USB evolved as the universal serial bus that can handle multiple devices (up to 127), loads device drivers automatically (standard USB drivers), has configurable operational details, can detect power configuration, and can determine the speed of operation. Most importantly, it detects the device state. If the device is unplugged from the system, it dynamically unloads the drivers to save system resources. All of these USB transactions are controlled through a single USB Host (master) that is responsible for all of the USB device (client or slave) support. The host provides power and loads the device drivers needed during enumeration, thus enabling a plug-and-play experience to end users. The devices are also known as “functional” devices in USB specifications, as they perform a specific functional class of operation like Mass Storage Device (MSD), Human Interface Device (HID), and Communications Device Class (CDC). Compound devices are formed when two different classes are operated on the same device. An HID device such as a keyboard, which also supports a flash drive interface (MSD), is a classic example of a compound device.