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From: roy@gitpyr.gatech.EDU (Roy Mongiovi)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Date does not change when PC is left on
Message-ID: <2763@gitpyr.gatech.EDU>
Date: Wed, 3-Dec-86 22:58:23 EST
Article-I.D.: gitpyr.2763
Posted: Wed Dec  3 22:58:23 1986
Date-Received: Fri, 5-Dec-86 01:21:54 EST
References: <289@rocky2.UUCP> <708@watmum.UUCP>
Organization: Georgia Institute of Technology, Atlanta
Lines: 32
Summary: A clear example of the sorts of bozos writing micro software


The reason for the date not changing may be twofold.  If you are using
DOS 2.0, there is a bug, but first let me describe how it can happen
on all PC's.

When IBM designed the timer tick interrupt that keeps track of the time
of day on the original PC (and hence, when they set the standard for
everyone else), they did it in a really shoddy way.  The BIOS keeps a
count of the number of timer ticks since midnight.  When the PC sees
that the tick count has gone past the number of ticks in a day, it
resets it to zero and sets a byte to one.  This byte is a flag that
says "the tick count has rolled over," not a count of the number of
times it has rolled over.  Now, in normal practice DOS reads the time
from the BIOS, which return and clears the rollover flag, and notes
that the date has changed.  But if you leave your PC on for more than
a day without causing DOS to access the date, the rollover count gets
set to one twice (which of course doesn't help much), and a day is
lost.  Leave it on over the weekend, and two days are lost.

The bug in 2.0 was that DOS would get the date and time whenever disk
I/O occurred so that it could record the modification date/time of the
file, and it would neglect to record the rollover flag.  So if you
happened to be accessing disk at midnight, the BIOS would clear the
flag but DOS would ignore it, and a day would be immediately lost.

Now ask me why IBM allocated an entire byte but didn't have the brains
to increment it rather than just setting it to one....
								Roy
-- 
Roy J. Mongiovi		Systems Analyst		Office of Computing Services
Georgia Institute of Technology		Atlanta GA  30332.	(404) 894-4660
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