INT 2fH 4001H: Notify Background Switch

                                                      Compatibility: Win 3.x 
 Expects: AX    4001H
 Returns: none
    Info: VM-aware applications can intercept this fn.  The VDD calls it
          when it is switching the application into the background.

          After being switched into the background, the application
          continues running, but will be frozen if it attempts to access
          video memory (and the screen is in a mode in which background
          access is not supported by the VDD--see INT 2fH 4000H).

See Also: INT 2fH 40xxH: Virtual Display Driver Functions
          INT 2fH: Multiplex Interrupt
          DOS Functions