The
piixpcib driver provides support for the Intel PIIX and compatible PCI-ISA Bridges with Intel's first generation SpeedStep.
Frequency scaling is supported on Pentium III with two voltage modes, used by SpeedStep as power states low and high. The driver will switch into low power state by reducing voltage and frequency of the CPU. The factor depends on the processor itself, but will always reduce power consumption about 1/2.
The user can manually control the CPU frequency with the
sysctl(8) program using the following node:
machdep.speedstep_state = [0/1]