Aux input (INT 21h, 03h)

        Read a character form the AUX-device. 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 = 03h
        Return:
          AL = character read