windows power manager

     Just as Windows Plug and Play features require support from a system’s hardware, its powermanagement capabilities require hardware that complies with the Advanced Configuration and Power Interface (ACPI) specification (available at www.teleport.com/~acpi/spec.htm).

The ACPI standard defines various power levels for a system and for devices. The six system power states are described in Table 7-8. They are referred to as S0 (fully on or working) through S5 (fully off ). Each state has the following characteristics:

■ Power consumption The amount of power the computer consumes

■ Software resumption The software state from which the computer resumes when moving to a “more on” state

■ Hardware latency The length of time it takes to return the computer to the fully on state

 

States S1 through S4 are sleeping states, in which the computer appears to be off because of reduced power consumption. However, the computer retains enough information, either in memory or on disk, to move to S0. For states S1 through S3, enough power is required to preserve the contents of the computer’s memory so that when the transition is made to S0 (when the user or a device wakes up the computer), the power manager continues executing where it left off before the suspend.

When the system moves to S4, the power manager saves the compressed contents of memory to a hibernation file named Hiberfil.sys, which is large enough to hold the uncompressed contents of memory, in the root directory of the system volume. (Compression is used to minimize disk I/O and to improve hibernation and resume-from-hibernation performance.) After it finishes saving memory, the power manager shuts off the computer. When a user subsequently turns on the computer, a normal boot process occurs, except that Bootmgr checks for and detects a valid memory image stored in the hibernation file. If the hibernation file contains saved system state, Bootmgr launches Winresume, which reads the contents of the file into memory, and then resumes execution at the point in memory that is recorded in the hibernation file.

On systems with hybrid sleep enabled (by default, only desktop computers), a user request to put the computer to sleep will actually be a combination of both the S3 state and the S4 state: while the computer is put to sleep, an emergency hibernation file will also be written to disk. Unlike typical hibernation files, which contain almost all active memory, the emergency hibernation file includes only data that could not be paged in at a later time, making the  suspend operation faster than a typical hibernation (because less data is written to disk). Drivers will then be notified that an S4 transition is occurring, allowing them to configure themselves and save state just as if an actual hibernation request had been initiated. After this point, the system is put in the normal sleep state just like during a standard sleep transition. However, if the power goes out, the system is now essentially in an S4 state—the user can power on the machine, and Windows will resume from the emergency hibernation file.

The computer never directly transitions between states S1 and S4; instead, it must move to state S0 first. As illustrated in Figure 7-43, when the system is moving from any of states S1 through S5 to state S0, it’s said to be waking, and when it’s transitioning from state S0 to any of states S1 through S5, it’s said to be sleeping.

 

Although the system can be in one of six power states, ACPI defines devices as being in one of four power states, D0 through D3. State D0 is fully on, and state D3 is fully off. The ACPI standard leaves it to individual drivers and devices to define the meanings of states D1 and D2, except that state D1 must consume an amount of power less than or equal to that consumed in state D0, and when the device is in state D2, it must consume power less than or equal to that consumed in D1. Microsoft, in conjunction with the major hardware OEMs, has defined a series of power management reference specifications that specify the device power states that are required for all devices in a particular class (for the major device classes: display, network, SCSI, and so on). For some devices, there’s no intermediate power state between fully on and fully off, which results in these states being undefined.