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  Description:  Write Character to Communications Port      [PC] [AT] [PS/2]
 
  Writes a character to the specified serial communications port, returning
  the current status of the port.
 
  Input                             Output
  ═════════════════════════════     ═════════════════════════════════════════
  AH=01H                            If function successful
  AL=character                      AH bit 7=0
  DX=communications port number     AH bits 0-6=port status
     (0=COM1, 1=COM2,   etc.)                   Bit(s)  Significance (if set)
                                                ══════  ═════════════════════
                                                6       transmit shift
                                                        register empty
                                                5       transmit holding
                                                        register empty
                                                4       break detected
                                                3       framing error
                                                        detected
                                                2       parity error detected
                                                1       overrun error
                                                        detected
                                                0       receive data ready
                                    AL=character (unchanged)
                                    If function unsuccessful (timed-out)
                                    AH bit 7=1
                                    AL=character (unchanged)
                                    -♦-