reactos/media/doc/INTERNALS

   1 A collection of articles on kernel internals, please add to this
   2
   3 ------------------------------------------------------------------------------
   4 IRQ level
   5 ------------------------------------------------------------------------------
   6
   7 IRQ level (IRQL) is a per-processor state in ReactOS used to coordinate
   8 execution between ISRs and between threads. There are several levels
   9
  10        PASSIVE_LEVEL, APC_LEVEL: The normal level for user mode and most
  11        kernel mode code. At the moment APC_LEVEL is unused.
  12
  13        DISPATCH_LEVEL: At this level all irqs are still allowed but thread
  14        rescheduling on the current processor is disabled. This is used by
  15        the spinlock synchronization primitive to implement its uniprocessor
  16        semantics (multiprocessor is more complex). It is also used for some
  17        other forms of synchronization, DPCs for example. Many APIs are
  18        unavailable at this IRQL, usually those that might have to wait. It
  19        is recommended that you don't spend too much time at this IRQL
  20        otherwise system responsiveness will be reduced.
  21
  22        > DISPATCH_LEVEL: Each irq is assigned a priority (which will be
  23        greater than DISPATCH_LEVEL). At an irq's priority level that irq,
  24        lower priority irqs and thread rescheduling are disabled. Higher
  25        priority irqs can still run. Very few APIs are available at IRQLs
  26        greater than DISPATCH_LEVEL.
  27
  28        HIGH_LEVEL: All irqs are disabled.
  29
  30 -------------------------------------------------------------------------------
  31 DPCs
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  33
  34 It is a design goal not to spend too much time in ISRs, for this reason
  35 ISRs should postpone most processing till it can run at a lower IRQL. The
  36 mechanism for this is a Deferred Procedure Call (DPC). When a DPC object is
  37 created, it is associated with a function. The DPC object can then be inserted
  38 in the DPC queue from an ISR. If the IRQL on return from the ISR is less than
  39 DISPATCH_LEVEL the DPC queue will be drained, otherwise this will happen when
  40 the IRQL level drops below DISPATCH_LEVEL or the processor becomes idle. When
  41 the DPC queue is drained each DPC object is removed and the associated
  42 function is called at DISPATCH_LEVEL. A DPC object can only be inserted once,
  43 further insertions before it is removed will have no effect.