Книга: Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

Processor Idle State

Processor Idle State

In addition to power-management-enabled applications and device drivers, the kernel also contributes to power management. The kernel calls the OEMIdle function, part of the OAL, when no threads are ready to run. This action switches the processor into idle state, which includes saving the current context, placing the memory into a refresh state, and stopping the clock. The processor idle state reduces power consumption to the lowest possible level while retaining the ability to return from the idle state quickly.

It is important to keep in mind that the OEMIdle function does not involve Power Manager. The kernel calls the OEMIdle function directly and it is up to the OAL to switch the hardware into an appropriate idle or sleep state. The kernel passes a DWORD value (dwReschedTime) to OEMIdle to indicate the maximum period of idle time. When this time passes or the maximum delay supported by the hardware timer is reached, the processor switches back to non-idle mode, the previous state is restored, and the scheduler is invoked. If there still is no thread ready to run, the kernel immediately calls OEMIdle again. Driver events, as in the response to user input via keyboard or stylus, may occur at any time and cause the system to stop idling before the system timer starts.

The scheduler is, by default, based on a static timer and system ticks at a one-millisecond frequency. However, the system can optimize power consumption by using dynamic timers and by setting the system timer to the next timeout identified by using the scheduler table content. The processor will then not switch back out of idle mode with every tick. Instead, the processor switches only to non-idle mode after the timeout defined by dwReschedTime expires or an interrupt occurres.

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