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
            0 means no timeout
        DX = I/O port address if AL bit 4 set
        ES:DI = pointer to user byte if AL bit 4 clear
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