Int 14h, 02h            Receive One Character

    Receives one character at the specified serial port.

        Entry   AH = 02h
                DX = Serial port number (0 - COM1, 1 - COM2, etc.)

        Return  AL = Character
                AH = Line status (See service 03h)

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    This service waits for a character. If no character is available
    or an error occurs, no character is returned and bit 7 of AH is
    set.

    If an error occurs, bit 7 of AH is 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).