A semiconductor memory input/output device includes selection pads used to input and output signals for multiple operation modes and having multiple functions, a control signal generator for outputting setting signals and a mask control signal, a lower input/output unit including a lower output buffer for outputting a read data strobe signal to a selection pad and a lower input buffer for receiving a lower data mask signal from the selection pad, and selecting one operation of the lower output buffer and the lower input buffer, and an upper input/output unit including an upper output buffer for outputting an inverted read data strobe signal to the second selection pad and an upper input buffer for receiving an upper data mask signal from the second selection pad, and selecting one operation of the upper output buffer and the upper input buffer.