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From: ralf+@cs.cmu.edu (Ralf Brown)
Newsgroups: comp.os.msdos.programmer
Subject: Re: How can a TSR detect if it has been installed?
Message-ID: <13599@pt.cs.cmu.edu>
Date: 24 Jun 91 02:09:47 GMT
References: <1991Jun23.145337.20537@netcom.COM> <4944@gumby.Altos.COM>
Organization: School of Computer Science, Carnegie Mellon
Lines: 21

In article <4944@gumby.Altos.COM> jesse@gumby.Altos.COM (Jesse Chisholm AAC-RjesseD) writes:
}Many TSRs chain their interrupt and so it might be installed but no longer
}be the first in the chain.  A way of checking for the signature you mention
}that gets around this is to check at that offset for ALL segments from 0000
}up to your current CS.  when you find the signature if the segment is not
}equal to the current CS, then your TSR has been installed before and is still
}in memory.  If the first time you find the signature is in your current
}CS segment, then your TSR has not yet been installed.  Go for it.

That will fail on an increasing number of machines, because more and more
people are loading things high (particularly now that DOS5 is out).  The
only acceptably reliable method for determining prior installation is to
reserve a call for an installation check.  Not 100% foolproof, either, but
it can be made arbitrarily reliable by returning a pointer to a signature
string--the longer the string, the less likelihood of a clash.

-- 
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