- XIO
"Warning: this page is mostly conjectural."
XIO is a packet-based, high-performance
computer bus employed by the SGI Origin 2000, Octane, Altix, Fuel and Tezro machines. The XIO forms a bus between high-performance system devices and thememory controller .XIO is usually used in a star topology, using a
router ASIC called Crossbow (Xbow) to connect up to eight fully-symmetrical devices in a system (one of them is usually 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 converterThe 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 (ie. in megabytes). 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).
It is probable that XIO uses STL (which SGI likes to call "SGI transistor logic") low-voltage single-ended I/O standard. CrossTown is a version of XIO utilizing PECL for differential I/O standard (like
NUMAlink ) for longer connections.Higher-level encapsulation differs from NUMAlink (like used to connect HUB chips in Origin-series machines). It is well suited for short memory transactions.
XIO uses very fragile compression connectors, which should be handled with extreme care.
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