Windows® Internals, Sixth Edition, Part 2

Just as Windows Plug and Play features require support from a system’s hardware, its power-management capabilities require hardware that complies with the Advanced Configuration and Power Interface (ACPI) specification (available at http://www.acpi.info).

The ACPI standard defines various power levels for a system and for devices. The six system power states are described in Table 8-8. 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.

Table 8-8. System Power-State Definitions

State

Power Consumption

Software Resumption

Hardware Latency

S0 (fully on)

Maximum

Not applicable

None

S1 (sleeping)

Less than S0, more than S2

System resumes where it left off (returns to S0)

Less than 2 seconds

S2 (sleeping)

Less than S1, more than S3

System resumes where it left off (returns to S0)

2 or more seconds

S3 (sleeping)

Less than S2; processor is off

System resumes where it left off (returns to S0)

Same as S2

S4 (hibernating)

Trickle current to power button and wake circuitry

System restarts from saved hibernatation file and resumes where it left off prior to hibernation (returns to S0)

Long and undefined

S5 (fully off)

Trickle current to power button

System boot

Long and undefined

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 8-45, 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.

Figure 8-45. System power-state transitions