Sleep States

Sleep states, or “S states”, describe the power applied to the host processor. The sleep state may relate to specific hardware components, but that information is usually not needed to manage Intel AMT-based platforms. When the host is fully powered up, it is said to be “in S0”. If the host is in some higher sleep state (i.e., lower power state), it is “in Sx”.

The Advanced Configuration and Power Interface (ACPI) specification defines the following states:

   G0 (S0): Working

   G1, Sleeping subdivides into the four states S1 through S4:

   S1 Sleeping State: The S1 sleeping state is a low wake latency sleeping state. In this state, no system context is lost (CPU or chip set) and hardware maintains all system context.

   S2 Sleeping State: The S2 sleeping state is a low wake latency sleeping state. This state is similar to the S1 sleeping state except that the CPU and system cache context is lost (the OS is responsible for maintaining the caches and CPU context). Control starts from the processor’s reset vector after the wake event.

   S3 Sleeping State: The S3 sleeping state is a low wake latency sleeping state where all system context is lost except system memory. CPU, cache, and chip set context are lost in this state. Hardware maintains memory context and restores some CPU and L2 configuration context. Control starts from the processor’s reset vector after the wake event.

   S4 Sleeping State: The S4 sleeping state is the lowest power, longest wake latency sleeping state supported by ACPI. In order to reduce power to a minimum, it is assumed that the hardware platform has powered off all devices. Platform context is maintained.

   G2 S5 Soft Off State: The S5 state is similar to the S4 state except that the OS does not save any context. The system is in the “soft” off state and requires a complete boot when it wakes. Software uses a different state value to distinguish between the S5 state and the S4 state to allow for initial boot operations within the BIOS to distinguish whether or not the boot is going to wake from a saved memory image.

 

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