by measuring an idle thread value of the cpu for a predetermined period of time or by detecting the cpu usage from the registry information of the system thereafter the measured cpu usage is compared with the minimum reference usage and maximum reference usage previously set using the display window of fig2 step s 32 or preset values if the measured cpu usage is less than the minimum reference usage the clock throttle rate of the cpu is stepwise increased thereby reducing the clock rate of the cpu step s 34 if the cpu usage measured after the reduction of the cpu clock throttle rate is still less than the minimum reference usage the cpu clock throttle rate is then adjusted to a next higher value thereby further reducing the clock rate of the cpu this procedure is repeated until the measured cpu usage is not less than the minimum reference usage if the measured cpu usage is between the minimum and maximum reference usages the current clock rate of the cpu is maintained as an optimum rate step s 33 if the measured cpu usage is more than the maximum reference usage the clock throttle rate of the cpu is initialized.