INT 2F - MS WINDOWS 3+ - BEGIN CRITICAL SECTION
        AX = 1681h
Notes:  used to prevent a task switch from occurring
        should be followed by an INT 2F/AX=1682h call as soon as possible
        nested calls are allowed, and must be followed by an appropriate number
          of "end critical section" calls
        not supported in Windows/386 2.x. Get INDOS flag with INT 21/AH=34h and
          and increment by hand.
SeeAlso: INT 15/AX=101Bh,INT 21/AH=34h