Good to know: What is ACPI?

ACPI is the abbreviation for "Advanced Configuration and Power Interface" and describes an open industry standard for the power management of PCs, laptops and servers. In order to be able to use ACPI, both ACPI-compatible hardware (mainboard, power supply unit, etc.) and an ACPI-compatible operating system (e.g. Windows) are required. ACPI was published in 1996 and is being developed by Intel, Microsoft, HP and other IT companies. In this case, the operating system has control over power management. This has a better overview of the power requirements of the computer and the possibilities for saving energy than the BIOS. In contrast to its predecessor APM, with ACPI the BIOS only has the task of communicating 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. With this standard, the energy-saving functions are mainly managed by the BIOS and the hardware. Since the introduction of ACPI, however, APM has only played a subordinate role.

How does ACPI work?

To better understand how power management works under ACPI, we need to go a little further and take a closer look at the different states a computer can be in with ACPI. The so-called G-State describes four possible states: The mode "G0" designates the active state ("Working") in which work can be carried out, "G1" the sleep state, "G2" is the so-called "Soft-Off" - a PC with ATX standby voltage, while "G3" describes the computer with the plug pulled out ("Mechanical off"). In the G1 state, a distinction is made between the various resting states or sleep states (S-states for short):

• S0: Working - System is switched on, functional and completely ready for use.
• S1: Sleep - Simple sleep mode in which the CPU is stopped.
• S2: Deeper Sleep - Extended sleep mode in which other components such as the CPU cache are switched off.
• S3: Standby mode - Most of the hardware on the motherboard is switched off. The mode is also referred to as "Suspend to RAM" (STR) or "Suspend to memory" (STM), i.e. the operating status is still stored in a volatile memory(RAM).
• S4: Hibernation - also known as "Suspend to disk" (STD) - This means that the operating status is saved on a non-volatile memory(hard disk or SSD). System could be disconnected from the power supply.
• S5: Soft-off mode - system is switched off (shut down), only the power supply unit supplies voltage. The system can be activated via a power button or, if necessary, via Wake on LAN.

In addition to the G and S states already mentioned, a distinction is also made between ten different processor states - CPU states, or "C states" for short - and five possible device states - device states, or "D states" for short.

What are the advantages of ACPI?

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

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