This chapter explains how a Windows PC manages communications with USB devices. The driver architecture described applies to Windows XP and Windows Vista, but much of the information also applies to other Windows editions.
A device driver is a software component that enables applications to access a
hardware device. The hardware device may be a printer, modem, keyboard, video display, data-acquisition unit, or just about anything controlled by circuits the CPU can access. Most USB devices are external devices that connect via cables (or wireless links). Some USB devices, such as fingerprint scanners, are in the box with the CPU.
USB communications under Windows use a layered driver model where each
driver in a series, or stack, performs a portion of the communication task. At the top of the stack is a client driver that the operating system has assigned to
the device. Another term for client driver is function driver. USB class drivers and vendor-specific device drivers are client drivers. Applications access a USB device by communicating with the client driver. The client driver in turn communicates with lower-level bus and port drivers that access the hardware. One or more filter drivers can supplement a client driver or bus driver. Dividing communications into layers is efficient because devices that have tasks in common can use the same driver for those tasks. For example, it makes sense to have one set of drivers that handle tasks common to all USB devices. An operating system can provide these drivers so device vendors don’t have to do so with much duplication of effort.
Tags: Communications, Under, USB, Windows
