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Function 4406h          Check Device Input Status

    Checks to see if a device or file is ready for input.

        Entry   AX = 4406h
                BX = Device or file handle

        Return  AL = FFh   Ready
                AL = 00h   Device not ready | File pointer at EOF
                or
                AX = Error code, if CF is set
                     | 01h Invalid function
                     | 05h Access denied
                     | 06h Invalid handle