Initialize USART (INT 14h, 00h)

        Entry:
          AH = 00h
          AL = initializing parameters
              7 - 6 - 5      4 - 3     2    1 - 0
              -BAUD RATE-    PARITY   STOP   WORD
                                      BITS  LENGTH
              000  110 BD    00 none  0-1   10 - 7
              001  150 BD    01 odd   1-2   11 - 8
              010  300 BD    11 even
              011  600 BD
              100 1200 BD
              101 2400 BD
              110 4800 BD
              111 9600 BD (4800 ON PCjr)
          DX = port number

        Return:
          AH RS-232 status code bits
            0: data ready
            1: overrun error
            2: parity error
            3: framing error
            4: break detected
            5: transmission buffer register empty
            6: transmission shift register empty
            7: time out--if set, other bits invalid
          AL = modem status bits
            0: delta Clear-To-Send
            1: delta Data-Set-Ready
            2: trailing edge of ring detected
            3: change in receive line signal detected
            4: Clear-To-Send
            5: Data-Set-Ready
            6: ring detected
            7: receive line signal detected