May I know which system register (I am using a core 2 duo E8400) should be used to control the possible voltage/freq settings for DVFS. I am not trying to over clock the cpu. I am trying to select one of the pairs of available voltage/freq combinations. ACPI drivers do it save power during idle periods. I went through manual vol 2 and 3 and was able to read performance registers using rdmsr/wrmsr commands. But, I was not able to find the address of the control register that can control the DVFS settings.