNELL.EDU (John Sahr) Newsgroups: comp.sys.next Subject: Re: NeXT Memory - No Error Checking or Parity ! Message-ID: <854@calvin.EE.CORNELL.EDU> Date: 16 Dec 88 16:25:19 GMT References: <549@gt-eedsp.UUCP> <8348@alice.UUCP> <1429@cpoint.UUCP> Reply-To: johns@calvin.ee.cornell.edu.UUCP (John Sahr) Organization: Cornell Space Plasma Physics Group Lines: 39 In an article, Alien Wells gives interesting information about MTBF for soft and hard errors of memory. Also, the following information about performance of single error correct/double error detect is given: In article <1429@cpoint.UUCP> alien@cpoint.UUCP (Alien Wells) writes: >Standard EDC will detect 2 or more errors and correct 1 in the word size it >deals with. The number of check bits required is log(2) of the word size. >Thus, the following chart shows the memory overhead required: > >Word Size EDC Check Bits 8-bit Parity Bits >--------- -------------- ----------------- > 8 5 1 > 16 6 2 > 32 7 4 > 64 8 8 > >As you can see, by the time you get to 64 bit memory - there really isn't a >reasonable excuse to not use EDC. (Of course, you could start using 16 bit >parity ... but the protection is significantly diluted) Even 32 bit memories >are seeing EDC used more and more often. The parity check versus EDC comparison is not quite fair, because they are really doing two different things. For 64 bit word, although EDC can detect 2 errors and correct 1, the parity check can detect up to 8 errors (while correcting none). So, the tradeoff is not quite so clear. Although single error correction and 2 error detection is straightforward, parity checking must be faster: in fact, the 2 error detect is just a global parity check built on top of the Hamming code (single error correct). As far as the absence of any checking on the Mac or NeXT, I think it is defensible: single error detect per word would be nice, however. (ps. error detection is a little hobby of mine; I've taken a few classes, that's all)-- John Sahr, School of Elect. Eng., Upson Hall Cornell University, Ithaca, NY 14853 ARPA: johns@calvin.ee.cornell.edu; UUCP: {rochester,cmcl2}!cornell!calvin!johns