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From: bayes@hpislx.HP.COM (Scott Bayes)
Newsgroups: comp.sys.mac
Subject: Re: $479.00 SIMMS!?? YIPES!!
Message-ID: <5880014@hpislx.HP.COM>
Date: 25 Aug 89 17:57:35 GMT
References: <9518215MES@MSU>
Organization: HP Measurement Systems Operation - Loveland, CO
Lines: 10

It sure seems likely to me that the extra chips are there for Error
Correcting Codes (ECC).  Parity (9-bit SIMMs) allows the machine to
detect a single bit error in a byte.  Parity checking can (will) miss 2
bit errors in the same byte.  ECC can (I forget the details) detect up
to 4(?) bit errors in a byte, and can _correct_ up to 2 or 3 bit errors
in the byte.

The extra chips provide info required for correcting the errors.

Scott Bayes