OS X and iOS Kernel Programming

USB Architecture

In a USB system, the host controller is the master and the USB devices are the subordinates. The USB topology is organized in a tree-like structure, which forms a bus, with the host controller being the root and the controller of the subordinated device. The host controller is responsible for coordinating activities on the bus, and a device is not able to perform I/O, or any other activity on the bus, without first being asked to do so by the host.

The tree structure branches out with the help of hubs, which allow connected devices to become part of the bus, and thus extend it, as a new branch. The subordinate devices of a hub can have other hubs connected to them— up to four levels deep is possible. The root hub is usually embedded ...