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From: markd@wolf.UUCP (Mark Divecchio)
Newsgroups: comp.sys.ibm.pc
Subject: Undocumented 34h Call
Message-ID: <558@wolf.UUCP>
Date: Sat, 14-Nov-87 17:01:40 EST
Article-I.D.: wolf.558
Posted: Sat Nov 14 17:01:40 1987
Date-Received: Sun, 15-Nov-87 20:16:43 EST
Organization: Systems'n'Software San Diego, CA
Lines: 28
Keywords: TSR,Critical Section Flag

The latest chapter in the saga of how to write TSR programs was played
out on page 77 of the September 1987 Microsoft Systems Journal.

In the Ask Dr. Bob column, he gives us some hints on how to use the
critical section flag returned by int 21h function 34h. He says
the what you do with it depends on the version of DOS which you are
running. Specifically : "Depending on the DOS version, you should
check the byte after (DOS 2.1), before (DOS 3.0 and 3.1, if not
Compaq-DOS), or 01AAH bytes before (Compaq-DOS 3.0). You should
check to make sure that you're not in the middle of a BIOS interrupt
13h (Disk I/O) or 16h (Keyboard Service Request)."

My first problem is the first sentence of the quote. It is so poorly
phrased that I can't figure it out. The second problem is how one
tells if we in the middle of a 13h or 16h interrupt.

I have a program called LPTx which tries to redirect the output
destined for the printer to a disk file. I use the 34h flag but I
always checked the byte which the flag pointed to, not one before or
after.

Has anyone figured this one out?

-- 
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Mark C. DiVecchio 10435 Mountain Glen Terrace, San Diego, CA 92131
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