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From: rde@ukc.ac.uk (R.D.Eager)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Date not advancing at midnight on my clone
Message-ID: <2666@eagle.ukc.ac.uk>
Date: Sun, 8-Mar-87 12:43:17 EST
Article-I.D.: eagle.2666
Posted: Sun Mar  8 12:43:17 1987
Date-Received: Mon, 9-Mar-87 19:12:19 EST
References: <4728@shemp.ucla-cs.UCLA.EDU> <4740007@hpvcla.HP.COM>
Reply-To: rde@ukc.ac.uk (R.D.Eager)
Organization: U of Kent at Canterbury, Canterbury, UK
Lines: 42


I  had this problem on MS-DOS 2.11 on a British PC clone. I also managed
to fix it, but the nature of the fix means that it works  ONLY  for  one
version (and OEM implementation) of DOS.

The  BIOS  time  of  day  call  (INT  1AH)  is  what DOS uses to get the
time. This is used by the DOS kernel only to get the  time  of  day. The
call returns the time in CX/DX, and a flag (indicating date rollover) in
AX. This flag is nonzero if the day has rolled over since the last call,
and by definition is returned ONCE ONLY after a given midnight.

Now,  the  real problem is that the OEM portion of DOS may also call the
INT 1AH function, and many implementations do this to provide a  timeout
on  disk access in order to make up for the lack of door latch status on
IBM style floppy drives. The details don't matter, but a call  from  one
of  these  just  after midnight clears the rollover flag so that the DOS
kernel never sees it.

My solution was messy (and doesn't handle the case of  an  idle  machine
being left for a weekend), but it does work.

First,  scan  thru the DOS file IO.SYS or IBMBIO.COM, looking to INT 1AH
calls.   Find  the  one  that  seems  to  do  something   with   AX   on
return. Figure out from this code (easier than it sounds) the word where
DOS obviously keeps the day number.

Second,  write  a little intercept (TSR) routine for INT 1AH. Do nothing
on the way in, but on the way out check AX. If it is nonzero,  clear  it
and increment the DOS day number yourself.

This WORKS. But, you need a different version for different versions and
flavours  of  DOS  (because  the  day  number  isn't  always in the same
place). It's a good idea to put some checks in to stop it being used  on
the wrong version and zapping some other word.
-- 
           Bob Eager

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