Advanced Configuration and Power Interface (ACPI)

The Differentiated System Description Table (DSDT) is the primary ACPI table containing the Definition Block for the base system. It is a persistent, compiled ACPI Machine Language (AML) bytecode table that describes:

• All system hardware (buses, devices, interrupts).
• The Power Management (PM) profile for the platform.
• The supported sleep states and power resources.

The operating system's ACPI driver interprets the DSDT's AML code to understand the platform's hardware configuration and the methods available to control it. It is loaded during boot and remains resident in memory.

Performance States (P-States) are ACPI-defined operational modes for a processor core that allow the OS to dynamically balance performance and power consumption. Key characteristics include:

• P0 is the highest-performance state (maximum voltage/frequency).
• P1, P2, P3... are successively lower-performance, more efficient states.
• Transitions are controlled by the OS power policy driver.

P-States are implemented using Dynamic Voltage and Frequency Scaling (DVFS). The OS selects a P-State based on CPU utilization, thermal conditions, and power policy (e.g., balanced vs. performance).

Processor Power States (C-States) are ACPI-defined idle sleep states for a processor core, enabling power savings during periods of inactivity by turning off internal circuits.

• C0: Active state; the core is executing instructions.
• C1 (Halt): Core clock is gated; quick exit latency.
• C2 (Stop-Clock): Core clocks are stopped; lower power than C1.
• C3 (Sleep): Core's caches are flushed and powered down; significant latency.
• C6+ (Deep Sleep): Core voltage is reduced to near-zero; state is saved to SRAM; longest exit latency.

Deeper C-States save more leakage power but incur higher wake-up latency. The OS idle scheduler decides when to enter deeper states.

Device Power States (D-States) define the power management states for individual system devices (e.g., PCIe, USB, SATA controllers).

• D0: Fully powered on and operational.
• D1, D2: Intermediate low-power states; device context may be partially lost.
• D3 (Subdivided into D3hot and D3cold):
  • D3hot: Device is soft-off; main power rail is on, enabling a software-initiated wake-up.
  • D3cold: Device is completely powered off; requires a full re-initialization to wake (e.g., via a wake signal).

The OS device driver, in coordination with the ACPI subsystem, manages transitions between D-States based on device usage.

ACPI defines global system sleep states that affect the entire platform.

Sleeping States (S-States):

• S1: CPU clocks stopped; RAM refreshed; quick resume.
• S2: CPU powered off; RAM refreshed.
• S3 (Suspend-to-RAM): System state saved in DRAM; most components powered off; low-power, fast resume.
• S4 (Suspend-to-Disk / Hibernation): System state saved to non-volatile storage; platform powered off; slower resume.
• S5 (Soft Off): System is off; requires full boot sequence.

Global States (G-States):

• G0 (S0): Working state.
• G1: Sleeping state (S1-S4).
• G2 (S5): Soft Off.
• G3: Mechanical Off (AC power disconnected).

The ACPI thermal model provides a standard interface for the OS to monitor and manage platform temperature. Its key objects include:

• Thermal Zones: Represent a discrete physical area of the platform (e.g., CPU package, GPU).
• Thermal Sensors: Report current temperature within a zone.
• Passive Cooling: OS-initiated cooling (e.g., lowering P-States, throttling performance). Defined by _PSV (Passive Trip Point) and _TC1/TC2 (thermal constants).
• Active Cooling: Hardware cooling (e.g., fan control). Defined by _ACx (Active Trip Points) and _ALx (Active Cooling Levels).
• Critical Trip Point (_CRT): Temperature at which the system must perform a hardware shutdown to prevent damage.

This model enables Dynamic Thermal Management (DTM) where the OS reacts to thermal events by invoking cooling policies.

P-States are ACPI-defined operational modes for a processor core that allow the operating system to dynamically adjust its voltage and frequency. Each P-State represents a specific performance level, with P0 being the highest performance (and power) state. The OS uses P-State transitions to match processor capability to the instantaneous workload, saving energy during periods of low demand. This is a primary mechanism for implementing Dynamic Voltage and Frequency Scaling (DVFS).

C-States are ACPI-defined idle sleep states for a processor core, ranging from C0 (fully active) to deeper states like C3 or C6. As the C-State number increases, more of the core's internal circuits are powered down to reduce leakage power, but the latency to wake back to C0 also increases. The OS places idle cores into progressively deeper C-States to minimize static power dissipation. C-States manage power when there is no work to do, complementing P-States which manage power during active execution.

DVFS is the foundational hardware technique implemented through ACPI P-States. It dynamically adjusts a processor's operating voltage and clock frequency in response to real-time computational load. Lowering the frequency allows a proportional reduction in voltage, which yields a cubic reduction in dynamic power (P ∝ C * V² * f). DVFS is the primary method for optimizing the performance per watt of a system under variable workloads.

A Power Management Unit is a dedicated hardware controller, often a microcontroller embedded within a System-on-Chip (SoC), that executes low-level power sequences. The PMU acts as the physical enforcer for ACPI commands. When the OS requests a P-State or C-State transition via ACPI, the PMU handles the precise timing of voltage regulator adjustments, clock gating, and power gate switching. It is the critical bridge between ACPI software policy and hardware power controls.

Thermal throttling is a protective mechanism triggered when a processor's temperature exceeds a safe threshold. While often managed independently by hardware, ACPI provides standardized interfaces (e.g., Thermal Zones) for the OS to participate. Throttling typically forces the processor into a lower P-State, reducing frequency and voltage to decrease power dissipation and allow the cooling system to recover. It is a reactive safeguard to prevent damage, distinct from proactive DVFS for efficiency.

RAPL is an Intel-specific hardware interface that provides software with the ability to monitor and enforce power consumption limits for processor packages, DRAM, and other domains over a specified time window. While not part of the core ACPI specification, RAPL often operates alongside it. The OS or a performance manager can use RAPL data to inform ACPI P-State decisions, ensuring the system stays within a defined power budget or Thermal Design Power (TDP) envelope.