What is IRQL and why is it important?

What is IRQL and why is it important?

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When people first hear the term IRQL (pronounced Er-kel) their thoughts sometimes turn to the sitcom "Family Matters" and Jaleel White's alter ego, Steve Urkel.  However, we're not going to be taking a trip down Television's Memory Lane today.  Instead we're going to talk about Interrupt Request Levels - aka IRQL's.  If you develop device drivers or spend a lot of time debugging, IRQL's are familiar territory for you.  An interrupt request level (IRQL) defines the hardware priority at which a processor operates at any given time. In the Windows Driver Model, a thread running at a low IRQL can be interrupted to run code at a higher IRQL.  The number of IRQL's and their specific values are processor-dependent.

Processes running at a higher IRQL will pre-empt a thread or interrupt running at a lower IRQL.  An IRQL of 0 means that the processor is running a normal Kernel or User mode process.  An IRQL of 1 means that the processor is running an Asynchronous Procedure Call (APC) or Page Fault.  IRQL 2 is used for deferred procedure calls (DPC) and thread scheduling.  IRQL 2 is known as the DISPATCH_LEVEL.  When a processor is running at a given IRQL, interrupts at that IRQL and lower are blocked by the processor.  Therefore, a processor currently at DISPATCH_LEVEL can only be interrupted by a request from an IRQL greater than 2.  A system will schedule all threads to run at IRQL's below DISPATCH_LEVEL - this level is also where the thread scheduler itself will run.  So if there is a thread that has an IRQL greater than 2, that thread will have exclusive use of the processor.  Since the scheduler runs at DISPATCH_LEVEL, and that interrupt level is now blocked off by the thread at a higher IRQL, the thread scheduler cannot run and schedule any other thread.  So far, this is pretty straightforward - especially when we're talking about a single processor system.

On a multi-processor system, things get a little complicated.  Since each processor can be running at a different IRQL, you could have a situation where one processor is running a driver routine (Device Interrupt Level - aka DIRQL), while another processor is running driver code at IRQL 0.  Since more than one thread could attempt to access shared data at the same time, drivers should protect the shared data by using some method of synchronization.  Drivers should use a lock that raises the IRQL to the highest level at which any code that could access the data can run.  We're not going to get too much into Locks and Deadlocks here, but for the sake of our discussion, an example would be a driver using a spin lock to protect data accessible at DISPATCH_LEVEL.  On a single processor system, raising the IRQL to DISPATCH_LEVEL or higher would have the same effect, because the raising of the IRQL prevents the interruption of the code currently executing.

That will actually wrap it up for this post.  It's a fairly short post, but hopefully you now have a basic understanding of IRQL.  Until next time ...

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- CC Hameed

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  • PingBack from http://liludrivers.net/2008/01/22/what-is-irql-and-why-is-it-important/

  • yep, the famous "IRQL not less or equal" bsod msg

  • Could you explain this in plain English.  I have a problem with IRQL rebooting my computer.  I know less now then when I started.

  • If the processor is running at an IRQL >=2 and a driver (or Windows at the request of a driver) tries to access memory that is paged out (in page file) or invalid then system will bugcheck with a stop 0xA IRQL_not_less_or_equal. This can happen because memory address the driver is using is incorrect or the driver didn't check and lower the IRQL before trying to access the data. There are many causes for this. The simplest answer is if a driver is mentioned in the stop error then update that driver.

  • What causes 'Worker Thread Returned At Bad IRQL'?  I get a fairly regular blue screen based on this, and want to know what causes it and how to avoid it.  If anyone knows the answer, could you post it please as it's really bugging me - 5th time in the past week.

  • I have just bought a new computer and uploaded windows 8.1 Pro. everything seems to be running fine very confusing but fine. Except my estimating software. Well the software will start up and run in demo mode just fine but it needs a dongle key to unlock the program so it can be fully utilized. Once I put the dongle into the USB port the computer screams at me and I get the IQRL_NOT_LESS_OR_EQUAL then it shuts down. I have called Microsoft on this issue and they say it is my issue with software not provided by them but it has always ran fine on other window products and runs fine on this product. I have tried to explain to them it is a issue with their system accepting the key/dongle to unlock the program. How can this be corrected?