Windows 3.0 Enhanced Mode Requires WINA20.386 (68655)

The information in this article applies to:
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22
    Microsoft Windows 3.0

This article was previously published under Q68655


When you use the Setup program to install Microsoft MS-DOS 5.0 or later, the read-only file WINA20.386 is copied to the root directory of the boot drive (C:\).

WINA20.386 is used by Microsoft Windows version 3.0 in enhanced mode to allow MS-DOS control of the A20 line. Enhanced mode Windows 3.0 cannot run in MS-DOS 5.0 or later without it, and the following error message is displayed if the file is not present:

You must have the file WINA20.386 in the root of your boot drive to run Windows in Enhanced Mode.
The file must be decompressed using Setup or the Expand utility. For information on using Expand, see your MS-DOS User's Guide and Reference.

NOTE: This information applies to Microsoft Windows version 3.0; it does not apply to later versions of Windows.


Windows 3.0 enhanced mode uses a modular architecture based on what are called virtual device drivers, or VxDs. VxDs allow pieces of Windows to be replaced to add additional functionality. WINA20.386 is such a VxD. (VxDs could be called "structured" patches for Windows.)

Windows 3.0 enhanced mode considers the state of the A20 line to be the same in all MS-DOS virtual machines (VMs). When MS-DOS is loaded in the high memory area (HMA), this can cause the machine to stop responding (hang) because of MS-DOS controlling the A20 line. If one VM is running inside the MS-DOS kernel (in the HMA) and Windows task switches to another VM in which MS-DOS turns off A20, the machine hangs when switching back to the VM that is currently attempting to execute code in the HMA.

WINA20.386 changes the way Windows 3.0 enhanced mode handles the A20 line so that Windows treats the A20 status as local to each VM, instead of global to all VMs. This corrects the problem.

For more information, query on the following words:

Windows and WINA20.386

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB68655