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DOS TSR's with file access (2)

Posted on 1998-08-31
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Last Modified: 2006-11-17
(for nietod...)
Hi, I want to code a DOS tsr. When the user press some
kind of "hot key" the tsr must open a text file and read
some lines from it and close it again. As soon as the file
is opened, the pc hangs. Any suggestions?
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Question by:Claude050897
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nietod earned 300 total points
ID: 1171674
My contributions From: http://www.experts-exchange.com/topics/comp/lang/cplusplus/Q.10075784
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Claude.  DOS was never really meant to have TSRs.  Most DOS procedures are
not reentrant.  Meaning if they are already running, they cannot be run a
second time (safely that is, they can always be run a second time).  This is
because DOS uses static memory to store parameters and other data structures
rather than storing them on the stack.  So if the DOS file procedures are
currently running when you interrupt and reinvoke them you will cause a
crash.

To prevent this you need to check the undocumented "indos" flag.  This is a
flag that DOS sets when it has entered a non-reentrant procedures.  You must
wait until that flag has cleared before calling DOS procedures..

Note that do not want to enable interrupts until you have executed all
non-reentrant code of your own.  Your file procedures use static variables,
so you don't want to enable interrupts while you are using them.  The DOS
handler is not reentrant either, so you don't want to enable interrupts
before calling it.

I see the old handler call now.  but you still need to wory about
reentrancy, both of your procedures and DOS.  The old handler will enable
interrupts so you don't need to.  However, once it does your procedure could
be called again (while already running).  Take that into account.

You can use DOS service function 34H to get a pointer to the indos flag.
Place 34H in AH and execute int 21.  ES:BX points to a 1 byte flag.  If it
is zero, you can safely use DOS functions, if not, you can't Given your
circumstances, you can use the keyboard to set a flag or record other
information, but not actually do the file i/O.  You can do the file I/O on a
timer interrupt.  That way if on one timer interupt you can't use DOS, you
can wait and do it on the next.

Another posibility is to forget DOS altogether and use the BIOS to perform
your I/O.  This is (well was) often done.
******************************************
Thanks
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