Windows Err Msg: Cannot Disable A20

• Microsoft Windows operating system versions 3.0, 3.0a

SUMMARY

You may get the message: "Cannot disable A20" when the HIMEM.SYS file is loaded.

The A20 line is the key to the first 64K of memory above the first megabyte of memory in the computer (also known as the High Memory Area or HMA). It is necessary that HIMEM.SYS enable this line; however, it is not necessary that HIMEM.SYS disable it. If the line cannot be disabled, there is a risk that other applications will use this line, possibly resulting in damage to HIMEM.SYS code and a loss of functionality.

MORE INFORMATION

There are two possible causes for this message:

• Another application is already using the A20 line when HIMEM.SYS tries to disable it. Check the CONFIG.SYS file for applications that are using the high-memory area (for example, VDISK, old versions of QEMM, or 386 to the Max). Either REMark these applications out or ensure HIMEM.SYS is loaded first in the CONFIG.SYS. Reboot the machine to check if this solves the problem.
• The hardware in the computer does not support disabling the A20 line. Although the problem can be in the design of the motherboard, usually the BIOS is responsible. An upgrade of the system BIOS may solve the problem. However, if the problem lies in the design of the motherboard, there is no workaround for this.