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From: resnicks@netcom.COM (Steve Resnick)
Newsgroups: comp.os.msdos.programmer
Subject: Re: How can a TSR detect if it has been installed?
Message-ID: <1991Jun23.145337.20537@netcom.COM>
Date: 23 Jun 91 14:53:37 GMT
References: <0094A83A.9AE8E1A0@MAPLE.CIRCA.UFL.EDU>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 29

In article <0094A83A.9AE8E1A0@MAPLE.CIRCA.UFL.EDU> sorrow@oak.circa.ufl.edu writes:
>I have written a  TSR that takes over the command line input for INT21H, in
>other words an INT21H replacement.
>
>How can it prevent itself from being re-installed?  I know programs such as
>NORTON SI will show you who "owns" an interrupt, and my program comes up 
>as the owner.  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.
The way I have handled this, when using an interrupt vector which may
already be used by some other utility/tsr/system function is to
grab a few bytes in the CS of the ISR as a "signature", then when installing
my TSR, look at that segment address and the specific offset. If my signature
is there, I know my TSR has been installed, otherwise it's not and I can
install it....



Hope this helps....
Steve

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 Real life: Steve Resnick. Chief Software Architect, Process Scientific, Inc
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