GUS Programmer's Digest Tue Jul 27 00:07 Volume 2: Issue 28 Today's Topics: GUS Programmer's Digest V2 #26 Standard Info: - Meta-info about the GUS can be found at the end of the Digest. - Before you ask a question, please READ THE FAQ. ---------------------------------------------------------------------- Date: Fri, 23 Jul 93 21:40:20 EDT From: n4zfd!frodo@rylos.n2idf.ampr.org (Jim Blakely) Subject: Re: GUS Programmer's Digest V2 #26 Message-ID: <743716272@rylos> > The operating principle behind NMI is a parity check - memory is > physically addressed by a row and grid method, so the computer adds up all > the 1s and 0s in each row, and the parity for that row is set to a number > (0 or 1) that when added to the row's sum, will result in either an 'even' > number or an 'odd' number... I'm not quite sure how even and odd apply to > binary terms, nor exactly what goes on during all that adding (bit by bit? > Byte by byte?). Actually, the parity bit is set or reset so that the *number* of 1's is either odd or even. Ie., a byte with bits 0000 0001 would have a parity bit of 0 added on it if odd parity; a 1 if even parity. Similarly, a byte with bits 0110 0011 would have a 1 as a parity bit if odd parity is in effect; a 0 if even. Parity is not the best way to test the validity of data, because it can only detect errors which affect an odd number of bits. Also, better ways of protecting data have been devised, which not only check for errors, but can correct them - all of your hard drives use an ECC - error correcting code to protect the data on them from loss of a few bits. I *have* seen RAM go bad. Unfortunately, some BIOS vendors do inappropiate things with NMI's, like putting nasty messages on your screen despite a handler being put in place... -Jim ------------------------------ End of GUS Programmer's Digest V2 #28 ************************************* To post to tomorrow's digest: To (un)subscribe or get help: To contact a human (last resort): FTP sites: archive.epas.utoronto.ca pub/pc/ultrasound wuarchive.wustl.edu systems/msdos/ultrasound Hints: - Get the FAQ from the FTP sites or the request server. - Mail to for info about other GUS related mailing lists (UNIX, OS/2, GUS-MIDI, etc.)