Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!wang!comm.wang.com!lws
From: lws@comm.wang.com (Lyle Seaman)
Newsgroups: sco.opendesktop,comp.unix.sysv386
Subject: Re: SCO Unix, ALR FlexCACHE losing time
Message-ID: <1991Jan3.204820.2619@comm.wang.com>
Date: 3 Jan 91 20:48:20 GMT
References: <1991Jan01.162400.6155@litwin.com> <1991Jan2.221527.15181@gsm001.uucp>
Organization: Wang Labs, Platform Comms.
Lines: 29

gsm@gsm001.uucp (Geoffrey S. Mendelson) writes:

>This is a very common bug in IBM PC design.  It comes from one of two
>"features" of the IBM pc that has been carried over from the original.

>2:  When UNIX (and MS-DOS) are booted, the read the battery backed up clock.
>    From then on they update the time on each "clock tick interupt".
>    Most device drivers, especially disk, turn off interupts while they 
>    are running.  

A lot of Unices check the battery backed clock periodically.  This 
should bound the drift caused by disbled interrupts so that the long
term clock drift is solely attributable to the battery-backed clock.

>Also a note on accuracy:
> 
>    1%  would be 864 seconds a day or 14 minutes 24 seconds
>   .1%  would be 86  seconds a day or  1 minute  26 seconds
>   .01% would be 8.6 seconds a day
>   1 second a day is 1/86400 or 1 in almost 1 part in one hundred thousand.

>How many scientific instruments can boast that accuracy?

Well, my free Disneyworld watch, for one.
My free Grimace watch from McDonald's, for another.

-- 
Lyle                  Wang           lws@capybara.comm.wang.com
508 967 2322     Lowell, MA, USA     Source code: the _ultimate_ documentation.