To access your hardware, your application calls the WinDriver kernel module using functions from the WinDriver USB API. The high-level functions utilize the low-level functions, which use IOCTLs to enable communication between the WinDriver kernel module and your user-mode application. The WinDriver kernel module accesses your USB device resources through the native operating system calls.
There are two layers responsible for abstracting the USB device to the USB device driver. The upper layer is the USB Driver (USBD) layer, which includes the USB Hub Driver and the USB Core Driver. The lower level is the Host Controller Driver (HCD) layer. The division of duties between the HCD and USBD layers is not defined and is operating system dependent. Both the HCD and USBD are software interfaces and components of the operating system, where the HCD layer represents a lower level of abstraction.
The HCD is the software layer that provides an abstraction of the host controller hardware, while the USBD provides an abstraction of the USB device and the data transfer between the host software and the function of the USB device.
The USBD communicates with its clients (the specific device driver, for example) through the USB Driver Interface (USBDI). At the lower level, the Core Driver and USB Hub Driver implement the hardware access and data transfer by communicating with the HCD using the Host Controller Driver Interface (HCDI).
The USB Hub Driver is responsible for identifying the addition and removal of devices from a particular hub. When the Hub Driver receives a signal that a device was attached or detached, it uses additional host software and the USB Core Driver to recognize and configure the device. The software implementing the configuration can include the hub driver, the device driver, and other software.
WinDriver USB abstracts the configuration procedure and hardware access described above for the developer. With WinDriver's USB API, developers can perform all the hardware-related operations without having to master the lower-level implementation for supporting these operations.