System and Device States
Windows defines six system power states and four device power states. Please be very clear about whether you are referring to system or device power states.
A POWER_STATE variable represents either a system or device power state. The POWER_STATE typedef is a union of the two enum typedefs, SYSTEM_POWER_STATE and DEVICE_POWER_STATE.
typedef union _POWER_STATE {
SYSTEM_POWER_STATE SystemState;
DEVICE_POWER_STATE DeviceState;
} POWER_STATE, *PPOWER__STATE;
System Power States
Table 10.1 shows the system power states along with the SYSTEM_POWER_STATE enum names and values. Sleeping state S1 has the lowest latency so the system can return to the fully on state in the quickest time possible. States S2 and S3 gradually increase power up latency and decrease power consumption.
Table 10.1 System power states
| ACPI State | Description | Enum name |
|---|---|---|
| S0 | Working/Fully on | PowerSystemWorking(1) |
| S1 | Sleeping | PowerSystemSleeping1(2) |
| S2 | Sleeping | PowerSystemSleeping2(3) |
| S3 | Sleeping | PowerSystemSleeping3(4) |
| S4 | Hibernating: Off, except for trickle current to the power button and similar devices. | PowerSystemHibernate(5) |
| S5 | Shutdown/Off | PowerSystemShutdown(6) |
Windows moves to and from only S0. There are never changes between the states S1 to S5 (e.g., from S4 to S2). Windows always assumes that it can power up to S0, so it does not have to ask drivers for permission (using the IRP_MN_QUERY_POWER request).
The IRP stack Parameters.Power.ShutdownType value gives extra information about system state changes. It is particularly useful for transitions to S5. PowerActionShutdownReset indicates a reboot while PowerActionShutdownOff means the computer is being switched off.
Device Power States
Table 10.2 shows the available device power states along with the DEVICE_POWER_STATE enum names and values. Again, state D1 has lower latency than D2.
Table 10.2 Device power states
| State | Description | Enum name |
|---|---|---|
| D0 | Fully working | PowerDeviceD0(1) |
| D1 | Sleeping | PowerDeviceD1(2) |
| D2 | Sleeping | PowerDeviceD2(3) |
| D3 | Off | PowerDeviceD3(4) |
Both the state values increase from 1 the sleepier they are.
A device need not support all the device states. DO for on and D3 for off are a basic minimum. A device is not limited in its state transitions, so a change from D1 to D3 is possible.
Later, I describe how each device (or its bus driver) provides a table for device states for each system state.