This article was previously published under Q68655
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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 bootdrive (C:\).
WINA20.386 is used by Microsoft Windows version 3.0 in enhanced mode toallow MS-DOS control of the A20 line. Enhanced mode Windows 3.0 cannot runin MS-DOS 5.0 or later without it, and the following error message isdisplayed 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. Forinformation on using Expand, see your MS-DOS User's Guide and Reference.
NOTE: This information applies to Microsoft Windows version 3.0; it doesnot apply to later versions of Windows.
Windows 3.0 enhanced mode uses a modular architecture based on what arecalled virtual device drivers, or VxDs. VxDs allow pieces of Windows to bereplaced to add additional functionality. WINA20.386 is such a VxD. (VxDscould be called "structured" patches for Windows.)
Windows 3.0 enhanced mode considers the state of the A20 line to be thesame in all MS-DOS virtual machines (VMs). When MS-DOS is loaded in thehigh memory area (HMA), this can cause the machine to stop responding(hang) because of MS-DOS controlling the A20 line. If one VM is runninginside the MS-DOS kernel (in the HMA) and Windows task switches to anotherVM in which MS-DOS turns off A20, the machine hangs when switching back tothe VM that is currently attempting to execute code in the HMA.
WINA20.386 changes the way Windows 3.0 enhanced mode handles the A20 lineso that Windows treats the A20 status as local to each VM, instead ofglobal to all VMs. This corrects the problem.
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