A method and apparatus for transitioning the CPU of a computer system to a lower performance level upon removal of AC power. Circuitry within the system input power control detects the removal of the AC power. The circuitry generates an event that causes power management software to immediately transition the CPU to a lower performance level sufficient to prevent the unwanted shutdown of the system. The AC power removal detection and CPU transition are effected while the system maintains an operational power level due to residual power in the system capacitors.