Method and apparatus for effecting BIOS independent power management of a personal computer system having a processor complex connected via a system bus to at least one I/O device capable of operating in a reduced power consumption state. A power control system comprising a dedicated power management microcontroller monitors the activity of various I/O devices. When a particular device or combination of devices has remained inactive for the preselected time interval, the power control system issues a bus request to a processor complex. When the processor complex acknowledges the bus request, the power control system asserts control as master of the system bus and performs the operations necessary to cause at least one device to enter a reduced power consumption state. The power control system then surrenders control of the bus to the processor complex. In one aspect, a CPU clock controller is utilized to reduce the processing speed of the processor complex CPU.