XIO is a packet-based, high-performance computer bus employed by SGI Octane, Origin, Altix, Fuel and Tezro machines. It forms a bus between high-performance system devices and the memory controller. XIO devices are called "widgets".
XIO is usually used in a star topology -- a router called Xbow (cross-bow) connects up to eight fully-symmetrical devices in a system (typically, one of them is the memory controller / CPU bridge, called HEART in Octane or HUB in Origin).
Other devices known to have XIO interfaces are:
- BRIDGE: XIO to PCI-64 bridge (Octane, Origin)
- XBRIDGE: XIO to PCI-X bridge (Altix)
- HQ4: command processor of ImpactSR cards (Octane)
- KTOWN: frontend for InfiniteReality2 cards (Onyx, Origin variation)
- XC: Crosstown converter
XIO employs two source-synchronous channels (one in each direction), each 8 or 16 bits wide. They are clocked at 400 MHz to achieve peak rates of 800 MB/s. Each of the devices can utilize the full bandwidth, as the XBow router prevents collisions by being able to route between any two points.
Transfer is organized into micropackets. These contain a total of 128 bits of data and 32 bits of control. The control information encapsulates an 8 bit sideband (used by higher layers for framing), sequence numbers (for go-back-n link-layer retransmissions) and check bits (CRC-16). The link layer is able to run both on 8- and 16-bit links, and if a upper half of a 16-bit link is non-functional, it will automatically revert to using the 8 lower bits.
XIO is usually used on a single-ended, source-synchronous physical layer (STL levels - SGI Transistor Logic). There is also a version called Crosstown, which uses normal cabling and differential Positive Referenced Emitter Coupled Logic (PECL) transmission.
XIO boards use very fragile compression connectors, which should be handled with extreme care.