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What is ACPI Power States?

You select an option or press a button, and your computer turns off. Ever wondered what goes on behind the scenes? The thing with electronics is- once you turn off the power, they lose any data stored in them. To solve this problem, power states are devised for most electronics with volatile memory. These power states make sure things don’t go bonkers when the power is turned off.

As you would have guessed, these power states exist on your computer too. They come into play when you press the power button and in some cases, even before that.

Also read: How to set a custom boot image in Windows?

What is the ACPI Power States?

ACPI stands for Advanced Configuration Power Interface. These are the states that your computer cycles between as and when your power it on, shut it down, put it to sleep or hibernation modes.

What are ACPI Power States? Everything you need to know
There’s a lot that goes on behind that power button.

It is an open industry standard designed for power management services developed jointly by Hewlett-Packard, Intel, Microsoft, Phoenix, and Toshiba. Although initially targeted at the PC, it now works across multiple operating systems.

ACPI provides developers with basic power management states while abstracting the hardware layer at the same time. It is essentially a specification of different power states of a computer system. It spans individual components or even your entire system if need be.

What are ACPI Power States? Everything you need to know
Linux Kernel Improvement: Toward Dynamic Power Management of Beowulf Clusters – Scientific Figure on ResearchGate | Source

There are six main power states under ACPI – S0 to S5. These states are defined under three global power states as follows:

  • G0/S1: The computer is active/running,
  • G1: Computer is in a sleep state
    • G1/S1: Power is suspended, but the CPU and CPU Caches are preserved and hence, powered. This is essentially how fast startup works.
    • G1/S2: CPU is entirely off. CPU Caches are lost.
    • G1/S3: Standby/Sleep. The system state is saved to the RAM which retains power.
    • G1/S4: Power is suspended, but CPU state is saved to a non-volatile storage device (HDD). RAM and all other major components are turned off. This is how hibernation works.
  • G2/S5: Soft off. Consider this the same when you switch off your PC but still leave the power switch on. No state is saved.
  • G3: Mechanical off. When you turn off all power to your system and turn off physical power as well, no state is saved, and hence a reboot is needed to get to G0. Only components like the Real-Time clock (RTC) with their batteries are powered.

How can they be helpful?

Well unless you’re a geek to some degree, you’re not even going to hear about them. However, if you’re a developer working with system-level applications, or even user level ones, knowing which state the PC is in can be crucial to providing the user experience you’re trying to.

Also, with people running dual boot systems off of the same drive, knowing which power state the primary system puts the PC can help troubleshoot any booting issues.

Also read: What is Secure boot and how to disable it?

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