USBFacts - Knowing Connectivity

The computer hardware that contains the host controller and the root hub has an interface geared toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer.
Dual Role Host Controller Devices switch to host mode when connected to a peripheral and to peripheral mode when connected to a PC. Most host/peripheral controllers support both full- and low-speed USB devices.
In the version 1.x age, there were two competing HCD implementations, Open Host Controller Interface (OHCI) and Universal Host Controller Interface (UHCI). OHCI was developed by Compaq, Microsoft and National Semiconductor; UHCI was by Intel.
 
A typical USB connector.
VIA Technologies licensed the UHCI standard from Intel; all other chip set implementers use OHCI. UHCI is more software-driven, making UHCI slightly more processor-intensive than OHCI but cheaper to implement. The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations, which increased cost.
HCD standards are out of the USB specification's scope, and the USB specification does not specify any HCD interfaces. In other words, USB defines the format of data transfer through the port, but not the system by which the USB hardware communicates with the computer it sits in.
During the design phase of USB 2.0, the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Enhanced Host Controller Interface (EHCI). Only EHCI can support hi-speed (480 Mbit/s[citation needed]) transfers. Most of PCI-based EHCI controllers contain other HCD implementations called 'companion host controller' to support Full Speed (12 Mbit/s[citation needed]) and may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device. Device classes are decided upon by the Device Working Group of the USB Implementers Forum.
During USB 3.0 development, Intel released the Extensible Host Controller Interface (xHCI) Draft Specification, which became a basis of USB 3.0 standard.