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From: wall@fortune.UUCP (Jim Wall)
Newsgroups: net.micro
Subject: Re:  net.digital: Is parity *really* worth it?
Message-ID: <3939@fortune.UUCP>
Date: Fri, 3-Aug-84 11:33:52 EDT
Article-I.D.: fortune.3939
Posted: Fri Aug  3 11:33:52 1984
Date-Received: Sat, 4-Aug-84 03:14:11 EDT
References: <641@sri-arpa.UUCP>
Organization: Fortune Systems, Redwood City, CA
Lines: 36


   As most people have stated already, parity is really determined
by the market you are trying to sell to. Board space, cost, complexity
are all really second to whether you win or lose sales of the final
product. We are of course looking at this from the perspective of a 
designer building a product to be marketed and sold, and not from the
viewpoint of a consumer looking to buy a machine. 

   But that isn't what this article is about, this one is about EDC
error detection and correction. Personally, I'm against it.  All the
simple EDC codes, such as Hamming codes, by adding three more bits
per byte, you are gaurenteed of correcting all single bit errors,
detecting all double bit errors and have a high probability of 
detecting multiple bit errors. Notice that you can only correct
single bit errors. In this era of ESD, power spikes, and electrical
noise, you are most often subjected to massive memory corruption, and
rarely is the end result of a memory hit a single bit error. THere 
have been studies done on this, but someone must have my copy; I'll
look for it.

   The other real drawback that I see with EDC is the performance hit,
the chips they have to perform these marvelous correction algorithms
are not what could be called real fast. Each memory read must have 
time allotted in it for an analysis of the data (including the code
bits) and time for any data correction in necessary. This isn't double
the nominal memory cycle time, but it is more than 50% additional.
The only way to circumvent this is with a CPU that can be aborted and
then restarted,...  but that has it's own unique brand of problems.


I should know this stuff, my whole life has been one parity error
after another....

					-Jim Wall
					!amd!fortune!wall