The control unit of a computer system comprises a first storing device for holding written data while the power is on, a second storing device for holding the written data, even when the power is off and a power-off preserving device for terminating power to the system after transferring the data held in the first storing device to the second storing device. The control system can be utilized to only store the effective contents of the operating data to thereby enable data reconstruction upon resumption of power. The control device can interface with a window system processing device which can enable the refreshing of graphic display information upon resumption of power. The second storing device may have limited memory capacity and the operator can be informed of its capacity prior to a final shut-off of power. The control unit can further identify those programs that are not capable of being restored to their original condition and accordingly canceling any program execution when such a condition is judged.