A rotation controller performs bit rotation in units of 2 bits for 8-bit data supplied from a common data bus in accordance with a mode selection signal from a mode selector and a column selection state of a column decoder and outputs the 8-bit data. The data bit-rotated by the rotation controller is supplied to a RAM via first, second, and third I/O gate blocks each having gates of 8 bits connected to a corresponding one of first, second, and third 8-bit column arrays of the RAM. First to third gate controllers are selectively activated by the column decoder. When a 6-bit write mode is selected by the mode selection signal from the mode selector, the first gate controller activates only gates of 6 upper bits in the first I/O gate block, the second gate controller activates only gates of 2 lower bits in the first I/O gate block and gates of 4 upper bits in the second I/O gate block, and the third gate controller activates only gates of 4 lower bits in the second I/O gate block and gates of 2 upper bits in the third I/O gate block.