iMac G5 Power Management Explained
(Energy Saver Preferences - Reduced, Automatic, Highest)
From Apple Developer Notes:
The power management techniques implemented in the iMac G5 are described in the following paragraphs.
Processor Power Step
To lower power consumption, the iMac G5 computer incorporates an automatic power management technique called power stepping. Power stepping is designed to run at high processor and bus speeds and high voltage when the demand on the processor is high, and to run at low processor and bus speeds and low voltage when the demand on the processor is low. The stepped processor speed is either 900 MHz or 1.8 GHz for the 1.8 GHz processor, 800 MHz or 1.6 GHz for the 1.6 GHz processor or 1GHz for the 2.0 GHz processor. Switching between different processor/bus speeds and voltages is achieved by a very fast transition that is designed to minimally impact system or application performance and typically operates seamlessly to the user.
In addition, the iMac G5 computer allows the user to control speed of the processor and bus. The options for specifying either high, reduced, or automatic processor and bus speeds are located at System Preferences>Energy Saver>Options; then select Automatic (default), 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 iMac G5 computer detects a system temperature that is too high, due to high ambient temperatures or other factors, it will automatically reduce the processor and bus speed regardless of the selected setting. If the temperature continues to be too high, the operating system will cause the system to enter sleep mode.
The following processor states are defined:
Run: The system is running at maximum processing capacity with the processor running at full speed.
Idle: The system is idling; this is the default state. All clocks are running and the system can return to running code within a few nanoseconds. If the system has no work to do, it will be in idle mode.
From idle, the processor will enter deep nap state when the system is power-stepped to half frequency. In this state, the processor’s internal frequency is reduced to 1/64 of the maximum frequency. The processor frequency is returned to normal on exit from idle. The exit from idle takes a bit longer than the regular idle nap but is still in the nanoseconds.
The Macintosh system has the following power-saving modes, which are set in the Energy Saver pane. If the system does not support full sleep, it will use the less efficient doze mode.
Run: The system is fully functional in normal operating mode with all components powered and operating.
Doze (default): The power to the disk drive motors and the display is turned off, but the power supply and fans are still on.
Full sleep: The main power supply is shut down. A trickle supply provides auxiliary power and keeps the DRAM state preserved for a quick recovery. The processor is powered off with its state preserved in DRAM. All non-essential clocks in the system are suspended. This mode allows the computer to meet Energy Star sleep requirements while providing the ability to start up without rebooting. This system may be awakened by administrative network packets, keyboard or mouse activity, USB device insertion or removal, or SMU scheduled wakeup.
Off: The iMac G5 meets FEMP (Federal Energy Management Program) requirements and supports FireWire ports functioning as repeaters when operating on AC power.