VDM - Ntvdm
In the MS Windows (NT/2000/XP) world is a acronym describing support for real-mode and protected-mode MS-DOS applications inside Win32 subsystem.
The VDM creates an environment where unmodified 16-bit MS-DOS binaries can execute without integrity of the 32-bit system and have ability to access I/O port (through IN and OUT instructions), writing directly to memory, DMA support and hardware interrupt simulation.