Abstract: Method and apparatus for dynamic control of power management circuitry in a microprocessor. A clock and power management subsystem within the microprocessor contains clock generation and control logic and a powered-down mode register. The register is controlled by register control logic in the microprocessor and determines the powered-down mode of the various hardware units that make up the microprocessor. The clock generation and control logic also receives a powered-down mode enable signal from each of the hardware units. The hardware unit receive a re-power-up signal which, when activated and deactivated, can cause the hardware units to de-activate and activate, respectively, the powered-down mode enable signal.