NTVDM
The NTVDM is present in all 32-bit NT-based Windows versions including Vista, 7 and 8, except for in 64-bit issues of these versions. 16-bit applications all run in their own thread within a single preemptively multithreaded 32-bit NTVDM process. The 16-bit processes are by default cooperatively multitasked with respect to each other, unless the "Run in separate memory space" option is checked in the Run box or the application's shortcut file. NTVDM emulates BIOS calls and tables as well as the Windows 3.1 kernel and 16-bit API stubs. The 32-bit WoW translation layer thunks 16-bit API routines. 32-bit DOS emulation is present for DOS Protected Mode Interface (DPMI) and 32-bit memory access. This layer converts the necessary extended and expanded memory calls for DOS functions into Windows NT memory calls. Wowexec.exe is the emulation layer that emulates 16-bit Windows. Windows 2000 and Windows XP added Sound Blaster 2.0 emulation. 16-bit virtual device drivers and DOS block device drivers (e.g., RAM disks) are not supported. Inter-process communication with other subsystems can take place through OLE, DDE and named pipes.
Since virtual 8086 mode is not available on RISC-based processors, NTVDM was instead implemented as a full emulator in these versions of NT. Up to Windows NT 3.51, only 80286 emulation was available. With Windows NT 4.0, 486 emulation was added.
Read more about this topic: Virtual DOS Machine