Method for decreasing power consumption of computer by dynamically changing voltage and frequency

PURPOSE: To decrease power consumption without letting a user recognize the exertion of serious influence upon performance by determining a required minimum power level based on the recent idle time of any specified operation or circuit and determining a voltage and clock velocity for applying the minimum power level to the circuit. CONSTITUTION: A power control subsystem 22 of a computer system 20 executes calculation for determining the power level required for executing the operation of the computer system 20. This power level is supplied to a sequencer 26 corresponding to a power select signal pwr-sel 24. The sequencer 26 selects the clock velocity and voltage value for providing the desired power level. This voltage is always requested when it is higher than or equal with a minimum voltage corresponding to the current clock velocity. A reference frequency ref-clk signal 34 is applied to a variable clock source 36, a clock signal 38 from the variable clock source 36 is applied to the computer system 20, and the desired operation is executed.