Från Apples diskutionsforum:
From Apple's PowerMac G5 Developer's Note:
Power Management
The basics of Power Mac G5’s power management techniques are described in the following paragraphs.
Processor and Bus Slewing To lower power consumption, heat generation, and fan noise, the Power Mac G5 computer incorporates an automatic power management technique called bus slewing. Bus slewing is designed to run at high processor/bus speeds and high voltage when the demand on the processor is high, and to run at low processor/bus speeds and low voltage when the demand on the processor is low.
Note: The memory bus runs at 400 MHz and does not slew.
Switching between different processor/bus speeds and voltages is achieved by a gradual transition that is designed to minimally impact system or application performance and typically operates seamlessly to the user. In slewing, the bus runs at half the speed of the processor.
The ranges of the slewed processor speeds are listed below:
1.8 GHz --> Processor Range 1.3 GHz to 1.8 GHz
2.0 GHz --> Processor Range 1.3 GHz to 2.0 GHz
2.5 GHz --> Processor Range 2.0 GHz to 2.5 GHz
In addition, the Power Mac G5 computer allows the user to control bus slewing mode. The options for specifying either high, reduced, or automatic processor and bus speeds are located at System Preferences>Energy Saver>Options; then select Automatic, Highest, or Reduced.
Note: For best performance when using professional applications with high demand on the system (such as Final Cut Pro or Logic), you may want to set the processor/bus speed option to highest.
If the Power Mac G5 computer detects a processor temperature that is high, due to high ambient temperatures or other factors, it will automatically enter bus slewing mode regardless of the selected setting.