A volatile memory device includes a memory cell array, a command decoder, a self-refresh circuit, and a register. The command decoder is configured to decode a self-refresh entry command, a self-refresh exit command, and a register read command based on external command signals received from outside the volatile memory device. The self-refresh circuit is configured to automatically refresh the memory cell array during a self-refresh mode which be entered in response to the self-refresh entry command and be exited in response to the self-refresh exit command. The register is configured to store an accessible state in response to the self-refresh exit command, and output the stored accessible state in response to the register read command. The accessible state indicates whether or not the memory cell array is ready to be read or written.