The OS/2 subsystem allows OS/2 16-bit character-based applications to run directly with Windows NT with essentially no modification. You can launch a character-based or video I/O (VIO) application from the Windows NT command prompt, from the File Manager, from the Program Manager, or indirectly from within a Win32 or OS/2 application. You can create a single batch file that can launch any combination of MS-DOS, Windows, or OS/2 programs. Windows NT recognizes an OS/2 application from information stored in the header of the executable file; it then calls the OS/2 subsystem to load the application.
If you never run an OS/2 application, the subsystem does not use any Windows NT resources. When you run an application, the OS2SRV process is loaded and continues to exist even after you've quit the application. To free up the minimal resources that the OS2SRV process uses, run the PViewer utility (which is provided with the Windows NT Resource Kit) and quit the OS2SRV process. If you later run another OS/2 application, the OS2SRV process is reloaded.