INT 2fH 4003H: Enter Critical Section

                                                      Compatibility: Win 3.x 
 Expects: AX    4003H
 Returns: none
    Info: This signals to the VDD that the VM-aware application is entering
          a critical section, during which time the video registers should
          not be reprogrammed.  It cause the VDD to delay that action for
          up to 1 second.

          You should use INT 2fH 4004H (exit critical section) within
          1000ms (1 second) of making this call.  It 1000ms elapse without
          the VDD getting an INT 2fh 40004H call, it goes ahead and
          reprograms the video registers anyway.

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