INT 14h,  03h (3)        Get Serial Port Status
 
    Returns line status and modem status information for a specified
    serial port.
 
       On entry:      AH         03h
                      DX         Serial port number (0 - COM1, 1 - COM2, etc.)
 
       Returns:       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.
 
 
       Notes:         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.