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From: jnw@mcnc.UUCP (John White)
Newsgroups: net.micro.pc
Subject: Re: Weirdness in PC???
Message-ID: <599@mcnc.mcnc.UUCP>
Date: Fri, 21-Jun-85 20:40:07 EDT
Article-I.D.: mcnc.599
Posted: Fri Jun 21 20:40:07 1985
Date-Received: Mon, 24-Jun-85 02:33:08 EDT
References: <256@timeinc.UUCP> <173@galbp.UUCP>
Organization: Microelectronics Center of NC; RTP, NC
Lines: 26


     When a program is executed in ms-dos 2.x it is given two blocks of
memory. One is the main block that starts at the begining of the 100h header
and contains the program, the other is the environment block whose starting
segment is given at offset 2Ch of the header. When the program terminates
these two blocks are freed. If the program terminates-and-stays-resident,
the environment block is not freed and the main block is shortened to the
length specified by the terminate call. I have written terminate-and-
stay-resident goodies that are removable. If the program is given the
"remove" switch then, instead of installing itself, it looks at the
interupt vector changed by a previous installation to find the address
of the installed versions main block. Then the old vector is loaded from
the appropriate offset in this block and the interupt vector is restored.
When the program installed itself it freed up its environment pointer
(using the dealloc dos call), so deallocing the main block of the
installed program now removes it completely. Memory segmentation can
still be a problem, so I usually remove my installed goodies in the
reverse order that I installed them. This isn't important unless there
is something installed that takes up a lot of memory (such as an
installable ramdisk). By the way, the extra code for doing the removal
is at the end of the program and is cut off by the terminate-and-stay-
resident dos call so it doesn't use up memory when installed.
Also, before removing the installed version, various offsets in the
suposed installed version are checked to make sure the interupt vector
is really pointing to an installed version.
- John N. White    {duke, mcnc}!jnw