INT 15 - SYSTEM - WAIT ON EXTERNAL EVENT (CONVERTIBLE)
        AH = 41h
        AL = condition type
            bits 0-2: condition to wait for
                     0 any external event
                     1 compare and return if equal
                     2 compare and return if not equal
                     3 test and return if not zero
                     4 test and return if zero
            bit 3:    reserved
            bit 4:    1=port address, 0=user byte
            bits 5-7: reserved
        BH = condition compare or mask value
        BL = timeout value times 55 milliseconds
            00h means no timeout
        DX = I/O port address if AL bit 4 set
        ES:DI -> user byte if AL bit 4 clear