New PowerBooks Use Processor/Bus "Slewing" like PowerMac G5s

Apple recently made available the Developer Notes for the new PowerBook models. This documentation reveals that the 15" and 17" models (but not the 12" model) support processor/bus"slewing." Slewing made its debut in the PowerMac G5 towers, and basically allows the computer to dynamically change the speed of processors and buses based on demand for processing power. According to Apple:

To lower power consumption and heat generation, the 15- and 17-inch PowerBook G4 incorporate an automatic power management technique called bus slewing. Bus slewing is designed to run at high processor speed, bus speeds, and voltage when the demand on the processor is high, and to run at low processor speed, bus speeds, and voltage when the demand on the processor is low. Switching between different processor/bus speeds and voltages is achieved by a transition that operates seamlessly to the user and should not impact system or application performance.

When the PowerBook is in slewing mode, the processor, processor bus, and memory bus adjust their speeds based on the current needs of the system. In other words, if you're doing something processor-intensive (such as rendering video), the PowerBook provides maximum performance. If you're doing something that doesn't require serious processing power (such as typing into a text editor), everything slows down. (Although that's too simplistic of an example; speed switching happens extremely quickly, and can occur numerous times within the same activity.) The benefits of slewing are lower power consumption and lower heat generation -- both of which are significant benefits in portable environments.

You enable slewing by selecting the "Automatic" setting in the Processor Performance sections of Energy Saver preferences. (The other two options, Highest and Reduced, force the PowerBook to use the fastest or lowest processor/bus speed, respectively.) In addition, if the PowerBook detects that it is running too hot, it will automatically go into slewing mode in order to cool down, even if you've selected the "Highest" processor setting instead of Automatic.

With slewing enabled, the range of speeds of the processors and buses are listed below:

PowerBook       Processor             Buses
15" 1.00 GHz    612 MHz - 1.00 GHz    102 - 167 MHz
15" 1.25 GHz    765 MHz - 1.25 GHz    102 - 167 MHz
17" 1.33 GHz    815 MHz - 1.33 GHz    102 - 167 MHz

Although processor slewing is similar to the "reduced processor" settings available on older PowerBooks, which also helped extend battery life, the biggest difference is that slewing allows the PowerBook to run at full speed when needed. Older PowerBooks set to "automatic" would use the "standard" (faster processor) setting when connected to AC power and the "reduced" (slower processor) setting when running off of the battery. With processor slewing, if you're operating off of battery power, you can still gain bursts of maximum processor/bus speed when needed, but benefit from lower speeds, and thus longer battery life and less heat, the rest of the time.

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