CMOS Access Causes Some Machines to Hang with EMM386.EXE (94356)

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 Q94356


When you use the EMM386.EXE HIGHSCAN switch, your machine may stop responding (hang) or reboot when you attempt to access the CMOS.

The following machines may stop responding (hang):
    Dell 486D/50 (accessed by CTRL+ALT+ENTER)
    DTK Keen 2000 (accessed by CTRL+ALT+ESC)
    Micronics VLB MB (accessed by CTRL+ALT+ESC)
    Leading Technology QE486/33
The following machines may reboot:
    Some Gateway 2000 computers that use Phoenix BIOS


The HIGHSCAN switch causes EMM386.EXE to scan the F000-F7FF upper memory range. On some machines, EMM386.EXE detects that this range is available.

You should not have problems using this range on the Dell 486D/50 except when you attempt to access the CMOS.

This problem is more apparent in MS-DOS 6.2 and later because EMM386.EXE version 4.48 zeros available upper memory blocks (UMBs). However, it can also happen in MS-DOS 6.0 if you use HIGHSCAN and subsequently load enough device drivers or terminate-and-stay-resident (TSR) programs into UMBs to fill enough of the F000-F7FF region.

EMM386.EXE version 4.48 no longer tries to claim the HIGHSCAN area (F000h-F7FFh) on older DELL systems that do not support the XBIOS function (used to return the available ROM space).


To access your CMOS settings, restart (reboot) the computer and press the F5 key or press and hold down the SHIFT key to bypass all commands in your CONFIG.SYS and AUTOEXEC.BAT files. You can then access your CMOS settings by pressing the appropriate key combination.

Modification Type: Major Last Reviewed: 11/16/1999
Keywords: KB94356