Backfill for EMM386.EXE Cannot Be Disabled (73322)
The information in this article applies to:
- Microsoft MS-DOS operating system 5.0
- Microsoft MS-DOS operating system 5.0a
- Microsoft Windows 3.0
- Microsoft Windows 3.1
- Microsoft Windows 3.11
This article was previously published under Q73322 SYMPTOMS
If you use EMM386.EXE on an 80386 machine with less than 640 kilobytes (K)
of conventional memory and an expansion memory card configured for extended
memory, conventional memory may be backfilled to 640K.
If you try to run Microsoft Windows in 386 enhanced mode on such a machine,
the following error message may be displayed:
Unable to start enhanced mode windows due to base memory backfill
STATUS
Microsoft has confirmed this to be a problem in EMM386.EXE version 4.20
(which is provided with MS-DOS 5.0) and version 4.33 (which is provided
with MS-DOS 5.00a). This problem does not occur with later versions of
EMM386.EXE.
CAUSE
Backfilling uses memory from the expansion card to fill the empty
conventional memory area up to 640K. EMM386.EXE redirects the empty
conventional memory addresses up to 640K to the chips on the expansion
card. This means that there are no physical memory chips on the motherboard
at these addresses; rather, the addresses are used for memory chips on the
expansion card.
There is no way to disable the backfilling that EMM386.EXE performs. The /X
switch excludes only within the range from 640K to 1024K. The /B switch
raises the base only for the backfill up to 256K, which is not useful here
because it does not extend to 640K. Effectively, this means that there is
no way to change the way EMM386.EXE handles memory between 256K and 640K.
For a machine running Windows, this means that you cannot run Windows in
386 enhanced mode, because the backfilling interferes with the operation of
the Windows memory managers. When you type win at the command prompt on such a machine, Windows returns an error
message stating that the memory from xxxK to 640K has been backfilled and
that the backfilling must be disabled to run Windows in 386 enhanced mode.
WORKAROUND
To work around this problem, use EMM386.EXE version 4.44 or later (Windows
3.1 contains version 4.44). Later versions do not backfill as the previous
two versions that shipped with MS-DOS 5.0 and 5.0a did (versions 4.20 and
4.33 respectively).
There is no other workaround for this problem other than to not use
EMM386.EXE, or to upgrade the motherboard so that it can provide
conventional memory up to 640K.
| Modification Type: | Minor | Last Reviewed: | 7/7/2005 |
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| Keywords: | KB73322 |
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