Every device driver must have one main entry point, like the main function in a C console application. This entry point is called DriverEntry in Windows and init_module in Linux. When the operating system loads the device driver, this driver entry procedure is called.

There is some global initialization that every driver needs to perform only once when it is loaded for the first time. This global initialization is the responsibility of the DriverEntry/init_module routine. The entry function also registers which driver callbacks will be called by the operating system. These driver callbacks are operating system requests for services from the driver. In Windows, these callbacks are called dispatch routines, and in Linux they are called file operations. Each registered callback is called by the operating system as a result of some criteria, such as disconnection of hardware, for example.