The following error message is generated when the device driver
HIMEM.SYS is being loaded and doesn't have a machine handler chosen:
ERROR: Unable to control A20 Line
There are two workarounds for this problem:
• Add the "/M:x" switch to the HIMEM.SYS line in the CONFIG.SYS file,
where x is number from the valid range of 1-14 and 16, shut down and
then restart your computer. For example:
NOTE: In Windows 95/98, The Himem.sys file is located in the Windows
folder, not the DOS folder.
NOTE: An incorrect handler may lock or hang the system at boot up.
You should have an MS-DOS version 5.0 bootable floppy disk
available to boot from before you experiment with different machine
Upgrade your machine's BIOS or contact your machine vendor for
assistance in modifying your CMOS settings. You may need to disable a
FastGate (or similar) option.
For more information, query on the following words in the Microsoft
The A20 line is the start of the first 64K of extended memory, known
as the high memory area (HMA). The HIMEM.SYS device driver must
control the A20 line to provide management of extended memory. The
error message is reported by HIMEM.SYS if it incorrectly identifies
the extended memory handling mechanism of the computer or if the
handling method of the machine's BIOS is unknown.
For more information concerning HIMEM.SYS, please refer to the
Microsoft MS-DOS version 5.0 "User's Guide and Reference" and to the
If you are using MS-DOS 6.x, type "help himem.sys" at the MS-DOS command
prompt, and then press ENTER.
Norton Utilities or Norton AntiVirus Installed
This behavior may also occur if you have Norton Utilities or Norton AntiVirus installed. For additional information, please see the following Symantec Web site:
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