An operational method of a memory module is provided. The method includes receiving, from an external of the memory module, a first command and a first address in synchronization with clock signals. Status information is output through a signal line, when first data corresponding the first address is available in a data buffer in response to the first command. A second command in synchronization with the clock signals after the transmitting the status information is received from the external of the memory module, a second command. In response to the second command, the first data being available in the data buffer is output through data lines.