Power saving functions are not only significant for the mobile use of laptops, but also for desktop systems. The main functions and their use in ACPI are:
This operating mode turns off the display. On some computers, the processor performance is throttled. This function corresponds to the ACPI state S1 or S2.
This mode writes the entire system state to the RAM. Subsequently, the entire system except the RAM is put to sleep. In this state, the computer consumes very little power. The advantage of this state is the possibility of resuming work at the same point within a few seconds without having to boot and restart applications. This function corresponds to the ACPI state S3. The support of this state is still under development and therefore largely depends on the hardware.
In this operating mode, the entire system state is written to the hard disk and the system is powered off. There must be a swap partition at least as big as the RAM to write all the active data. Reactivation from this state takes about 30 to 90 seconds. The state prior to the suspend is restored. Some manufacturers offer useful hybrid variants of this mode, such as RediSafe in IBM Thinkpads. The corresponding ACPI state is S4. In Linux, suspend to disk is performed by kernel routines that are independent from ACPI.
ACPI checks the battery charge status and provides information about it. Additionally, it coordinates actions to perform when a critical charge status is reached.
Following a shutdown, the computer is powered off. This is especially important when an automatic shutdown is performed shortly before the battery is empty.
Switching off the hard disk is the greatest single aspect of the power saving potential of the overall system. Depending on the reliability of the overall system, the hard disk can be put to sleep for some time. However, the risk of losing data increases with the duration of the sleep periods. Other components, like PCI devices that can be put into a special power saving mode, can be deactivated with ACPI (at least theoretically) or permanently disabled in the BIOS setup.
In connection with the CPU, energy can be saved in three different ways: frequency and voltage scaling (also known as PowerNow! or Speedstep), throttling, and putting the processor to sleep (C states). Depending on the operating mode of the computer, these methods can also be combined.