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Chapter 23. Maintaining and Optimizing S... > VDM: The Virtual DOS Machine

VDM: The Virtual DOS Machine

Windows XP provides support for old MS-DOS and Windows 3.x applications through a program subsystem called the Virtual DOS Machine, or VDM. VDM is a program that mimics the hardware and software of a computer running MS-DOS. This program then loads and executes MS-DOS and Windows 3.x programs in a controlled environment. The “virtual” part signifies that an old program running in the VDM thinks it can directly control hardware such as the video card and keyboard, when in fact, the VDM intercepts all hardware control attempts and uses Windows XP to carry out the desired operations safely. An illegal or dangerous hardware operation can simply be rejected. No user application program can directly manipulate hardware on Windows XP—that’s why it’s so sturdy. When you run a Windows 3.x application or an MS-DOS program, Windows XP runs it through the VDM, where it can do whatever it wants without the risk of actually crashing the computer.

The Virtual DOS Machine can be configured by several settings and setup files, which are discussed in the following sections.


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