A memory module is provided which includes a printed circuit board; first semiconductor packages provided on one surface of the printed circuit board; and second semiconductor packages provided on the other surface of the printed circuit board, the first semiconductor packages and the second semiconductor packages having semiconductor dies that form ranks. A number of the ranks formed by the first semiconductor packages being different from a number of the ranks formed by the second semiconductor packages. Semiconductor packages forming a same one of the ranks receive a chip selection signal in common and semiconductor packages forming other ranks receive a different chip selection signal.