Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site fortune.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!fortune!wall 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