ACPI Advanced Configuration and Power Interface

• ACPI is an OS-based power management standard that replace the BIOS-based APM (Advanced Power Management).
• ACPI allows the operating system rather than the BIOS  to control power management tasks and to configure the hardware configuration platform.
• It is open-standard and platform-independent.
• ACPI was developed and released by Intel, Microsoft, and Toshiba in December of 1996.
• It supports hot-swappable devices and controls power modes.
• There are four requirements for ACPI to function:
• System Management Mode (SMM) capable CPU
• APM compliant BIOS
• Energy Star devices that can be shut off when not in use
• An operating system that will shut off devices
• ACPI is capable of handling all five of the APM power usage levels plus a few more:
• Full On
• All components are running at full power.
• The OS is not running in a power saving mode.
• APM Enabled
• The CPU and RAM are running at full power.
• Power management is turned on.
• The CPU clock can be slowed down or turned off as needed.
• When not in use, devices can be powered down.
• APM Standby
• The CPU and all peripherals are shut down.
• The RAM stores the current configuration settings.
• Recovery is quick.
• This is short term power savings.
• APM Suspend
• The system using minimum power.
• Most devices are turned off.
• The CPU clock is stopped.
• The configuration settings are saved to disk.
• Recover time is slow.
• This is long term power savings.
• Off
• The PC is powered down.
• Configuration settings are not saved.
• Four ACPI Global States:
• G0: normal, working, active state
• G1: Sleeping
• Four Submodes
• S1: Sleep (Low Power) Mode
• S1 uses the most power of the S states.
• The CPU clock is stopped.
• Ram is continually refreshed.
• There is a 2 second latency before a return to the G0 state.
• S2:
• The CPU and its caches are turned off.
• This state is not usually used.
• S3:
• S3 is also known as Sleep, Standby, Suspend to RAM, or Instantly Available PC.
• Only the RAM has power.
• If there is a power failure, all user data is lost.
• There is a 5-6 second latency.
• S4:
• This is also known as Hibernation or Suspend to Disk.
• Everything but real-time mode (RTM) is turned off.
• Main memory is saved to disk.
• There is very low power consumption.
• S5:
• This is the same as G2 Soft Off.
• A reboot is required.
• G2: Soft Off
• A reboot is required to restart the computer.
• G3: Power Off, Mechanical Off, or Hardware Off
• There is zero power consumption.
• The main power switch is turned off.
• It is when the PC is in this state that a tech can safely dissemble the computer.
• Four ACPI Processor States:
• CO: normal operational state
• C1: Halt
• This is the powered-down state.
• This state has the least latency time.
• C2: Stop-Clock
• C3: Sleep
• The PC is powered down and unresponsive.
• Four ACPI Device States:
• D0: fully on
• This state consumes the most power.
• D1: 
• intermediate power state
• varies by device
• D2:
• intermediate power state
• varies by device
• D3: off
• The PC is powered down and unresponsive.
• This devices state consumes the least power and has the longest restoration time.
• Performance States
• P0:
• maximum power consumption, frequency, and performance
• P1:
• limited power and performance
• Pn:
• N is a number between 1 and 16.