What are ACPI devices

ACPI - Advanced Configuration and Power Interface

ACPI is an open industry standard for power management. In addition, it provides interfaces for hardware detection, device configuration and energy management.
ACPI was introduced by Intel and Microsoft in the late 1990s. Today, a large number of PC manufacturers, operating systems, drivers, BIOS, firmware and software developers are involved in the ACPI specification and implementation. This specification is correspondingly extensive. Despite this, or precisely because of this, errors can be found in many implementations. That is why there are always problems with the various operating states of the PC hardware.
For full ACPI support, both the motherboard with the chipset, timer and BIOS as well as the operating system and, in some cases, the processor must be ACPI-capable. Because all components come from different manufacturers, incompatibilities cannot be ruled out.

Energy management

ACPI is mainly known for energy management, which has replaced Advanced Power Management (APM). With ACPI, unlike the older APM standard, control over energy management lies entirely with the operating system.
ACPI enables the operating system to switch off the computer in order to be able to switch it on again at a later point in time so that it can continue working in exactly the same state as it was when it was switched off. But as great as it sounds, it still does not work properly to this day.

Operating states

ACPI differentiates between the operating states of the entire system (S-States), the processor (C- and P-States) and other components (D-States). The identification consists of an initial letter followed by a number. While the letter indicates the area of ​​responsibility for the operating status, the number refers to the savings effect. The lower the sleep state of the CPU cores and the other components of the processor, the lower the power consumption. The larger the number, the more economical the operating state and the longer it takes to restore the normal state.

ACPI S0

The operating state S0 stands for the normal state of a running computer. It is possible to switch the processor to the C1 state. It is also possible to put other components, such as hard disks, to sleep while the system is running (D3 / D4).

ACPI S1 - S2 - S3

At S1 and S3 the system goes to sleep and then wakes up when, for example, the keyboard is pressed. There is practically no difference in the power consumption between S0 and S1.
The S2 operating state is imprecisely defined and therefore never occurs.

ACPI S3 - Suspend-to-RAM

If you select "Standby" under Windows XP or "Save Energy" in Windows Vista, the system goes into S3 mode. If the necessary hardware, BIOS and driver support is missing, the system switches to S1 mode. If the buttons for "Standby" and "Save energy" are missing, then the necessary drivers are missing or have been installed incorrectly. ACPI-S1 / S3 could also be switched off in the BIOS / UEFI.
In this state, the power supply unit only supplies the motherboard from the 5V standby line. As long as a computer has not been de-energized, it wakes up within seconds when it is started.

ACPI S4 - Suspend-to-Disk

With the ACPI S4, the main memory is saved in a file on the hard disk. The computer may then be disconnected from the power supply. The system start then takes a little longer than with ACPI S3. Because the data must first be written back from the hard disk to the main memory. If the main memory is very large and the hard disk is slow, waking up from the S4 operating state can take longer than if the system was completely restarted.

Not every device is able to wake up any PC from S3 and S4. That depends on the drivers, the power management settings, the BIOS and its settings, and the structure of the motherboard. That means it is different in every PC.

ACPI S5

The S5 operating mode is referred to as "shutdown" under Windows XP and Windows Vista.

Windows: save energy

In Windows 7 and 8 there is a standby mode that combines ACPI S3 and S4 and is called "energy saving". The hibernation, also known as hibernation, suspend-to-disk or ACPI S4, cannot be selected directly. The system automatically switches to "Save energy" after a set time if no user entries are made.
With "Save energy" Windows first switches to ACPI S3. Before this, important data is written from the RAM to the "hiberfil.sys" file on the system partition. After a certain time, the system then changes to ACPI S4. When "waking up" Windows copies the content of "hiberfil.sys" back into RAM and then starts from ACPI S3.
The problem with ACPI S3 (Suspend-to-Disk) is that you don't necessarily look at the computer to see whether it is in this mode or whether it is switched off. By combining ACPI S3 and S4, Windows ensures that there is no loss of data if the power supply fails (accidentally pulling a plug or an empty notebook battery).

Windows: shut down

Windows 8 switches the system to ACPI S4 when it "shuts down", with the RAM content being written to the hard disk. During the next boot process, the operating system loads its memory image from RAM. The system saves loading the kernel and drivers, which is why it starts up again more quickly.

ACPI C0 ... C7

The C-states identify the operating or sleep states of the processor or a processor core. C-States belong to the central energy-saving mechanisms of modern processors and form the basis for the Turbo mode (automatic overclocking) from Intel and AMD. Not all processors can handle all operating states. But the same stages do not work the same for every processor.
In general, one can say that higher numbers after the C indicate a deeper sleep phase. But this also means that the deeper the sleep state, the longer it takes for a core to wake up again.
The processor not only cuts down the core voltage and clock frequency, but also disconnects caches and entire cores from the power supply. This saves a lot of electricity and is particularly interesting for battery-operated devices.

ACPI operating states for power management of a PC

ACPIdescriptionPower consumption
S0Normal state, individual components can be in standby mode, power consumption depending on the load 40 to 200 watts
S1Screen is off. Hardly more economical than S0. But CPU: C1 to C3 40 to 100 watts
S2Like S1. Almost never used in practice.40 to 100 watts
S3System sleeps, data is in the main memory, wake-up time within a few seconds. Operating status is as Suspend-to-RAM designated.2 to 15 watts
S4System sleeps, data is saved on the hard disk, wake-up time within a minute. Operating status is as Suspend-to-Disk designated.1 to 10 watts
S5Switched off state. PC can be started with a button.0 to 10 watts
G3Power supply unit is disconnected from the mains.0 watts
C0Processor works normally. Different P-States possible.10 to 130 watts
C1Processor is waiting for work and is in a light sleep state (halt command). Change between C0 and C1 several times per second possible.10 to 30 watts
C2 / C3Processor in deeper sleep states. At C3, clocks are disconnected and caches are emptied (sleep mode). This reduces power consumption.7 to 15 watts
C4 ... C6Processor in even deeper sleep states. Mainly used in mobile processors to save power5 to 10 watts
C7The core voltages of the processor are completely switched off.<0.5 watt

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The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memories, interfaces, data storage devices, drives and important hardware components.

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