The thread can only be interrupted by code which needs to run at a higher IRQL on the same processor. In a multi-processor system, each processor operates independently at its own IRQL. This is the lowest IRQL. No interrupts are masked off and this is the level in which a thread executing in user mode is running. Pagable memory is accessible. In a processor running at this level, only APC level interrupts are masked. This is the level in which Asynchronous Procedure Calls occur. Pagable memory is still accessible.
This, in turn, also disables other APCs from occurring. The processor running at this level has DPC level interrupts and lower masked off. Pagable memory cannot be accessed, so all memory being accessed must be non-paged. If you are running at Dispatch Level, the APIs that you can use greatly decrease since you can only deal with non-paged memory.
Generally, higher level drivers do not deal with IRQLs at this level, but all interrupts at this level or less are masked off and do not occur. This is actually a range of IRQLs, and this is a method to determine which devices have priority over other devices. However, it is necessary for you to be aware of what IRQL is, if you intend to continue writing device drivers.
For more information on IRQLs and thread scheduling, refer to the following documentation , and another good source of information is here. This is a data structure that allows drivers to communicate with each other and to request work to be done by the driver. The IRP includes information about the operation that is being requested.
A description of the IRP data structure can be found here. The description and usage of an IRP can go from simple to complex very easily, so we will only be describing, in general, what an IRP will mean to you. That article can be found here. If they were going to build a house, they could have a common overall design and perhaps a common set of tools like their tool box. This includes things like power drills, etc. All of these common tools and overall design of building a house would be the IRP.
Each of them has an individual piece they need to work on to make this happen, for example, the plumber needs the plans on where to put the pipe, how much pipe he has, etc. Once everyone has completed their job, they then complete the IRP.
The device driver we will be building will not be that complex and will basically be the only driver in the stack. There are a lot of pitfalls that you will need to avoid but they are mostly unrelated to our simple driver. It is hard to digest theory or even how code is supposed to work, without actually doing anything. You need some hands on experience so you can bring these ideas out of space and into reality.
The prototype for the DriverEntry is the following. We will simply be creating one device. The driver can use this location to store driver specific information. The next part is to actually put things in the DriverEntry routine. The first thing we will do is create the device. You may be wondering how we are going to create a device and what type of device we should create. This is generally because a driver is usually associated with hardware but this is not the case.
There are a variety of different types of drivers which operate at different levels, not all drivers work or interface directly with hardware. Generally, you maintain a stack of drivers each with a specific job to do. The highest level driver is the one that communicates with user mode, and the lowest level drivers generally just talk to other drivers and hardware.
There are network drivers, display drivers, file system drivers, etc. Each place in the stack breaks up a request into a more generic or simpler request for the lower level driver to service. The highest level drivers are the ones which communicate themselves to user mode, and unless they are a special device with a particular framework like display drivers , they can behave generally the same as other drivers just as they implement different types of operations.
As an example, take the hard disk drive. The driver which communicates to user mode does not talk directly to hardware.
The high level driver simply manages the file system itself and where to put things. It then communicates where it wants to read or write from the disk to the lower level driver which may or may not talk directly to hardware. There may be another layer which then communicates that request to the actual hardware driver which then physically reads or writes a particular sector off a disk and then returns it to the higher level.
It could then determine what sector read requests to service, however, it has no idea what the data is and does not interpret it. The first thing you will notice is the DbgPrint function. This data structure contains basically three entries. The first is the size of the current Unicode string, the second is the maximum size that the Unicode string can be, and the third is a pointer to the Unicode string.
This is used to describe a Unicode string and used commonly in drivers. Most Unicode strings passing into your driver will not be NULL terminated, so this is something you need to be aware of. Devices have names just like anything else. The second parameter we passed 0, and it says to specify the number of bytes to create for the device extension. This is basically a data structure that the driver writer can define which is unique to that device.
This is how you can extend the information being passed into a device and create device contexts, etc. We will not be using this for this example. These requests are called IRP Major requests. There are also Minor requests which are sub-requests of these and can be found in the stack location of the IRP.
What do these refer to? When communicating with the user-mode application, certain APIs call directly to the driver and pass in parameters! So as you can see, when a user mode application uses these functions, it calls into your driver. That is true, these APIs can talk to any device which exposes itself to user mode, they are not only for accessing files. In the last piece of this article, we will be writing a user mode application to talk to our driver and it will simply do CreateFile , WriteFile , CloseHandle.
You can technically omit this function but if you want to unload your driver dynamically, then it must be specified. If you do not specify this function once your driver is loaded, the system will not allow it to be unloaded.
We are simply setting the flags. I will explain this in the section on handling user-mode write requests. However, if you create a device in any function outside of the DriverEntry , you need to manually clear this flag for any device you create with IoCreateDevice. This flag is actually set by the IoCreateDevice function. The last piece of our driver is using both of the Unicode strings we defined above. To put this into perspective, different vendors have different drivers and each driver is required to have its own name.
You cannot have two drivers with the same NT Device name. Say, you have a memory stick which can display itself to the system as a new drive letter which is any available drive letter such as E:. If you remove this memory stick and say you map a network drive to E:. Writing a device driver can be pretty simple, or it can be almost arbitrarily complicated. For instance, I've been involved in a project where it took six of us almost three years to solve ONE bug in a device driver. Of course, we cleared out dozens of other bugs while looking for it The fix turned out to be an eight line patch, that cost, conservatively, about a million dollars.
But, as a side project to that, I wrote an ethernet driver from the chip data sheet in a week, and took another week to debug it. Haven't needed to touch it since. There's no way to say in general how much work a driver will be; a GPU driver could cost hundreds of millions, a driver for a single LED costs a couple of hours work at the most.
In order to find unsupported hardware pieces for which you could write a driver, ask on the Linux mailing lists. Maybe some USB 3. It's think and heavy but fun to read. For Linux take a look at Linux Device Drivers, 3rd Edition - it's lighter free PDFs online : and is really device driver-oriented, might be a better start. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How should I get started on writing device drivers? Asked 11 years, 11 months ago. Implement the functions required by your software driver. As you implement and organize your functions, you might decide to add header files and additional. It is extremely unlikely that you'll want to use WDM for a software driver.
But if you do, follow these steps. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Select OK. In this exercise, the hardware ID does not identify a real piece of hardware. It identifies an imaginary device that will be given a place in the device tree as a child of the root node. The hardware ID is located under [Standard. On the Build menu, choose Deploy Solution.
Visual Studio automatically copies the files required to install and run the driver to the target computer. This may take a minute or two. If something goes wrong during deployment, you can check to see if the files are copied to the test computer.
Verify that the. For more information about deploying drivers, see Deploying a Driver to a Test Computer. With your Hello World driver deployed to the target computer, now you'll install the driver.
When you previously provisioned the target computer with Visual Studio using the automatic option, Visual Studio set up the target computer to run test signed drivers as part of the provisioning process. Now you just need to install the driver using the DevCon tool. For example, look in the following folder:. On the target computer, install the driver by navigating to the folder containing the driver files, then running the DevCon tool.
Open a Command Prompt window as Administrator. Navigate to your folder containing the built driver. If you get an error message about devcon not being recognized, try adding the path to the devcon tool.
A dialog box will appear indicating that the test driver is an unsigned driver. Select Install this driver anyway to proceed. Now that you have installed your KmdfHelloWorld driver on the target computer, you'll attach a debugger remotely from the host computer. On the host computer, open a Command Prompt window as Administrator.
Change to the WinDbg. We will use the x64version of WinDbg. Here is the default path to WinDbg.
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