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From: xsession@McRCIM.McGill.EDU (X session)
Newsgroups: comp.os.msdos.programmer
Subject: Re: How can a TSR detect if it has been installed?
Message-ID: <1991Jun27.041153.17579@thunder.mcrcim.mcgill.edu>
Date: 27 Jun 91 04:11:53 GMT
References: <0094A83A.9AE8E1A0@MAPLE.CIRCA.UFL.EDU> <PSHUANG.91Jun23164405@beeblebrox.mit.edu>
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Use int 2fh

quoting Ping-Shun Huang:

(1) Grab a completely unused interrupt/service and have your TSR return
identification codes when that interrupt/service is called: e.g.
Interrupt $21, AX=$FEED is probably unused, your TSR could grab it and
return non-trivial data in the registers, BX=$DEAF and CX=$BEAD and
DX=$FEED, the combination of which is unlikely to be accidentally
duplicated by another program.  Your TSR's installation routine can then
check before continuing.  Problem with this method: potential (although
darn unlikely) conflicts with other TSR's if by chance two programmer
decide on the same interrupt/service and ID codes.


But use AH=2fh  (I hope I got it right) instead DOS uses this one 
especially for this purpose  (print graftabl append etc.)
I might be able to find which program use which code, if I find it I'll 
post it.
I always use anything above 80h and it seems to work.

Hope this helps

                        grtnx  Martijn

                               nykerk@mcrcim.mcgill.ca