INT 15h,  41h (65)       Wait for External Event                  Convertible

    This service waits for a specified status change after an external
    event. The external event can be an interrupt or the DMA controller.
    While waiting for the specific status, the system clocks are stopped
    in order to conserve battery power. This service is available only for
    the PC Convertible.

       On entry:      AH         41h
                      AL         Event Type code (see below)
                      BH         Condition compare or mask value
                      BL         Time-out (in units of 55 ms); 0 means no
                                 time-out
                      ES:DI      Pointer to user byte for event determination
                                 (if AL is 01h-04h)
                      DX         I/O Port Address for event determination (if
                                 AL is 11h-14h)

       Returns:       Carry      0 - external event occurred
                                 1 - time-out

  --------------------------------------------------------------------------

  AL -- Event Type Code (AL)

       00h            Return after any event has occurred
       01h            Compare value; return if equal
       02h            Compare value; return if not equal
       03h            Test bit; return if non-0
       04h            Test bit; return if 0

    Event Type Codes 11h-14h are the same as 01h-04h, except the event
    determination is coming from the I/O Port Address in DX, instead of
    the user byte specified by ES:DI.

    If the event type is 01h-02h or 11h-12h, then the byte signaling the
    event determination is compared (using the CMP instruction) against
    BH. If the event type is 03h-04h or 13h-14h, then the byte signaling
    the event determination is tested (using the TEST instruction) against
    BH.

       Notes:         This service is available only for the PC
                      Convertible.