Port 3FE  -  Modem status register

        1xxx xxxx  1=DCD  (Data Carrier Detect)
        x1xx xxxx  1=RI   (Ring indicator - Set when a ring is detected)
        xx1x xxxx  1=DSR  (Data set ready)
        xxx1 xxxx  1=CTS  (Clear to send)
        xxxx 1xxx  1=.DCD (DCD has changed state)
        xxxx x1xx  1=.RI  (RI has changed state)
        xxxx xx1x  1=.DSR (DSR has changed state)
        xxxx xxx1  1=.CTS (CTS has changed state)

        Note: When bit 0-3 is set and change-in-modem-status interrupt
        is enabled, an interrupt is generated.