Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!rutgers!iuvax!bsu-cs!dhesi
From: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Newsgroups: comp.sys.ibm.pc
Subject: Re: unexpected "TSR's"
Message-ID: <1254@bsu-cs.UUCP>
Date: Sat, 10-Oct-87 15:12:31 EDT
Article-I.D.: bsu-cs.1254
Posted: Sat Oct 10 15:12:31 1987
Date-Received: Mon, 12-Oct-87 07:20:24 EDT
References: <307@gvgspd.UUCP>
Reply-To: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Organization: CS Dept, Ball St U, Muncie, Indiana
Lines: 40
Summary: not TSR;  poor technique

In article <307@gvgspd.UUCP> mrk@gvgspd.UUCP (Michael R. Kesti) writes:
>Since I have begun to use Bob Frolick's AT program, I have discovered that
>some programs (notably Procomm and TurboC) appear to attach themselves to
>DOS in TSR fashion.  Especially, they reassign the vector for interrupt 16
>(keyboard services) in the course of their execution, and do *NOT* restore
>this vector upon termination.  

I doubt very much that Turbo C traps INT 16h without restoring it.
That would cause chaos, and Turco C hasn't caused chaos on my system so
far.

>This causes failure of AT, as it compares the
>code pointed to by this vector to determine whether it is already installed.
>When this vector is changed, it fails to recognize itself, and re-installs
>itself, losing any previously scheduled events!

This is purely AT's fault.

A program that traps a keyboard interrupt must always be prepared to
have other programs trap it too.  Programs that claim exclusive use of
vectors such as the keyboard software interrupt are using a bad
programming technique.  (Aside:  Prokey used to use this bad
programming technique and would therefore not work with SideKick.
David Rose, the author, refused to fix it.  One Sunday morning, in a
fit of exasperation, Phillippe Kahn declared he would write his own
keyboard enhancer.  The rest is history.)

The correct technique for a terminate-and-stay-resident program
to avoid loading itself twice is to trap an unused interrupt vector
(or MS-DOS function call), and install a service routine that
returns a known value.  Then, when it is loaded, the TSR program
first calls this vector and examines the return value.  Return
values for unused interrupt vectors and MS-DOS calls are usually
well-defined though not always well-documented.  If the value returned
is that expected to be returned by an already-installed TSR program,
the newly-loaded program prints a message and gracefully exits.  For
safety's sake, allow the user to change the value of the unused vector
used in case other software happens to use the value you choose.
-- 
Rahul Dhesi         UUCP:  <backbones>!{iuvax,pur-ee,uunet}!bsu-cs!dhesi