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 .