Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!phri!roy
From: roy@phri.UUCP (Roy Smith)
Newsgroups: comp.unix.wizards,comp.arch
Subject: Double-bit errors and ECC memory
Message-ID: <2891@phri.UUCP>
Date: Thu, 10-Sep-87 10:23:49 EDT
Article-I.D.: phri.2891
Posted: Thu Sep 10 10:23:49 1987
Date-Received: Sat, 12-Sep-87 10:42:53 EDT
References: <1184@itm.UUCP> <797@spar.SPAR.SLB.COM>
Reply-To: roy@phri.UUCP (Roy Smith)
Organization: Public Health Research Inst. (NY, NY)
Lines: 26
Xref: mnetor comp.unix.wizards:4186 comp.arch:2135
Original-Subject: Re: Computers: The New Generation (was: Re: Free Software Foundation)


In article <797@spar.SPAR.SLB.COM> hunt@spar.UUCP (Neil Hunt) writes:
> Does anyone know about soft failure modes of DRAMs ? How likely is it to
> find double bit errors ? With denser and denser memory chips, one might
> expect that one day soon, background alpha particles will be able to flip
> several adjacent bits.

	The way most (all?) modern memory systems are built is to have each
chip contribute a single bit to each of many words.  Thus, a typical 1
Mbyte ECC board (small by today's standards) might consist of 39 256k
chips, each chip contributing a single bit to each of the 256k 39-bit words
(32 data plus 7 EEC bits) on the board.  If several bits in a given chip
were to go bad, you would see errors in the same bit of several different
words.  If an entire chip were to die, you would see an error in the same
bit of *every* word on the board.  The memory controller would be able to
correct any of these problems.

	Note that the typical-but-mythical memory board described above
has 7 check bits per 32 bit data word.  Since you need 2N+1 check bits to
correct an N-bit error, this board should be able to detect and correct as
many as 3 bad bits in any 32-bit word.  Thus, you could, if you wanted, go
so far as to pluck out any 3 RAM chips on the board without loosing any
function (other than, maybe, access speed).
-- 
Roy Smith, {allegra,cmcl2,philabs}!phri!roy
System Administrator, Public Health Research Institute
455 First Avenue, New York, NY 10016