The "Unable to control A20 Line" error message (73713)

Important This article contains information that shows you how to help lower security settings or how to turn off security features on a computer. You can make these changes to work around a specific problem. Before you make these changes, we recommend that you evaluate the risks that are associated with implementing this workaround in your particular environment. If you implement this workaround, take any appropriate additional steps to help protect your system.

INTRODUCTION

The following error message is generated when the device driver HIMEM.SYS is being loaded but does not 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, and then restart your computer. For example:
```
DEVICE=C:\DOS\HIMEM.SYS /M:1
```
Note In Microsoft Windows 98, the Himem.sys file is located in the Windows folder, not the DOS folder.

Note An incorrect handler may cause the computer to stop responding during startup. We recommend that you have an MS-DOS version 5.0 startup floppy disk available before you use different switches.
Upgrade your computer's BIOS or contact your computer vendor for help with the modification of 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:
FAST GATE A20

MORE INFORMATION

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 manage extended memory. The HIMEM.SYS driver reports the error if it incorrectly identifies the extended memory handling mechanism of the computer or if the handling method of the computer's BIOS is unknown.

For more information about HIMEM.SYS, see 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 is installed

Warning This workaround may make your computer or your network more vulnerable to attack by malicious users or by malicious software such as viruses. We do not recommend this workaround but are providing this information so that you can implement this workaround at your own discretion. Use this workaround at your own risk.

Note An antivirus program is designed to help protect your computer from viruses. You must not download or open files from sources that you do not trust, visit Web sites that you do not trust, or open e-mail attachments when your antivirus program is disabled.

For additional information about computer viruses, click the following article number to view the article in the Microsoft Knowledge Base:

129972Computer viruses: description, prevention, and recovery

This behavior may also occur if you have Norton Utilities or Norton AntiVirus installed. For additional information, visit the following Symantec Web site:

http://www.symantec.com

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