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 3.0a
    Microsoft Windows 3.1
    Microsoft Windows 3.11

This article was previously published under Q73322


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


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.


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.


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: Major Last Reviewed: 11/25/2003
Keywords: KB73322