The IT industry profited greatly from its experience with APM. In 1996, five major companies joined together to design a replacement to APM that addressed its shortcomings and improved its capabilities. Hewlett-Packard, Intel, Microsoft, Phoenix, and Toshiba drafted an open standard that also encompassed mobile systems and servers.
The primary architectural difference between APM and ACPI is that where APM was BIOS based, the Advanced Configuration and Power Interface (ACPI) standard is based in the operating system. Moving control to the operating system provides space for extensive logic to support immediate and future requirements. In addition, there are fewer operating systems than there are computer manufacturers. This provides the entire industry with a more consistent interface for hardware and software designers.
An open standard is very important. It reduces the amount of redundant investment in power management throughout the industry. As an open standard, it benefits from the sharing of many different perspectives across hardware and software development industries. Manufacturers are free to focus their efforts and investments on innovation rather than on arguing over variations in interpreting some other company's technical approach.
ACPI hit the streets with Windows 98. Currently, it is found in notebook, server, desktop, and special purpose computers—and for good reason. It is simple to implement. For the technical details of ACPI, download the standard from http://www.acpi.info/spec.htm.
The ACPI architecture places control of all of the power management functions (APM, Plug and Play, legacy, device controlled, etc.) into one place. It is broken into two major components. The hardware devices provide information about their power requirements and status, and the operating system provides the software logic for managing power. Separating hard ware and software components frees the manufacturers of each to creatively focus on what they know best, with an assurance that their product will work with the other. An open source model prevents one company or another from gaining a marketplace advantage by modifying the standard to the advantage of its product and the detriment of its competitors.
When a computer boots, the hardware information table (along with its associated ACPI controls) is read from the BIOS into the ACPI work space. This information is called the Discrete System Descriptor Table (DSDT). Added to this table, is device information about attached peripherals connected via Plug and Play, PCI, IEEE 1394, and USB. These data tables are used by power management software to control individual devices. It also establishes a table of information on their power level in a central location. The ACPI tables describe the power planes and clock sources used by a specific device, as well as the controls for turning them off. An example would be the different device requirements between device state "DO" (full power) and state "D1" (reduced power).
The current ACPI specifications version is 3.0b and was last updated in August 2006. Some of the design requirements include:
▲ Every component in the system or attached to it must be capable of power management. The standardization of interfaces enables support of a wide range of equipment, both known and which may be created in the future.
▲ Power management is handled by the operating system for flexibility and consistency across computer manufacturers.
▲ The operating system interfaces with the ACPI data tables that describe the power characteristics of each device, along with the ACPI Machine Language (AML) commands to control it.
▲ Wake On LAN is required for remote management of computers.
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