Int 14h, 03h            Get Serial Port Status

    Returns line status and modem status information for a specified
    serial port.

        Entry   AH = 03h
                DX = Serial port number (0 - COM1, 1 - COM2, etc.)

        Return  AX = Line and Modem status (See below)

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    Status information is returned in AX, as follows:

            AH (Line status)
            7 6 5 4 3 2 1 0
            1 . . . . . . .       Time-out error
            . 1 . . . . . .       Transfer shift register empty
            . . 1 . . . . .       Transfer holding register empty
            . . . 1 . . . .       Break-detect error
            . . . . 1 . . .       Framing error
            . . . . . 1 . .       Parity error
            . . . . . . 1 .       Overrun error
            . . . . . . . 1       Data ready

            AL (Modem status)
            7 6 5 4 3 2 1 0
            1 . . . . . . .       Received line signal detect
            . 1 . . . . . .       Ring indicator
            . . 1 . . . . .       Data set ready
            . . . 1 . . . .       Clear to send
            . . . . 1 . . .       Change in receive line signal detected
            . . . . . 1 . .       Trailing edge ring detector
            . . . . . . 1 .       Change in data set ready
            . . . . . . . 1       Change in clear to send

    If bit 7 of the Line status byte (AH) is set, then the rest of the
    bits in AH and AL are unpredictable.


    Note
    Early versions of the ROM-BIOS for the original PC had a
    programming error that would cause "time-out" errors to be
    reported as "transfer shift register empty" and "break-detect"
    errors. This has been corrected in all other versions of the
    ROM-BIOS.