CPU Power States | Raman's Hub

Definition

CPU power states refer to the operating modes defined by the Advanced Configuration and Power Interface (ACPI) specification. These states control the power consumption of a system or its components, allowing it to conserve energy when idle or under low load conditions.

Types of Power States

ACPI defines several power states for the entire system and individual components, such as CPUs. The two primary categories are:

1. System Power States (S-States)

State	Name	Description
S0	Working	The system is fully operational. The CPU, memory, and peripherals are powered and active.
S1	Sleep (Low Wake Latency)	A low-power state where the CPU halts but remains in a low-power mode. Memory is still powered for context.
S2	Sleep (Deeper Sleep)	Similar to S1, but with the CPU powered down completely, and only memory is retained.
S3	Suspend-to-RAM	Also known as “Standby” or “Sleep.” The system stops almost all operations, saving the state to RAM.
S4	Hibernate	Known as “Suspend-to-Disk,” the system saves its state to the disk and powers down almost entirely.
S5	Soft Off	The system is fully powered down, but certain components like the BMC or wake-on-LAN (WOL) features remain active.

2. CPU Power States (C-States)

State	Description
C0	Active state. The CPU is fully powered and executing instructions.
C1	Halt state. The CPU is idle but can return to C0 almost immediately.
C2	Stop-clock state. Some parts of the CPU are powered down for slightly lower power consumption.
C3	Sleep state. The CPU stops all internal operations and requires more time to transition back to C0.
C6	Deep power-down. The CPU core power is removed, and the state is saved externally (depends on architecture).

3. Device Power States (D-States)

These states apply to individual devices (e.g., GPUs, storage, network adapters) and define their activity or power-saving modes:

State	Description
D0	Fully on and operational.
D1	Low-power state with reduced functionality (optional).
D2	Deeper low-power state with more functionality reduction.
D3	Powered off, except for wake-up circuitry (if available).