Event wait (INT 15h, 83h)

        Entry:
          AH = 83h
          AL = SUBSERVICE
              0 = set interval
              1 = cancel
          ES:BX = pointer to event flag (bit 7 set when interval expires)
          CX,DX = number of microseconds to wait (only accurate to 977 us)

        Return:
          CF set if function already busy