WRITING STABLE TERMINATE AND STAY RESIDENT(TSR) SOFTWARE:
                    SOMETHING THEY'RE NO TELLING US?
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Through the research and development of our Task Switching program we stumbled
onto a problem and couldn't understand why it was occurring. It was the dreaded
system lockup (crash) for no indentified reason. We contacted several past
Microsoft people as well as continued to study the problem closely.

We then discovered that stable programs such as SideKick, Windows, Novell's
IPX, and TSR Utilities written by past DOS authors were hooking interrupts for
no apparent reason, and were just chaining them back to the original vectors.
We again contacted prior employees at Microsoft, and got the run around. Now
we suspect, they were under Trade secret agreements and could not answer
our questions.

It seems that all stable TSR programs (or ones that simulate a TSR, like the
Menu System I Plus Program Manager 3.0) must hook and chain the following
interrupts back to their original vectors (as well as the normal interrupts
which must be hooked):

 INT C8, INT CE, INT CF, INT F0, INT F3, INT F5, INT FD, and INT FF

It is not clear why this must be done, but if you use one of the MCB chain
walkers that are included with the book UNDOCUMENTED DOS (Authored by Andrew
Schulman), you will see that all of the above mentioned programs do hook these
interrupts, and happen to be the most stable on the market.

This article as written by Mark Vitt on October 1st, 1991, EMAIL 70053,2236