INT 14h,  01h (1)        Send One Character

    Sends one character to the specified serial port.

       On entry:      AH         01h
                      AL         Character
                      DX         Serial port number (0 - COM1, 1 - COM2, etc.)

       Returns:       AH         Line status (see Service 03h)

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    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).