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From: john@xanth.UUCP (John Owens)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Date not advancing at midnight on my clone
Message-ID: <720@xanth.UUCP>
Date: Thu, 19-Mar-87 01:12:16 EST
Article-I.D.: xanth.720
Posted: Thu Mar 19 01:12:16 1987
Date-Received: Sat, 21-Mar-87 02:48:59 EST
References: <4728@shemp.ucla-cs.UCLA.EDU> <4740007@hpvcla.HP.COM>
Organization: Old Dominion University, Norfolk Va.
Lines: 43
Summary: The Real Story - multiple compounding bugs

Well, I thought I had explained this at length some time ago, but I
can't find the article, so....

There is a mis-design in IBM's BIOS, both XT and AT, such that when the
time-of-day clock passes midnight, the date wrap flag is *set*.  This
gives no indication of how many days have passed, just that *at least
one* has passed.  Once the time is read, which returns this flag in a
register, the flag is cleared.  These have been propagated for the sake
of compatibility.

So, DOS would need to read the time at least once a day, or lose days.
So far, so good, but there was another bug in the device drivers
supplied to IBM by Microsoft.  It would read the clock to decide whether
or not a disk might have been changed, but it would not note the setting
of the date-changed flag, so the fact that this flag had been set would
be lost.  The next time that the clock driver checks, the flag isn't
set, so the date doesn't get changed.

This bug was fixed a while back.  I'm not sure what version, but
certainly the 3's would have it.

So much for the "lose date change while accessing floppies at midnight
bug", but that still leaves the "multiple date rollover bug", wherein
you leave your system idle, over a weekend, say, and the flag gets set
several times.  You come back in on Monday, create a file or somesuch,
and it says "Oh - the midnight flag!  It must be Saturday now!"  This
still exists in PC-DOS 3.2, the "generic" "retail" MS-DOS 3.2, and the
MS-DOS 3.2 OEM kit.

*However*, Microsoft does have a fix for the problem, but it turns out
that the fix breaks something to do with the runtimes for the BASIC
Compiler, so they took it out.  I would not be at all surprised if
various OEMs got the original fix or wrote their own....

Disclaimer:  All this was discovered through unconscious psychic
communication with the MS-DOS systems software group, and I take no
responsibility for the implications of its correspondence to the real
world.  (Whatever that is.)

-- 
John Owens		Old Dominion University - Norfolk, Virginia, USA
john@ODU.EDU		old arpa: john%odu.edu@RELAY.CS.NET
+1 804 440 3915		old uucp: {seismo,harvard,sun,hoptoad}!xanth!john