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Power Savings: Advanced Configuration And Power Interface

Power Supply Reference: Consumption, Savings, And More
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As power-management techniques continued to develop, maintaining the complex information states necessary to implement more advanced functions became increasingly difficult for the BIOS. Therefore, another standard was developed by Intel, Microsoft, and Toshiba. Called Advanced Configuration and Power Interface (ACPI), this standard was designed to implement power-management functions in the OS. Microsoft Windows 98 and later automatically use ACPI if ACPI functions are found in the system BIOS. The need to update system BIOSs for ACPI support is one reason many computer vendors have recommended performing a BIOS update before installing Windows 98 or later on older systems.

ACPI was initially released in 1996 and first appeared in the Phoenix BIOS around that time. ACPI became a requirement for the Intel/Microsoft “PC’97” logo certification in 1996, which caused developers to work on integrating ACPI into system designs around that time. Intel included ACPI support in chipsets starting with the PIIX4E South Bridge in April 1998, and ACPI support was included in Windows starting with the release of Windows 98 (June 25, 1998) as part of what Microsoft called its “OnNow” initiative. By the time Windows 2000 came out (February 17, 2000), ACPI had universally replaced APM on new systems. ACPI 4.0a was released in April 2010, and the ACPI 5.0 Specification is currently under development. The official ACPI specifications can be downloaded from www.acpi.info.

Placing power management under the control of the OS enables a greater interaction with applications. For example, a program can indicate to the OS which of its activities are crucial, forcing an immediate activation of the hard drive, and which can be delayed until the next time the drive is activated for some other reason. For example, a word processor may be set to automatically save files in the background, which an OS using ACPI can then delay until the drive is activated for some other reason, resulting in fewer random spin-ups of the drive.

ACPI goes far beyond the previous standard, APM, which consisted mainly of processor, hard disk, and display control. ACPI controls not only power but also all the Plug and Play (PnP) hardware configuration throughout the system. With ACPI, system configuration (PnP) and power-management configuration are no longer controlled via the BIOS Setup; they are instead controlled entirely within the OS.

ACPI enables the system to automatically turn internal peripherals on and off (such as CD-ROM drives, network cards, hard disk drives, and modems) as well as external devices such as printers, monitors, or any devices connected to serial, parallel, USB, video, or other ports in the system. ACPI technology also enables peripherals to turn on or wake up the system. For example, a telephone answering machine application can request that it be able to respond to answer the telephone within 1 second. Not only is this possible, but if the user subsequently presses the power or sleep button, the system only goes into the deepest sleep state that is consistent with the ability to meet the telephone answering application’s request.

ACPI enables system designers to implement a range of power-management features that are compatible with various hardware designs while using the same OS driver. ACPI also uses the Plug and Play BIOS data structures and takes control over the Plug and Play interface, providing an OS–independent interface for configuration and control.

ACPI defines several system states and substates. There are four Global System states, labeled from G0 through G3, with G0 being the fully operational state and G3 being mechanically turned off. Global System states are immediately obvious to the user of the system and apply to the entire system as a whole. Within the G0 state, there are four CPU Power states (C0–C3) and four Device Power states (D0–D3) for each device. Within the C0 CPU Power state, there are up to 16 CPU Performance states (P0–P15).

Device Power states are states for individual devices when the system is in the G0 (Working) state. The device states may or may not be visible to the user. For example, it may be obvious when a hard disk has stopped or when the monitor is off; however, it may not be obvious that a modem or other device has been shut down. The Device Power states are somewhat generic; many devices do not have all four Power states defined.

Within the G1 Global Sleep state are four Sleep states (S1–S4). The G2 Global Soft Off state is also known as the S5 Sleep state, in which case the system is powered off but still has standby power. Finally, G3 is the Mechanical Off state, where all power is disconnected from the system.

The following list shows the definitions and nested relationship of the various Global, CPU/Device Power, and Sleep states:

G0 Working—This is the normal working state in which the system is running and fully operational. Within this state, the Processor and Device Power states apply. The Device Power states are defined as follows:

  • G0/D0 Fully-On—The device is fully active.
  • G0/D1—Depends on the device; uses less power than D0.
  • G0/D2—Depends on the device; uses less power than D1.
  • G0/D3 Off—The device is powered off (except for wakeup logic).


The Processor Power states are defined as follows:

  • G0/C0 CPU On—Normal processor operation.
  • G0/C1 CPU Halted—The processor is halted.
  • G0/C2 CPU Stopped—The clock has been stopped.
  • G0/C3 CPU/Cache Stopped—The clock has been stopped and cache snoops are ignored.


G1 Sleeping
—The system appears to be off but is actually in one of four Sleep states—up to full hibernation. How quickly the system can return to G0 depends on which of the Sleep states the system has selected. In any of these Sleep states, system context and status are saved such that they can be fully restored. The Sleep states available in the Global G1 state are defined as follows:

  • G1/S1 Halt—A low-latency idle state. The CPU is halted; however, system context and status are fully retained.
  • G1/S2 Halt-Reset—Similar to the S1 sleeping state except that the CPU and cache context is lost, and the CPU is reset upon wakeup.
  • G1/S3 Suspend to RAM—All system context is lost except memory. The hardware maintains memory context. The CPU is reset and restores some CPU and L2 context upon wakeup.
  • G1/S4 Suspend to Disk (Hibernation)—The system context and status (RAM contents) have been saved to nonvolatile storage—usually the hard disk. This is also known as Hibernation. To return to G0 (Working) state, you must press the power button, and the system will restart, loading the saved context and status from where they were previously saved (normally the hard disk). Returning from G2/S5 to G0 requires a considerable amount of latency (time).


G2/S5 Soft Off
—This is the normal power-off state that occurs after you select Shutdown or press the power button to turn the system off. The system and all devices are essentially powered off; however, the system is still plugged in and standby power is coming from the power supply to the motherboard, allowing the system to wake up (power on) if commanded by an external device. No hardware context or status is saved. The system must be fully rebooted to return to the G0 (working) state.

G3 Mechanical Off—Power is completely removed from the system. In most cases this means the system must be unplugged or the power turned off via a power strip. This is the only state in which it is safe to disassemble the system. Except for the CMOS/clock circuitry, power consumption is completely zero.

In normal use, a system alternates between the G0 (Working) and G1 (Sleeping) states. In the G1 (Working) state, individual devices and processors can be power-managed via the Device Power (D1–D3) and Processor Power (C1–C3) states. Any device that is selectively turned off can be quickly powered on in a short amount of time, from virtually instantaneous to only a few seconds (such as a hard disk spinning up).

When the system is idle (no keyboard or mouse input) for a preset period, the system enters the Global G1 (Sleeping) state, which means also selecting one of the S1–S4 sleep states. In these states, the system appears to be off, but all system context and status are saved, enabling the system to return to exactly where it left off, with varying amounts of latency. For example, returning to the G0 (Working) state from the G1/S4 (Hibernation) state requires more time than when returning from the G1/S3 (Suspend) state.

When the user presses the power button to turn the system off or selects Shutdown via the OS, the system enters the G2/S5 (Soft Off) state. In this state, no context is saved, and the system is completely off except for standby power. Fully disconnecting AC or battery power causes the system to be in the Global G3 (Mechanical Off) state, which is the only state in which the system should be disassembled.

During the system setup and boot process, ACPI performs a series of checks and tests to see whether the system hardware and BIOS support ACPI. If support is not detected or is found to be faulty, the system typically reverts to standard Advanced Power Management control, which is referred to as legacy power management under ACPI. Virtually all ACPI problems are the result of partial or incomplete ACPI implementations or incompatibilities in either the BIOS or device drivers. If you encounter any of these errors, contact your motherboard manufacturer for an updated BIOS or the device manufacturers for updated drivers.

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