In an OnNow system, the operating system directs power management and integrates the activities of other components. The operating system's goal is to conserve energy while the PC is working and to put the PC to sleep when it is not doing work. The decisions that determine how to save energy and when to go to sleep are referred to as the power policy. In OnNow, power policy is based on end-user preferences, application needs, and system hardware capabilities.
The implementation of power policy is distributed throughout the system, with different policy owners for different aspects. The operating system itself is responsible for the policy of when the system as a whole should go to sleep and how deep a sleep to go into, as well as how to operate the processor to obtain energy conservation and to meet thermal and audible noise goals.
System power states derive from the Advanced Configuration and Power Interface (ACPI) specification. They are defined as follows:
* S0/Working - The CPU is fully up and running, devices are powering up and down as needed.
* S1 - The CPU is stopped; RAM is refreshed; the system is running in a low power mode.
* S2 - The CPU has no power; RAM is refreshed; the system is in a lower power mode than S1.
* S3 - The CPU has no power; RAM is in slow refresh; the power supply is generally in a reduced power mode (for example, it can't supply much power and is running in a lower power efficiency mode).
* S4 - The hardware is completely off, system memory has been saved to disk.
* S5/Off - the hardware is completely off, the operating system has shut down; nothing has been saved.
System power states are primarily defined in terms of motherboard/CPU/core logic characteristics and don’t relate to individual devices themselves.
Every device in the system has its own power states. These are defined as follows:
* D0 - fully on
* D1 - low power; context may be saved in hardware (depends on the device class)
* D2 - low power; context may be lost by hardware (depends on the device class)
* D3 - power may be lost; all context lost by hardware
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