Regions Scanned by the EMM386.EXE HIGHSCAN Switch (96522)

The information in this article applies to:
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22

This article was previously published under Q96522


The EMM386.EXE version 4.45 and 4.48 HIGHSCAN switch allows EMM386.EXE to map expanded memory pages or upper memory blocks (UMBs) over portions of the upper memory area (UMA) used by system read-only memory (ROM).

Choosing "Yes" in response to the MemMaker prompt "Scan the upper memory area aggressively?" causes MemMaker to add HIGHSCAN to the EMM386.EXE device= line.


If you use the HIGHSCAN switch on the DEVICE=C:\DOS\EMM386.EXE line in the CONFIG.SYS file, EMM386.EXE examines the system ROM area starting at memory location F000:0000. If EMM386.EXE determines that ROM is duplicated between F000h-F7FFh and F800h-FFFFh, EMM386.EXE uses the F000h-F7FFh region for expanded memory page mapping or UMB memory. (This adds up to 32 kilobytes to the UMA.

On Micro Channel (MCA) systems with ROM BASIC (for example, IBM PS/2 systems), the HIGHSCAN switch allows EMM386.EXE to map expanded memory or UMB memory over the ROM BASIC code in the system ROM. Typically this is the region from F600 to FDFF.

Note: On some systems, EMM386.EXE uses the ROM area and the system does not operate correctly. The symptoms of this condition vary. For example, the system may stop responding (hang) or appear to operate normally until you use a floppy disk drive. Because of these potential problems, HIGHSCAN is not used by default.

Modification Type: Major Last Reviewed: 11/22/1999
Keywords: KB96522