Aux output (INT 21h, 04h)

        Writes a character form the AUX-device, and checks for Ctrl-Break
        afterwards. At startup, the AUX device is initialized to COM1,
        No parity, 8 databits and one-stop bit. We don't recomend use
        of this function, since it don't support any status reading
        from the comm-port. Use INT 14 instead, or even better - write
        your own communication drive that directly accesses the ports.

        Entry:
          AH = 04h
          DL = character to send