1999-01-17 17:17:09 +00:00
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A collection of articles on kernel internals, please add to this
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------------------------------------------------------------------------------
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IRQ level
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------------------------------------------------------------------------------
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IRQ level (IRQL) is a per-processor state in ReactOS used to coordinate
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execution between ISRs and between threads. There are several levels
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PASSIVE_LEVEL, APC_LEVEL: The normal level for user mode and most
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kernel mode code. At the moment APC_LEVEL is unused.
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DISPATCH_LEVEL: At this level all irqs are still allowed but thread
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rescheduling on the current processor is disabled. This is used by
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the spinlock synchronization primitive to implement its uniprocessor
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semantics (multiprocessor is more complex). It is also used for some
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other forms of synchronization, DPCs for example. Many APIs are
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unavailable at this IRQL, usually those that might have to wait. It
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is recommended that you don't spend too much time at this IRQL
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otherwise system responsiveness will be reduced.
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> DISPATCH_LEVEL: Each irq is assigned a priority (which will be
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greater than DISPATCH_LEVEL). At an irq's priority level that irq,
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lower priority irqs and thread rescheduling are disabled. Higher
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priority irqs can still run. Very few APIs are available at IRQLs
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greater than DISPATCH_LEVEL.
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HIGH_LEVEL: All irqs are disabled.
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-------------------------------------------------------------------------------
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DPCs
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-------------------------------------------------------------------------------
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It is a design goal not to spend too much time in ISRs, for this reason
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ISRs should postpone most processing till it can run at a lower IRQL. The
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2005-07-17 12:24:15 +00:00
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mechanism for this is a Deferred Procedure Call (DPC). When a DPC object is
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1999-01-17 17:17:09 +00:00
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created, it is associated with a function. The DPC object can then be inserted
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in the DPC queue from an ISR. If the IRQL on return from the ISR is less than
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DISPATCH_LEVEL the DPC queue will be drained, otherwise this will happen when
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the IRQL level drops below DISPATCH_LEVEL or the processor becomes idle. When
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the DPC queue is drained each DPC object is removed and the associated
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function is called at DISPATCH_LEVEL. A DPC object can only be inserted once,
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further insertions before it is removed will have no effect.
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