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From: yawei@bronze.ucs.indiana.edu (mr. yawei)
Newsgroups: comp.os.msdos.programmer
Subject: Re: TSR PROGRAMMING PROBLEM
Message-ID: <1991Jan30.050646.7067@bronze.ucs.indiana.edu>
Date: 30 Jan 91 05:06:46 GMT
References: <1991Jan29.173752.10045@bronze.ucs.indiana.edu> <5369@trantor.harris-atd.com> <2168@njitgw.njit.edu>
Organization: Indiana University, Bloomington
Lines: 32

In article <2168@njitgw.njit.edu> cd5340@mars.njit.edu (Charlap) writes:
>>>This is called the DOS re-entrancy problem which anyone who is teaching
>>>you to write TSRs should have told you about. Simply speaking, DOS
>>>cannot be entered twice.
>
>Not quite.  DOS re-enters itself all over the place.  There is a scheduler
>interrupt (I forget what it is, though) which is very undocumented, which
>many hackers use for making re-entrant TSR's.  Most notable of these is the
>DOS PRINT command, which uses the scheduler interrupt in order to do background
>printing.

You didn't read the rest of my paragraph. :-)  It said "actual situation
is more complicated as DOS uses several stacks..." 

To be exact (and make my ends covered), DOS uses 3 stacks for its
various function calls. So if you stretch it, you can re-enter DOS
for a miximum of 3 times. A few other glitches (restrictions) reduce
the maximum to 2 times. 

DOS itself does not detect that you are trying to trash its stack,
so it takes elaborate programming to ensure that the re-entrancies
are safe. I am not going to elaborate how to do this as it probably
will take a couple of chapters. :-)

The interrupt you just mentioned is int 28h. Within interrupt 28h it
is safe to access some of the DOS functions which mainly deal with
disk i/o etc. If you set the DOS critical error flag you can access 
all the DOS functions within int 28h.

The problem in the original question was DOS re-entrancy, however.

yawei