Using MS-DOS 5.x/6.x with Win 3.0 Enhanced Without WINA20.386 (77983)

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
    Microsoft Windows 3.0a

This article was previously published under Q77983


Microsoft Windows version 3.0 operates under MS-DOS 5.x, 6.0, and 6.2 without the WINA20.386 file if you include the line "SWITCHES=/W" in the CONFIG.SYS file. Although it is possible to run Windows 3.0 this way, doing so may cause Windows to stop responding (hang) if the following conditions are true:
    Windows 3.0 is running in 386 enhanced mode
    MS-DOS (or another program) is using the high memory area (HMA)
This information does not apply to later versions of Microsoft Windows.


Windows 3.0 enhanced mode considers the status of the A20 line to be identical in all MS-DOS virtual machines (VMs). Changing the status of the A20 line can have dangerous results, as in the following example. While one VM is accessing the MS-DOS kernel located in the HMA, Windows task switches to another VM in which MS-DOS turns off the A20 line. When Windows switches back to the original VM that is currently attempting to execute code in the HMA, Windows hangs (stops).

The WINA20.386 file is designed to force Windows 3.0 enhanced mode to treat the A20 line status as local to each VM, instead of global to all VMs. This is why it is not recommended to force Windows 3.0 enhanced mode to run without WINA20.386.

For more information about Windows and WINA20.386, query on the following words in the Microsoft Knowledge Base:

Windows and WINA20.386

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB77983