DOS

The NT Virtual DOS Machine (aka Windows on Windows),is a Win 16 subsystem that runs under Windows NT, which allows 16 bit applications to run as if they were being executed on a DOS machine,with that machine's multitasking and segmented memory model.

The NT-based 32-bit operating system provides a virtual DOS machine (VDM) within the Microsoft? Win32? subsystem to support real-mode and protected-mode MS-DOS applications. The VDM creates an environment where unmodified 16-bit MS-DOS binaries can execute without threatening the security and integrity of the 32-bit system. The VDM environment serves programs such as:
MS-DOS applications 16-bit drivers 16-bit Windows applications in the Windows on Windows (WOW) subsystem Terminate and stay resident (TSR) programs The NT-based operating system launches a VDM as necessary to support 16-bit applications launched by the user. There can be several VDMs running at any one time, each in its own process. Every VDM sets up a self-contained virtual DOS machine environment. Each individual VDM is dynamically configured to run either a Windows 3.x application or an MS-DOS application. When any Windows 3.x application is launched, the VDM in which it runs loads the WOW subsystem, which is a simulation of the Windows? 95 16-bit environment. Any Windows 3.x application can run either in a separate VDM or in the default shared-WOW VDM. In addition, a console session launches a VDM whenever the user starts an MS-DOS application from the commandprompt.

When the VDM is launched, it:

Allocates memory in which to create a virtual DOS machine.
Reads the files \system32\config.nt and \system32\autoexec.nt.
Loads drivers and executes batch files as specified.
Loads the application and executesit.
The application runs as it would in a native MS-DOS environment because the VDM provides virtual equivalents of all the functionality MS-DOS would supply to the application. All drivers and batch files execute within the virtual DOS environment set up by the VDM.

Given the functionality of the VDM, most MS-DOS applications can run without any modification or additions in the Windows NT/Windows 2000 environment. However, there are some MS-DOS applications that depend on custom hardware,such as a fax board or a 3270 communications board. Generally, these applications have a plug-in hardware board they manipulate directly through 80386/80486 I/O port IN and OUT instructions and by writing directly to memory on the board. These applications might also use DMA to transfer data to and from the board. In an MS-DOS environment, these operations allow the application or its 16-bit driver to get direct access to the hardware device

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