MS-DOS Error Msg: Unable to Control A20 Line (73713)

The information in this article applies to:
    Microsoft MS-DOS operating system
    Microsoft Windows 95
    Microsoft Windows 98
    Microsoft Windows Millennium Edition

This article was previously published under Q73713


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:
          DEVICE=C:\DOS\HIMEM.SYS /M:1
    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 switches.
    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 Knowledge Base:



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 README.TXT file.

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: Microsoft provides third-party contact information to help you find technical support. This contact information may change without notice. Microsoft does not guarantee the accuracy of this third-party contact information.

The third-party products that are discussed in this article are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, regarding the performance or reliability of these products.

Modification Type: Major Last Reviewed: 5/10/2003
Keywords: kbenv kberrmsg kbinfo KB73713