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Q78609: Troubleshooting HIMEM.SYS

Article: Q78609
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:5.x,6.0,6.2,6.21,6.22
Operating System(s): 
Keyword(s): 
Last Modified: 17-DEC-2000

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The information in this article applies to:

- Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 
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SUMMARY
=======

This article describes how to troubleshoot HIMEM.SYS.

MORE INFORMATION
================

HIMEM.SYS Auto-detection Strategy
---------------------------------

Selection of A20 handlers is based on an auto-detection strategy. HIMEM runs
through all of its auto-detection schemes trying to find a match. If found, the
particular handler is installed. If no match is found, the default A20 handler
is installed. There are a number of different auto-detection routines and these
are done in a certain order. Sometimes, due to the non-standard nature of the
machine, the auto-detection fails and the default handler is installed. However,
the default handler does not work too well. In this case, the user has to give
an explicit override and ask HIMEM to install a particular A20 handler. In other
cases, HIMEM may identify a machine incorrectly and install the wrong A20
handler. Basically, the machine identification strategies are not, and cannot
be, foolproof. Troubleshooting this kind of problem can be done only by trial
and error.

HIMEM.SYS and /INT15=xxxx Switch
--------------------------------

When HIMEM starts, it grabs all of the INT 15h memory present in the system. It
then provides extended memory to different applications via the XMS interface.
However, there are some old applications that do not understand XMS and need
some INT 15h memory to be present. To support these applications, HIMEM.SYS
provides a /INT15=xxxx switch on its command line. Using this, you can ask
HIMEM.SYS to leave xxxx Kbytes of extended memory on the INT 15h interface so
that applications can use INT 15h to allocate extended memory. For example,
Pharlap DOS Extender needs this switch. The use of this switch is documented in
the version 5.0 "Microsoft MS-DOS User's Guide and Reference."

Additional query words: tshoot 6.22 5.00 5.00a 6.00 6.20

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Keywords          :  
Technology        : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a
Version           : MS-DOS:5.x,6.0,6.2,6.21,6.22

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