Abstract: Power down circuitry in a processor for controlling power delivered to functional units of the processor, comprising first and second power down circuits. The first power down circuit comprises a state machine having a decoded instruction as input and a control signal as output. The control signal disables a clock signal to a floating point unit (FPU) when the decoded instruction is not a floating point instruction. The second power down circuit comprises a prediction circuit that generates a predict signal when a cache access cannot occur. The predict signal disables a clock signal to a cache.

Abstract: A micro processor including a bus fraction register with an encoding which when decoded indicates either a bus fraction encoding or a stop clock function, data processing logic that includes a number of units including a bus unit, arranged as an instruction pipeline. The units are clocked by an internal clock running at a first frequency and operating with an I/O bus clocked by an I/O clock running at a second frequency which is a fraction of the first frequency. A stop clock signal is generated upon the condition that the bus fraction register contains the stop clock encoding. A bus unit busy (BBSY) signal line is polled to ensure that all pending bus cycles in the pipeline are completed, the polling being initiated in response to the stop clock signal. A special cycle encoded to indicate the stop clock function is run to inform the units of the microprocessor that the internal and I/O clocks are going to stop toggling.