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The NT Insider The only publication dedicated entirely to Windows® system software development A publication by OSR Open Systems Resources, Inc. Not endorsed by or associated with Microsoft Corporation.

January — February 2009

Volume 16 Issue 1

Common Topics

Happy 2009?

Deferred Procedure Call Details

D

eferred Procedure Calls (DPCs) are a commonly used feature of Windows. Their uses are wide and varied, but they are most commonly used for what we typically refer to as "ISR completion" and are the underlying technology of timers in Windows. If they're so commonly used, then why are we bothering to write an entire article on them? Well, what we've found is that most people don't really understand the underlying implementation details of how DPCs work. And, as it turns out, a solid understanding is important in choosing the options available to you when creating DPCs and is also a life saver in some debug scenarios.

Introduction

This article is not meant to be a

comprehensive review of why or how DPCs are used. It is assumed that the reader already knows what a DPC is or, even better, has used them in a driver. If you do not fall into this category, information at that level is readily available on MSDN.

In addition, Threaded DPCs, which are a special type of DPC available on Windows Vista and later, will not be covered in any detail. As a basis of our discussion, let's briefly review some basic DPC concepts. A working definition of DPCs is that they are a method by which a driver can request a callback to an arbitrary thread context at IRQL DISPATCH_LEVEL. The DPC object itself is nothing more than a data structure with a LIST_ENTRY, a callback pointer, some

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ell, that remains to seen for many folks, regardless of the industry. Profits are down, job losses are up, belts are being tightened...all is not right in the world. For those of you who are not aware, OSR provides job forums for both those companies looking to hire permanent, experienced driver developers, and of course, those looking to be hired by said companies. Check out Driver Jobs in the left-hand navigation menu at OSR Online (www.osronline.com). If you’re still gainfully employed, great. If you’re not, rest assured The NT Insider will still keep coming your way.

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Common Topics — Deferred Procedure Call Details

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