Windows® Internals, Sixth Edition, Part 2

Many power commands have corresponding query commands. For example, when the system is moving to a sleep state, the power manager will first ask the devices on the system whether the transition is acceptable. A device that is busy performing time-critical operations or interacting with device hardware might reject the command, which results in the system maintaining its current system power-state setting.

EXPERIMENT: Viewing the System Power Capabilities and Policy

You can view a computer’s system power capabilities by using the !pocaps kernel debugger command. Here’s the output of the command when run on an ACPI-compliant laptop:lkd> !pocaps PopCapabilities @ 0x82114d80 Misc Supported Features: PwrButton SlpButton Lid S3 S4 S5 HiberFile FullWake VideoDim Processor Features: Thermal Disk Features: SpinDown Battery Features: BatteriesPresent Battery 0 - Capacity: 0 Granularity: 0 Battery 1 - Capacity: 0 Granularity: 0 Battery 2 - Capacity: 0 Granularity: 0 Wake Caps Ac OnLine Wake: Sx Soft Lid Wake: Sx RTC Wake: S4 Min Device Wake: Sx Default Wake: Sx

The Misc Supported Features line reports that, in addition to S0 (fully on), the system supports system power states S1, S3, S4, and S5 (it doesn’t implement S2) and has a valid hibernation file to which it can save system memory when it hibernates (state S4).

The Power Options page, shown here (available by selecting Power Options in Control Panel), lets you configure various aspects of the system’s power policy. The exact properties you can configure depend on the system’s power capabilities, which we just examined.

By changing any of the preconfigured plan settings, you can set the idle detection timeouts that control when the system turns off the monitor, spins down hard disks, goes to standby mode (moves to system power state S1), and hibernates (moves the system to power state S4). In addition, selecting the Change Plan Settings option lets you specify the power-related behavior of the system when you press the power or sleep buttons or close a laptop’s lid.

The settings you configure by clicking the Change Advanced Power Settings link directly affect values in the system’s power policy, which you can display with the !popolicy debugger command. Here’s the output of the command on the same system:lkd> !popolicy SYSTEM_POWER_POLICY (R.1) @ 0x82107994 PowerButton: Sleep Flags: 00000000 Event: 00000000 SleepButton: Sleep Flags: 00000000 Event: 00000000 LidClose: Sleep Flags: 00000000 Event: 00000000 Idle: Sleep Flags: 00000000 Event: 00000000 OverThrottled: None Flags: 00000000 Event: 00000000 IdleTimeout: 384 IdleSensitivity: 90% MinSleep: S3 MaxSleep: S3 LidOpenWake: S0 FastSleep: S0 WinLogonFlags: 1 S4Timeout: fd20 VideoTimeout: 300 VideoDim: 0 SpinTimeout: 258 OptForPower: 0 FanTolerance: 0% ForcedThrottle: 0% SpinTimeout: 258 OptForPower: 0 MinThrottle: 0% DyanmicThrottle: None

The first lines of the display correspond to the button behaviors specified on the Advanced Settings tab of Power Options, and on this system both the power and the sleep buttons put the computer in a sleep state, just as closing the lid does.

The timeout values shown at the end of the output are expressed in seconds and displayed in hexadecimal notation. The values reported here directly correspond to the settings you can see configured on the Power Options page. (The laptop is on battery.) For example, the video timeout is 300, meaning the monitor turns off after 300 seconds, or 5 minutes, and the hard disk spin-down timeout is 0x258, which corresponds to 600 seconds, or 10 minutes.

Driver and Application Control of Device Power

Besides responding to power manager commands related to system power-state transitions, a driver can unilaterally control the device power state of its devices. In some cases, a driver might want to reduce the power consumption of a device it controls when the device is left inactive for a period of time. Examples include monitors that support a dimmed mode and disks that support spin-down. A driver can either detect an idle device itself or use facilities provided by the power manager. If the device uses the power manager, it registers the device with the power manager by calling the PoRegisterDeviceForIdleDetection function.