MS-DOS 5.0 UMBs, Enhanced Mode Windows, and LIM 4.0 Support (69516)

SUMMARY

If you use the upper memory block UMB support included with MS-DOS version 5.0 and you try to run an LIM 4.0-aware application under enhanced mode Windows version 3.0, Windows may not detect any expanded memory, even if you set up a PIF file telling Windows to allocate expanded memory for it.

This problem occurs if you are running EMM386.EXE with the NOEMS option. When this switch is used, EMM386 fills all available addresses between 640K and 1 MB with UMBs. Because all expanded memory managers operate by swapping pages of memory into unused addresses in this range, when all the available space is filled, there is no room for the pages and no expanded memory is available.

WORKAROUND

There are two ways to work around this:

You can use the RAM option instead of the NOEMS option. You can allocate as little as 16K (one page) of expanded memory. Windows is then able to give expanded memory to applications that can access LIM 4.0 memory.
You can use the X=xxxx-yyyy parameter on EMM386 to exclude enough space for a page frame. Reserving a 16K page of memory is enough for Windows to allocate expanded memory.

With the NOEMS option on, EMM386 Windows creates a page frame below 640K through handle 0000h. This is a special handle that is normally available only to the operating system. This handle has a set of pages allocated to it when EMM386 installs. The pages that are automatically allocated to handle 0000h are those that backfill conventional memory. Typically, this backfill occurs between addresses 40000h (256K) and 9FFFFh (640K).

An application that is aware of handle 0000h is able to make use of expanded memory through the handle. However, most applications are not written to take advantage of handle 0000h because the LIM 4.0 specifications specify that this handle is reserved for the operating system. Any applications that use handle 0000h are incompatible with future versions of Microsoft operating systems and environments.