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Missing Himem.sys or A20 error in Windows95 or 98

by Roxanne Pierce


When you reboot the computer, it displays one of the following (or similar) error messages:

  • "Windows XMS driver Unable to control A20 line; XMS driver not installed."
  • "HIMEM.SYS is missing."

This error message does not appear when first applying power to the computer, only when rebooting it once it is already powered on. It is more likely to affect laptop computers than desktop computers. This error message will frequently appear for the first time after installing one of several versions of Norton Anti-Virus.


Turn the computer off completely, then follow these steps to make changes to the computer's DOS startup files, autoexec.bat and config.sys:

  1. Turn on the computer.
  2. For Windows 95 When the screen says "Starting Windows 95", immediately press the F8 key. This displays the Windows 95 Startup Menu. For Windows 98: as soon as you turn on the computer, press and hold the CTRL key until the Windows 98 Startup Menu is displayed.
  3. Select Safe Mode - Command Prompt Only either by highlighting that option with your down arrows or by typing in the number of that menu option, and then press the Enter key on your keyboard.
  4. At the C:\> prompt, type EDIT AUTOEXEC.BAT [Enter]
  5. You should now have a blue screen which you can edit text in. At the very top of the window, add the following line SET DOS16M=2
  6. Press Alt+F to open the File menu. Choose Exit and press the Enter key.
  7. When prompted to save changes, use your arrow keys to highlight "Yes" and press Enter.
  8. You will now be back at the C:\> prompt. Type EDIT CONFIG.SYS [Enter]
  9. In the text editor, find a line which refers to a file called HIMEM.SYS and add /M:1 to the very end of that line. Be sure there is a space following the end of the existing line and the /M:1
  10. If you do not have a HIMEM.SYS line, add the following line: DEVICE=C:\Windows\HIMEM.SYS /M:1
  11. Press Alt+F to open the File menu. Choose Exit and press Enter. When prompted to save changes, answer Yes and press Enter.
  12. Reboot the computer. It should now boot normally.

Note1 You may need to alter the path if your version of Windows 95 or Windows 98 is not located in the Windows directory.

Note2 The /M:1 line depends upon the hardware version of the A20 handler. You may need to try the numbers 1-17 (for instance, /M:12) to find the correct version for the computer. PS/2 computers and Prolinea computers, for example, tend to require /M:2 instead of /M:1.


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