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        Port 3FA  -  Interrupt identification register (prioritized)

        1111 1xxx  Forced to 0
        xxxx x11x  Interrupt ID (see below)
        xxxx xxx1  0=Interrupt pending - 1=No interrupts pending


   ID  Priority  Type                   Source               Reset Control
   -------------------------------------------------------------------------
   00  Lowest    Modem Status           Clear to Send        Read Modem
                                        Data Set Ready       Status
                                        Ring indicator       Register
                                        Carrier Detect
   -------------------------------------------------------------------------
   01  Low       Transmitter Holding    Ready to send next   -
                 Register Empty         character
   -------------------------------------------------------------------------
   10  High      Received Data          Something was        Read Receive
                 Available              received             Buffer Register
   -------------------------------------------------------------------------
   11  Highest   Receiver Line status   Overrun Error        Read Line
                                        Parity Error         Status Register
                                        Framing Error
                                        Break Interrupt