Windows Virtual PC in Windows 7. A redirector in a "guest" VDM or NTVDM allows access on the disks of the OS/2 or NT "host". Applications in a "guest" can use named pipes for communication with their "host". Due to a technical limitation, DOS and 16-bit Windows applications under OS/2 were unable to see more than 2 GB of hard drive space, this was fixed in ArcaOS 5.0.4. Windows NTVDM NTVDM was a system component of all IA-32 editions of the Windows NT family since 1993 with the release of Windows NT 3.1. It allows execution of 16-bit Windows and 16-bit / 32-bit DOS applications. The Windows NT 32-bit user-mode executable which forms the basis for a single DOS (or Windows 3.x) environment is called . In order to execute DOS programs, NTVDM loads which in turn loads , which executes a modified in order to run the application that was passed to NTVDM as command-line argument. The 16-bit real-mode system files are stripped down derivations of their MS-DOS 5.0 equivalents , and with all hard-wired assumptions on the FAT file system removed and using the invalid opcode 0xC4 0xC4 to bop down into the 32-bit NTVDM to handle the requests. Originally, NTDOS reported a DOS version of 30.00 to programs, but this was soon changed to report a version of 5.00 at and 5.50 at to allow more programs to run unmodified. This holds true even in the newest releases of Windows; many additional MS-DOS functions and commands introduced in MS-DOS versions 6.x and in Windows 9x are missing. 16-bit Windows applications by default all run in their own thread within a single NTVDM process. Although NTVDM itself is a 32-bit process and pre-emptively multitasked with respect to the rest of the system, the 16-bit applications within it are cooperatively multitasked with respect to each other. When the "Run in separate memory space" option is checked in the Run box or the application's shortcut file, each 16-bit Windows application gets its own NTVDM process and is therefore pre-emptively multitasked with respect to other processes, including other 16-bit Windows applications. 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