Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!netcom!wolf From: wolf@netcom.COM (Buckskin Tech.) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: Parity chip on Intel Computers Message-ID: <26249@netcom.COM> Date: 1 Mar 91 01:21:05 GMT References: <1991Feb27.140227.9246@panix.uucp> <1991Feb27.173721.3963@cs.mcgill.ca> Organization: Netcom - Online Communication Services UNIX System {408 241-9760 guest} Lines: 25 storm@cs.mcgill.ca (Marc WANDSCHNEIDER) writes: > What is the ninth chip on a Standard PC SIMM for (ie, the so call >'parity' chip) ...? Why is it that the Intel computers require these and the >AMIGAs and other 680x0 computer do not...? Simple. The ninth chip stores parity-checking information for the other eight. The PC architecture stores one parity bit for each byte, so you end up needing nine physical bits for each eight bits of storage. The designers of the original IBM PC did this because they felt that since their processor was at the "cutting edge" of technology, they couldn't trust the current crop of memory chips to keep up. It was touted as a "security feature", supposedly making the machine more reliable. The parity checking is done in hardware, so there's no way to "reclaim" the extra bits. If the hardware subsystem detects a parity incongruity, it generates an interrupt that normally halts the processor (wouldn't want your processor to chew on any un-parity-matched bytes, now, would you?) In my opinion, the concept of parity-checked memory is bogus. Given the state of software these days, it's more likely that your operating environment will crash than your memory will fail. 'Course, I could be (and often am) wrong... - Phil @ Buckskin Technologies