Path: utzoo!attcan!uunet!overload!dillon From: dillon@overload.UUCP (Matthew Dillon) Newsgroups: comp.sys.amiga.tech Subject: Re: Parity Checking / ECC RAM on the A3000 Message-ID: Date: 29 May 90 15:21:21 GMT References: <1641@lpami.wimsey.bc.ca> <3620@tymix.UUCP> Lines: 43 >In article <3620@tymix.UUCP> pnelson@hobbes.uucp (Phil Nelson) writes: >In article <1641@lpami.wimsey.bc.ca> lphillips@lpami.wimsey.bc.ca (Larry Phillips) writes: > >>Right.. ECC is not parity, and vice versa. Parity checking is totally, >>completely, and utterly useless. > >Oh really? Please explain why parity checking would not have saved me much > > The advantage of parity checking is diagnostic, intermittent problems on I have a tendancy to agree. ECC is cute but expensive. It takes 7 bits of ECC to detect and correct 1 bit in a 32 bit wide word. The way you think about 1-bit-ECC is that you need enough codes to generate the address of the incorrect bit, plus a no-error code, plus a parity bit. Unfortunately, that no-error code takes us from 5 to 6 bits, then one more to parity-check the ECC code itself. A 1-bit ECC can correct 1 bit errors and detect 2-bit errors. 2+ bit correct is MUCH more difficult (think of the # of codes required... at least double the number of bits as for 1 bit ECC but the analogy I used above no longer holds, so it's even more!). When you get into >1 bit ECC you generally switch to burst-error correction (which requires fewer correct-codes and thus fewer bits of ECC). Unfortunately, burst error correction is useless when the medium is memory. A simple 1-bit parity check is sufficient to detect the problem that ECC would have corrected, and allow the processor to map the page out with its MMU. In anycase, this kind of failure occurs less often than you think. What most people come up against is a BAD DRam (i.e. cause of problem is not alpha radiation), in which case it is not reliable anyway and you simply have to replace the chip. DRAMs these days are much more reliable than 10 years ago... even 5 years ago. -- Matthew Dillon uunet.uu.net!overload!dillon 891 Regal Rd. Berkeley, Ca. 94708 USA