A semiconductor memory device and a method therefor for changing an access right to access a shared memory area according to an external command and a refresh mode is provided. In one embodiment, the semiconductor memory device includes a plurality of input/output ports for inputting command signals for first or second mode refresh operation, a memory array divided into a plurality of different memory areas including a shared memory area that is accessible via at least two of the plurality of input/output ports, and a grant control block for assigning an access right to access the shared memory area in response to an external command signal. The grant control block may also generate grant control signals for preferentially assigning the access right to access the shared memory area to the input/output port for inputting the command signals for the first mode refresh operation.