Requirements for UMB Support in MS-DOS 5.0 (71865)

The information in this article applies to:
    Microsoft MS-DOS operating system 5.0

This article was previously published under Q71865


MS-DOS version 5.0 provides systems the ability to free up conventional memory by running device drivers and other memory resident programs in the upper memory blocks (UMBs) located in the upper memory area (UMA). The UMA is address space located between 640K and 1024K that normally contains the system ROM BIOS and the video, network, and other hardware adapters. To access MS-DOS 5.0's ability to support UMBs, Microsoft recommends the following system conditions be met:
    A 80386 or 80486 microprocessor
    350K of XMS memory
    The following CONFIG.SYS statements:
          dos=umb                    or     dos=high,umb
          device=emm386.exe noems    or     device=emm386.exe ram
Any DEVICEHIGH statements in the CONFIG.SYS file must follow the two device statements previously mentioned.


80386 or 80486 Microprocessor

The 80386 and 80486 are the first processors that have paging hardware capability. EMM386.EXE uses this capability to map device drivers or programs into the UMA where these programs run in real-mode addresses that are below the 1024K line. Therefore, EMM386.EXE can currently work only with these microprocessors. For any earlier processor, a different upper memory manager is necessary for access to that area.

350K of XMS Memory

XMS is the Lotus/Intel/Microsoft/AST eXtended Memory Specification, which is a standard way for programs to use extended memory. 350K is the recommended amount of XMS memory necessary to use all of the UMA for UMBs. However, it is possible to have a smaller amount of XMS memory for UMB support. Most often, 350K of UMBs will never be available due to the system ROM BIOS and other hardware adapters that may be present in the UMA.

CONFIG.SYS Statements

The following MS-DOS 5.0 statements are needed for UMB support:
    The DEVICE=HIMEM.SYS statement loads the extended memory manager driver that provides access to extended memory.
    DOS=UMB provides MS-DOS the link to the UMA providing UMB support.
    The EMM386.EXE statement needs one of the following switches to manage the UMA:

    noems (Does not provide expanded memory support)
    ram (Provides both upper memory and expanded memory support)

    Examples: device=c:\dos\emm386.exe noems
    device=c:\dos\emm386.exe ram

More information on this topic can be found in the version 5.0 "Microsoft MS-DOS User's Guide and Reference," pages 313-330.

Modification Type: Major Last Reviewed: 11/26/1999
Keywords: KB71865