Windows NT Virtual DOS Machine (NTVDM)

MS-DOS-based applications run in a special Win32-based component referred to as a Windows NT virtual DOS machine (NTVDM). The NTVDM component translates MS-DOS operating system calls into calls used by the WIN32 subsystem.

If you print from a locally-run MS-DOS-based application or from a Windows NT command-line utility that sends data to a printer port, NTVDM receives the job. What happens next depends on whether the Windows NT redirector or the spooler manages the port:

• First, NTVDM queries the redirector to find out whether it is managing the target port. This would be the case if someone had issued a NET USE LPT1:\\SERVER\SHARE command. If so, the redirector takes control of the print job, and none of the spooler options affect the job.

• If the redirector is not managing the port, and someone has assigned a printer to that port using the Add Printer wizard, the job goes to that printer. In this case, the default data type of the print server and its spooling options are in effect.

• If neither the redirector nor printers are managing the port, the job goes to the port device driver unaltered. Naturally, this presumes a print device is physically connected to the port. If not, you'll get an error message.

For example, suppose one printer prints to COM1, and none of the printers print to LPT1 (but a print device is connected to LPT1). Also, suppose that you issue a net use command to redirect output from LPT2 to a network print share. If an MS-DOS-based application prints to LPT2, the job is sent to the network print share. If the application prints to COM1, NTVDM submits the job to the printer that prints to COM1, and that printer's default data type is used. If the application prints to LPT1, NTVDM submits the job directly to the parallel port device driver, and no print job alteration occurs.