Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!fluke!ssc-vax!uvicctr.UVic.ca!sbanner1
From: sbanner1@uvicctr.UVic.ca.UUCP (S. John Banner)
Newsgroups: comp.sys.ibm.pc
Subject: Re: DOS memory allocation
Message-ID: <674@uvicctr.UVic.ca.UUCP>
Date: 29 May 89 01:07:20 GMT
References: <8261@phoenix.Princeton.EDU> <670@spudge.UUCP> <1989May20.132014.5230@ziebmef.uucp>
Reply-To: sbanner1@uvicctr.UVic.ca.UUCP (S. John Banner)
Organization: University of Victoria, Victoria B.C. Canada
Lines: 46

In article <1989May20.132014.5230@ziebmef.uucp> stephen@ziebmef.UUCP (Stephen M. Dunn) writes:
>In article <670@spudge.UUCP> johnm@spudge.UUCP (John Munsch) writes:
>$DOS versions past 3.0 have the INT 21h function call 58h which sets the DOS
>$memory allocation strategy.  One of the available options for this is to
>$allocate from the top down.  Consult a DOS technical manual for details on
>$the parameters of the function call.
>
>   Yes, and it also allows you to tell DOS to use the "best-fit" strategy.
>
>   But back to the original point - allocating memory from the top down.
>DON'T DO THIS WITH TSRs!  If you do, you'll end up rebooting your  machine.
>Why?  Let me explain (DOS gurus can skip the rest of this article)

>   Now, let us say that you have set the memory allocation strategy to
>get DOS to put your programs high in memory, and that you have just run
>a TSR.  It will, of course, have wiped out the high-memory part of
>COMMAND.COM.  The low-memory part will notice this and will try to
>put it back.  Now, as I understand it, what will happen is that your TSR
>will be overwritten and the machine has at least a 99.9% chance of crashing.
>(the other alternative is that maybe DOS will realize that the memory is
>unavailable and will crash ... I've never tried it, so I don't know which
>is the case.)

   This is was a very good explanation of the point, most informative.
Unfortunately, it has one little flaw in it.  I have written a TSR
that I have never had any complaints about, that uses exactly this
sort of a strategy.  I have this program that needs to leave a chunk
of itself available after it has been used.  The program is written
in C, and the easyest way to prevent memory fragmentation is to put
things into High memory, so...  The only reason that the problems
described here should ever happen, is if you don't properly allocate
the memory (I use Dos's memory allocation calls, and have had none
of the problems you describe, not even while I was developing the
program).

   The moral to this story is (I think), follow the rules whenever
you can, even if you have to break the rules (as TSRs often have to),
follow the rules as much as possible.

                      S. John Banner

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