| How Does a Device Driver Work? |
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Device drivers are codes that the operating system for interacting with the various pieces of hardware and attached devices. Instead of dealing directly with the hardware, it loads the device driver, calling out functions for each device as needed.
Device drivers serve their purpose with little input from users. Once installed, you may never deal with your device driver directly. Instead, it sits in the background waiting until the operating system calls on it.
Drivers are similar to Dynamic Link Library files in that they contain segments of codes that various applications can call on as needed. Device drivers let the operating system know about the hardware that the driver communicates with and its features. Windows allocates a portion of memory for this driver object which describes address information and other details. The driver fills entries into the Function Dispatch Table. Windows references the Function Dispatch Table whenever device functions are required and selects the appropriate function as needed.
For example, let’s say you want to print a document to your printer. Windows accesses the Function Dispatch Table for the function code required to carry out this task. It then sends a request to the appropriate device, in this case your printer. Requests are called IRPs or Input/Output Request Packets. When the driver receives and IRP, it does one of three things: it either understands and does its function followed by sending a confirmation notice that the job is complete; it accepts the request and places it in a queue (if the device is busy doing something else); or it notifies Windows that there’s a problem and it cannot perform the request.
Drivers generally operate in a mode called “kernel” mode. Windows handles program codes in either user or kernel mode. User mode is typically used for programs. When using user mode, programs can call and use services provided by the operating system but cannot access the hardware directly. Processes running in kernel mode can interact with system memory and hardware.
Like computer programs, device drivers and hardware can cause computer crashes and instability which is why sometimes replacing or updating a device driver can solve computer and hardware problems.
Microsoft tests and digitally signs many device drivers. When a device driver passes a series of tests conducted by the Windows Hardware Quality Lab, they are digitally signed and recognized by the Windows Operating System. You can also load unsigned drivers depending on what Signature Checking Level your computer is set on. Level 0 disables signature checking. Level 1 checks for a digital signature and warns the user if one is not found. The user may go ahead and proceed despite the warning. Level 2 blocks the installation of all unsigned device drivers.
To work with signature checking, go to the Start button, click on Run, and type in: sigverif. This will run a scan of all of your device drivers to search for any unsigned device drivers. Digitally signed drivers tend to be more robust and have been thoroughly tested.
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