Int 14h, 01h            Send One Character

    Sends one character to the specified serial port.

        Entry   AH = 01h
                AL = Character
                DX = Serial port number (0 - COM1, 1 - COM2, etc.)

        Return  AH = Line status (see Service 03h)

    ──────────────────────────────────────────────────────────────────

    If an error occurs, bit 7 of AH will be set. Since bit 7 is used
    as a general error flag, this service is unable to identify a
    time-out error. For complete diagnostic information, use Service
    03h (Get serial port status).