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Q82712: HIMEM.SYS /EISA Switch

Article: Q82712
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.1,3.11
Operating System(s): 
Keyword(s): win31
Last Modified: 21-SEP-1999

3.10 3.11

WINDOWS

kbenv

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The information in this article applies to:

- Microsoft Windows versions 3.1, 3.11 
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SUMMARY
=======

The HIMEM.SYS driver that ships with Windows 3.1 (version 3.07, dated 3/10/92,
size 13824 bytes), provides an additional command-line switch that allows
HIMEM.SYS to use all of the Extended Industry Standard Architecture (EISA)
memory installed in the system.

If you have an EISA system that has memory available, and HIMEM.SYS and/or
Windows is not using all the memory on the system, adding the /EISA switch to
the CONFIG.SYS file as follows may make more memory available for use:

  DEVICE=C:\HIMEM.SYS /EISA

NOTE: This command may cause conflicts if any drivers using extended memory are
loaded from the CONFIG.SYS file before HIMEM.SYS.

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

The /EISA command-line switch has been added to allow HIMEM.SYS to use all the
EISA memory installed in an EISA system. The switch is ignored on non- EISA
systems. HIMEM.SYS maintains its former behavior when this switch is not used.

When the /EISA switch is not used (or when the MS-DOS version 5.0 version of
HIMEM.SYS is used), only the amount of contiguous extended memory identified by
Int 15H/88H (get extended memory size) is allocated by HIMEM.SYS. If a system
has 24 MB of memory installed (and all extended memory was in one contiguous
block starting at 1 MB) and Int 15H/88H reports 16 MB available, HIMEM will use
only 16 MB, leaving the remainder unused and unavailable. Several original
equipment manufacturer's (OEM) BIOS implementations never indicate more than 16
MB of available memory through Int 15H/88H regardless of how much more memory
may be installed. Therefore, on one of these machines with more than 32 MB of
memory installed, most of it is not usable with HIMEM.SYS, EMM386.EXE, and
Windows 3.0.

This behavior is not a problem in HIMEM.SYS. HIMEM.SYS behaves this way (without
the /EISA switch) because it cannot be sure there isn't some other driver loaded
before HIMEM.SYS that is already hooking the Int 15H/88H interrupt and
allocating extended memory for its own use.

If no other drivers using extended memory are loaded before HIMEM.SYS in the
CONFIG.SYS file, the /EISA switch can safely be used to force HIMEM.SYS to
allocate all EISA memory, not just the amount returned by Int 15H/88H. This can
make a lot more extended memory that conforms to the Extended Memory
Specification (XMS) available.

Some older programs that used extended memory (such as Windows 3.0) do not work
properly with extended memory addresses above 16 MB. Using the /EISA switch on
machines with more than 16 MB may allow Windows 3.1 to use more memory, but will
cause Windows 3.0 to fail. Users that plan to switch back and forth between
Windows 3.1 and Windows 3.0 may not want to use the /EISA switch.

Additional query words: 3.10 3.1 XMS DPMI

======================================================================
Keywords          : win31 
Technology        : kbWin3xSearch kbZNotKeyword3 kbWin310 kbWin311
Version           : WINDOWS:3.1,3.11

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