mirror of
https://github.com/reactos/reactos.git
synced 2024-05-16 02:01:59 +00:00
44 lines
2.3 KiB
Plaintext
44 lines
2.3 KiB
Plaintext
A collection of articles on kernel internals, please add to this
|
|
|
|
------------------------------------------------------------------------------
|
|
IRQ level
|
|
------------------------------------------------------------------------------
|
|
|
|
IRQ level (IRQL) is a per-processor state in ReactOS used to coordinate
|
|
execution 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 reason
|
|
ISRs should postpone most processing till it can run at a lower IRQL. The
|
|
mechanism for this is a Deferred Procedure Call (DPC). When a DPC object is
|
|
created, it is associated with a function. The DPC object can then be inserted
|
|
in the DPC queue from an ISR. If the IRQL on return from the ISR is less than
|
|
DISPATCH_LEVEL the DPC queue will be drained, otherwise this will happen when
|
|
the IRQL level drops below DISPATCH_LEVEL or the processor becomes idle. When
|
|
the DPC queue is drained each DPC object is removed and the associated
|
|
function is called at DISPATCH_LEVEL. A DPC object can only be inserted once,
|
|
further insertions before it is removed will have no effect.
|