Supporting D3cold in a Driver
Starting with Windows 8, the D3 (off) device power state is divided into two distinct substates, D3hot and D3cold. D3 is the lowest-powered device power state, and D3cold is the lowest-powered substate of D3. Moving idle devices to the D3cold substate can reduce power consumption and extend the time that a mobile hardware platform can run on a battery charge.
In D3hot, the device is mostly turned off. However, the device is not disconnected from its main power source, and the parent bus controller can detect the presence of the device on the bus. In D3cold, the main power source is removed from the device, and the bus controller cannot detect the presence of the device. For more information, see the descriptions of D3hot and D3cold in Device Low-Power States.
In earlier versions of Windows, the D3 device power state is implicitly divided into D3hot and D3cold substates, but a device cannot enter D3cold unless the computer is preparing to exit the S0 system power state and enter one of the sleeping states, S1 through S4. The low-power Dx states that a device can enter when the computer is to remain in S0 are limited to D1 through D3hot.
Windows 8 is the first version of Windows to support device-power-state transitions to the D3cold substate when the computer is in S0 and is not preparing to enter a sleeping state. A device that supports D3cold in this way helps to save power in the following ways:
- The device consumes less power in D3cold than in any other low-power Dx state.
- If this device shares a bus with other devices, and all these devices support D3cold, then after all the devices on the bus enter D3cold, the bus controller can enter a low-power Dx state.
- If this device shares a power source with other devices, and all these devices support D3cold, then when the last of these devices enters D3hot, the power source can be removed, at which time these devices all enter D3cold in unison.
Conversely, a device that cannot idle in D3cold can prevent other devices from entering D3cold or other low-power Dx states.
For more information, see the following topics:
- Enabling Transitions to D3cold
- D3cold Capabilities of a Device
- Using the GUID_D3COLD_SUPPORT_INTERFACE Driver Interface
- Surprise Wake-Up
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Build date: 5/2/2013
