Nice to know: What is ACPI?

We all like to put our PC into hibernation instead of always shutting it down right away. But how does this work? ACPI makes it possible. Let’s take a closer look at this standard.

ACPI is the abbreviation of "Advanced Configuration and Power Interface" and describes an open industry standard for power management of PCs, laptops and servers. In order to use ACPI, an ACPI compatible hardware (motherboard, power supply etc.) and an ACPI compatible operating system (for example Windows) are both required. ACPI was released in 1996 and is being developed by Intel, Microsoft, HP and other IT-companies. The control over the power management lies in the operating system. It has a better overview of the power requirements of the computer and the possibilities to save energy than the BIOS. In contrast to its predecessor APM, with ACPI the BIOS only has the task to communicate with the hardware.

What is APM?

APM stands for "Advanced Power Management". The term also describes a standard for energy-saving methods for PCs. APM was developed by Intel and Microsoft in the early 1990s. The power management features of this standard are mostly managed by the BIOS and hardware. However, since the appearance of ACPI, APM has played only a minor role.

How does ACPI work?

To better understand how ACPI handles power management, we need to go further and look at the different states in which a computer with ACPI can be. The so-called G-state describes four possible states: the mode "G0" designates the active state ("Working"), in which one can work, "G1" the sleep state, "G2" is the so-called "soft-off", a PC with ATX standby voltage, while "G3" describes the unplugged computer ("Mechanical off"). In the case of the "G1" state, you can now distinguish between the various sleep states (s-states):

  • S0: Working – The system is switched on, functional and fully operational.
  • S1: Sleep – Simple sleep mode in which the CPU is stopped.
  • S2: Deeper Sleep – Extended sleep mode, where other components such as the cache of the CPU are turned off.
  • S3: Standby Mode – Much of the motherboard’s hardware is turned off. This mode is also referred to as "Suspend to RAM" (STR) or "Suspend to Memory" (STM), that is, the operating state is still stored on a volatile memory (RAM).
  • S4: Hibernation – Also "Suspend to disk" (STD) – that is, the operating state is backed up to non-volatile memory (hard disk or SSD). The system can be disconnected from the power.
  • S5: Soft-Off Mode – System is switched off (shut down), only the power supply provides power. The system can be activated via a power button or optionally via Wake on LAN.

In addition to the G and S-states, a further distinction is made between ten different CPU states (short "C-states") and five possible device states (short "d-states".

What are the Advantages of ACPI?

The advantage of standby and hibernation is, that the PC does not have to reboot completely and is ready for use more quickly. During these modes, we can also keep programs and files we are working with, open.

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