A memory module includes a random access memory (RAM) device that includes a first storage region and a second storage region, a nonvolatile memory device, and a controller that controls the RAM device or the nonvolatile memory device under control of a host. The controller includes a data buffer that temporarily stores first data received from the host, and a buffer returning unit that transmits first release information to the host when the first data are moved from the data buffer to the first storage region or the second storage region of the RAM device and transmits second release information to the host when the first data are moved from the second storage region to the nonvolatile memory device.