PURPOSE: To enable to prevent the error in computer systems, by saving the internal information of CPU at the switching of clock and returning the internal information saved after clock switching.

CONSTITUTION: If a failure is taken place to a main memory control unit 2-0, a service processor 5 saves the internal information of CPUs 3-0, 3-1 and it reconnects the CPUs 3-0, 3-1 and channel processors 4-0, 4-1 to a main memory control unit 2-1. As a result, the oscillator of a main memory control unit 2-1 supplies clock to the system. After the reconnection, the processor 5 resets the internal information of CPUs 3-0, 3-1 and channel processor 4-0, 4-1, and returns the internal information of the CPUs 3-0 and 3-1 saved to the original location after that. Then, the processor 5 to bring the main memory 1 ON-line connects the main memory 1 to the main memory control unit 2-1 to start the CPUs 3-0, 3-1.