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        Serial Communication (RS-232)

        The PC supports up to 4 serial communication ports at one
        time.

                Name    Interrupt    Ports 
                COM1       0Ch      3F8-3FF
                COM2       0Bh      2F8-2FF
                COM3       0Ch      3E8-3EF
                COM4       0Bh      2E8-2EF

        A serial communication adapter has 8 ports:

                Port    Name                                            
                xx8     Transmit/Receive buffer or Baud rate divisor LSB
                xx9     Interrupt enable or Baud rate divisor MSB
                xxA     Interrupt identification register
                xxB     Line control register
                xxC     Modem control register
                xxD     Line status register
                xxE     Modem status register
                xxF     Scratch pad register

        When programming the RS-232 port, a lot of steps has to be
        made, and a lot af things has to be considered.

          1.  The communications parameters has to be set.
              a.  The baud rate (port xx8, xx9 and xxB).
                  Remember to reset bit 7 on port xxB afterwards.
              b.  The parity and number of stop-bits (port xxB).
          2.  Interrupt parameters has to be set.
              a.  Set interrupt vectors (int 0Ch and 0Bh).
              b.  Select which interrupts to answer (port xx9).
              c.  Enable interrupt (port 021).
          3.  The serial port has to be reset.
              Read port xxA, xxD and xxC.

        The interrupt handler has to do the following things:

          1.  Get interrupt type.
              Read port xxA.
          2.  Multiplex on interrupt type.
              a.  If change-in-modem status
                  Read xxC and take action upon it.
              b.  If transmit-register-empty
                  Write next character to xx8 (if any).
              c.  If data-ready
                  Read data from xx8.
              d.  If line-status-change
                  An error has occurred - read port xxD and take
                  action upon it.

          3.  Return from interrupt
              a.  Send 20h to port 020 (signal end-of-interrupt).
              b.  Return from interrupt.

        When the program terminate, it must do the following things:

          1.  Disable interrupt (port 021).
          2.  Restore interrupt vector.