What is ACPI and what does ACPI mean?

acpi

ACPI - Advanced Configuration Power Interface
ACPI refers to a device control and power management specification developed by Dell, HP, Intel, Microsoft, Phoenix, Toshiba, and Dell.
The ACPI specification allows an operating system to communicate with, and to control the power distribution to peripherals such as hard drives. It was originally intended for laptops/notebooks where power saving is vital.

Since Windows 98, ACPI has replaced the older APM (Advanced Power Management) specification (used in Windows 95). Microsoft stopped supporting the APM specification in their operating systems from Windows Vista onwards.

ACPI allows the operating system to communicate with the computer's BIOS and instruct the BIOS to power down peripherals. For example, when your computer goes into hibernation mode, the operating system is using the ACPI specification to control the power to the internal components.

ACPI power states
ACPI specifies various modes, referred to as states, that control the power to the components of a PC.
  • The 4 Global states:
  • Working (G0)
  • Sleeping (G1)
  • Soft Off (G2)
  • Mechanical Off (G3).
Within these Global states are various different Sleep states: S0, S1, S2, S3, S4 and S5.

For example, when you have not moved your mouse or touched a key for a certain period of time the computer's monitor will power-down, this is actually Global state G0 with Sleep state S0.

Another example is when your computer is put into hybernate mode, this is actually Global state G1 with Sleep state S4.

The Global state G2 (Soft Off) is when the system is powered-down but can be brought back to life by the case's soft power button (usually located at the front of the PC) or other method such as "Wake-on-LAN". When a machine is in this state the case covers should never be removed as there is still residual power to the motherboard.

The Global state G3 (Mechanical Off), as its name suggests, is when the system is completely powered-down by using the main power button (usually a rocker switch on the back) of the computer's case. When in this mode, the PC can be disconnected from the power source and any maintenance, repairs or upgrades carried out.
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