🚀 go-pugleaf

In article <0094A83A.9AE8E1A0@MAPLE.CIRCA.UFL.EDU> sorrow@oak.circa.ufl.edu writes:

 > If there was a way to see if the owner was NOT DOS (i.e. another
 > program, such as a TSR, owned it) and to not install itself in that
 > case, I would like to know.

I think there are two fairly common ways to do it.

(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.

(2) In your program, make sure somehow that near your ISR (interrupt
service routine) you place some identifying text string.  This is not
trivial to do, except in assembler, because your compiler is permitted
to move things like constant strings from one place to another, so
having the string adjacent to the routine in your source code does not
guarantee anything.  If you can do this, the installation routine would
need to do a search on memory locations near to where the interrupt
points.  Problem: if another program hooks the interrupt between the
previous installation of the TSR and now, you're scrod; in this case, if
someone loads CED after your INT21H program, the pointer now points into
CED code and not yours, and you have no easy way of tracing back the
interrupt chain since you don't know for sure where CED stored the old
copy of the Int $21 pointer.

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Above text where applicable is (c) Copyleft 1991, all rights deserved by:
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