the mainboard peripheral bus. The miniport drivers, like the miniclass drivers, are supplied by the hardware vendors and support only device specific operations of peripheral hardware that is connected to a port on the mainboard. Each driver that processes an I/O request for a device has a corresponding object, which is loaded into main memory. A device object is created by the Windows operating system from the associated device class. Device objects contain structures of type DEVICE_OBJECT, which store pointers to their driver. At run time these pointers are used to locate a driver's dispatch routine and member functions. In the WDM driver stack, the filter driver device object, known as the upper filter, will receive an I/O request packet (IRP) for a device from the I/O manager. If the upper filter driver can not serve the request, it will locate the object of the driver one step down in the driver stack. The IRP is passed down the driver stack by calling the function IoCallDrive(), and processed by the function driver device object, also known as functional device object. The function driver device object in turn may pass the IRP to the lower filter, another filter device object. Then the IRP may be passed down to the bus driver, which operates as the physical device object. The bus driver object is at the bottom of the driver stack, and interacts with the hardware abstraction layer, which is part of the Windows operating system kernel and allows Windows operating systems to run on a variety of processors, different memory management unit architectures, and a variety of computer systems with different I/O bus architectures. The execution of an IRP is finished when any of the driver objects in the stack returns the request back to the I/O manager, with the result and a status flag. Device drivers for different Windows operating systems The WDM framework was developed by Microsoft to simplify the communication between the operating system and drivers inside the kernel. In Windows operating systems, drivers are implemented as Dynamic Link Libraries .DLL or .SYS files. WDM compliant drivers must follow rules of design, initialisation, plug-and-play, power management and memory allocation. In practice WDM driver programmers reuse large pieces of code when building new object-oriented drivers. This means that drivers in the WDM stack may contain residual functionality, which is not documented in specifications. Drivers that have passed the Microsoft