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From: manes@dasys1.UUCP (Steve Manes)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Standard date bug
Message-ID: <2049@dasys1.UUCP>
Date: Mon, 23-Nov-87 03:04:06 EST
Article-I.D.: dasys1.2049
Posted: Mon Nov 23 03:04:06 1987
Date-Received: Thu, 26-Nov-87 22:23:26 EST
References: <7457@eddie.MIT.EDU>
Organization: The Big Electric Cat, NYC, NY
Lines: 57
Summary: The BIOS Timer Bug

In article <7457@eddie.MIT.EDU>, nathan@eddie.MIT.EDU (Nathan Glasser) writes:
> 
> This is a question about the standard bug that many people know
> about regarding the date maintained in memory on standard
> pc's and many clones. If your computer is on when midnight comes
> around, the date will not be changed. I don't know whether this
> always happens or only sometimes.
> Does anybody have any simple solution that they've been using?

Yes, although I believe that DOS 3.2 fixed this problem (I could be
wrong; I still use 3.1).  The problem lies in a "sticky flag" maintained
by the BIOS timer in the ROM_BIOS_DATA segment.  The structure of this
segment is:

	ROM_BIOS_DATA SEGMENT @40h
		org 6Ch
	timer_low	dw ?	; count up to 65535...(18.2x/sec)
	timer_hi	dw ?	; then increment this (every hour) until...
	timer_ofl	db ?	; we hit midnight and we inc' this.
	ROM_BIOS_DATA ENDS

Whenever DOS calls BIOS to read the current real-time clock, it runs
Interrupt 1Ah (see Tech Ref) and a single line of code that is executed
during any TIME_OF-DAY "read time" call:

	mov	timer_ofl,0	; get overflow and reset flag

In other words, every time Int 1Ah is called to read the time, whether from
the DOS TIME/DATE routines or from your own homebrew to count the number of
timer ticks, 'timer_ofl' is reset to zero and its previous contents
returned in AL by Int 1Ah.  The sticky widget: DOS needs this flag so it
knows that the date has rolled over.  If your application calls Int 1Ah
before DOS gets it (and remember that this flag is set only ONCE every 24
hours and read only ONCE and then reset) DOS will simply roll the clock
over but not the day.

There are a few fixes that come to mind but the easiest by far is to write
your own Int 1Ah replacement that DOESN'T reset 'timer_ofl'.  Fortunately,
this is an easy task since Int 1Ah doesn't do very much.  What I do is use
this routine whenever I need to read the timer block:

	push	es
	mov	ax,40h
	mov	es,ax
	mov	ax,word ptr es:[6Ch]
	mov	my_timerlo,ax
	mov	ax,word ptr es:[6Eh]
	mov	my_timerhi,ax
	pop	es

That eliminates the problem.

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