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Reactos
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1234567891011121314151617181920212223242526272829303132333435363738394041424344A collection of articles on kernel internals, please add to this------------------------------------------------------------------------------IRQ level------------------------------------------------------------------------------IRQ level (IRQL) is a per-processor state in ReactOS used to coordinateexecution between ISRs and between threads. There are several levels PASSIVE_LEVEL, APC_LEVEL: The normal level for user mode and most kernel mode code. At the moment APC_LEVEL is unused. DISPATCH_LEVEL: At this level all irqs are still allowed but thread rescheduling on the current processor is disabled. This is used by the spinlock synchronization primitive to implement its uniprocessor semantics (multiprocessor is more complex). It is also used for some other forms of synchronization, DPCs for example. Many APIs are unavailable at this IRQL, usually those that might have to wait. It is recommended that you don't spend too much time at this IRQL otherwise system responsiveness will be reduced. > DISPATCH_LEVEL: Each irq is assigned a priority (which will be greater than DISPATCH_LEVEL). At an irq's priority level that irq, lower priority irqs and thread rescheduling are disabled. Higher priority irqs can still run. Very few APIs are available at IRQLs greater than DISPATCH_LEVEL. HIGH_LEVEL: All irqs are disabled.-------------------------------------------------------------------------------DPCs-------------------------------------------------------------------------------It is a design goal not to spend too much time in ISRs, for this reasonISRs should postpone most processing till it can run at a lower IRQL. Themechanism for this is a Deferred Procedure Call (DPC). When a DPC object iscreated, it is associated with a function. The DPC object can then be insertedin the DPC queue from an ISR. If the IRQL on return from the ISR is less thanDISPATCH_LEVEL the DPC queue will be drained, otherwise this will happen whenthe IRQL level drops below DISPATCH_LEVEL or the processor becomes idle. Whenthe DPC queue is drained each DPC object is removed and the associatedfunction is called at DISPATCH_LEVEL. A DPC object can only be inserted once,further insertions before it is removed will have no effect.