OHCI

A host controller interface (HCI) is a register-level interface that enables a host controller for USB or IEEE 1394 hardware to communicate with a host controller driver in software. The driver software is typically provided with an operating system of a personal computer, but may also be implemented by application-specific devices such as a microcontroller.

On the expansion card or motherboard controller, this involves much custom logic, with digital logic engines in the motherboard's controller chip, plus analog circuitry managing the high-speed differential signals. On the software side, it requires a device driver (called a Host Controller Driver, or HCD).

Open Host Controller Interface (OHCI) is an open standard.

When applied to a IEEE 1394 (also known as FireWire; i.LINK or Lynx) card, OHCI means that the card supports a standard interface to the PC and can be used by the OHCI IEEE 1394 drivers that come with all modern operating systems. Because the card has a standard OHCI interface, the OS does not need to know in advance exactly who makes the card or how it works; it can safely assume that the card understands the set of well-defined commands that are defined in the standard .

The OHCI standard for USB is similar to the OHCI standard for IEEE 1394 , but supports USB 1.1 (full and low speeds) only; so as a result its register interface looks completely different. Compared with UHCI, it moves more intelligence into the controller, and thus is accordingly much more efficient; this was part of the motivation for defining it. If a computer provides non-x86 USB 1.1, or x86 USB 1.1 from a USB controller that is not part of an Intel or a VIA chipset, it probably uses OHCI (e.g. OHCI is common on add-in PCI Cards based on an NEC chipset). It has many fewer intellectual property restrictions than UHCI. It only supports 32-bit memory addressing, so it requires an IOMMU or a computationally expensive bounce buffer to work with a 64-bit operating system. OHCI interfaces to the rest of the computer only with memory-mapped I/O.

...
Wikipedia