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Advanced Configuration and Power Interface

lastmod 2022-07-20created 2020-06-274 edits

The Advanced Configuration and Power Interface (ACPI) specification is the first open standard, developed by HP, Intel, Microsoft, Phoenix, and Toshiba and first released in December 1996. It defines common interfaces for hardware discovery, motherboard and device configuration, and power management. According to the specification, "ACPI is the key element in OSPM."

States

Global states

The ACPI specification defines the following (global) states that can be used on an ACPI-compliant computer system.

  • G0 (S0): Working.
  • G1: Sleeping.
    • S0ix: Modern Standby.
    • S1: A sleep mode placed in a state that still requires power.
    • S2: A sleep mode that saves more power than S1. It cuts power to the CPU, but this feature is rarely used.
    • S3: Sleep mode, suspend. Main memory is not turned off.
    • S4: Hibernation
  • G2 (S5): Soft Off
  • G3: Mechanical Off

Device states

Device states D0-D3 vary by device:

  • D0: Fully on, operating
  • D1 and D2: Intermediate power states (definition varies by machine)
  • D3: Off, the device is powered off and does not respond to the computer bus

Processor states

CPU power states C0-C3 are defined as follows:

  • C0: Operating
  • C1: Halt. The processor executes no instructions but can return to an executing state immediately. Some processors, such as the Pentium 4, support an enhanced C1 state (C1E) to save power.
  • C1E: Enhanced Halt. Stops the CPU internal clock via software and lowers the CPU voltage. The bus interface unit and APIC run at full speed
  • C2: Stop-Clock. Takes a long time to return to the original state.
  • C3: The processor does not keep its caches coherent but maintains other state. Several processors support C3, but each of them takes a different amount of time to return to the normal operating state.
  • C4: Deeper Sleep. Lowers the CPU voltage
  • C4E/C5: Enhanced Deeper Sleep. Lowers the CPU voltage and turns off the memory cache
  • C6: Deep Power Down. Reduces the CPU internal clock and lowers the CPU voltage

Performance states

While a device or processor operates in D0 or C0 respectively, it runs in one of several power performance states. These states can vary depending on the added features, but P0 is always the highest-performance state, P1 through Pn-1 are successively lower-performance states, and the number of states n is limited to a maximum of 16.

ACPI tables

The tables below are used by the operating system to obtain hardware information.

  • RSDP (Root System Description Pointer)
  • RSDT (Root System Description Table)
  • DSDT (Differentiated System Description Table): Provides configuration information about the base system.
  • XSDT (Extended System Description Table)
  • FADT (Fixed ACPI Description Table)
  • FACS (Firmware ACPI Control Structure)
  • SBST (Smart Battery Table)
  • ECDT (Embedded Controller Boot Resources Table)
  • MADT (Multiple APIC Description Table)
  • SRAT (System Resource Affinity Table)
  • SLIT (System Locality Distance Information Table)
  • SLIC (Software Licensing Description Table)
  • SSDT (Secondary System Description Table)

Utilities

See also

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