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        Port 3F9  -  Interrupt enable register or baud rate divisor MSB

        If 3FB, bit 7 = 1:  Baud rate divisor MSB (see 3FB)
                      = 0:  Interrupt enable

        Interrupt enable register (write only):
                1111 xxxx  Forced to 0
                xxxx 1xxx  1=Enable Modem Status interrupt
                xxxx x1xx  1=Enable Receiver Line Status interrupt
                xxxx xx1x  1=Enable Transmitter Register Empty interrupt
                xxxx xxx1  1=Enable Received Data Available interrupt