XIO
Encyclopedia
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 the memory 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:
The 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 (i.e. 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 delicate compression connectors, which should be handled with extreme care.
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
employed by the SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...
Origin 2000
SGI Origin 2000
The SGI Origin 2000, code named Lego, is a family of mid-range and high-end servers developed and manufactured by SGI and introduced in 1996 to succeed the SGI Challenge and POWER Challenge. At the time of introduction, these systems ran IRIX 6.4 and later, IRIX 6.5. A variant of the Origin 2000...
, Octane
SGI Octane
The Octane and the similar Octane2 are UNIX workstations marketed by SGI. Both are 2-way SMP-capable workstations, originally based on the MIPS R10000 microprocessor. Newer Octanes are based on MIPS R12000 and R14000. The Octane2 has four improvements compared to Octane, a revised power supply,...
, Altix, Fuel
SGI Fuel
The SGI Fuel is a mid-range workstation developed and manufactured by Silicon Graphics, Inc. . It was introduced in January 2002, with a list price of US$11,495. Together with the entire MIPS platform, general availability for the Fuel ended on 29 December 2006...
and Tezro
SGI Tezro
The SGI Tezro was a series of high-end computer workstations sold by SGI from 2003 until 2006. It was the immediate successor to the SGI Octane line. The systems were available in both rack-mount and tower versions, and the series was released in June 2003 with a list price of $20,500...
machines. The XIO forms a bus between high-performance system devices and the memory controller
Memory controller
The memory controller is a digital circuit which manages the flow of data going to and from the main memory. It can be a separate chip or integrated into another chip, such as on the die of a microprocessor...
.
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
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
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
The XIO employs two source-synchronous
Source-synchronous
Source-Synchronous clocking refers to the technique of sourcing a clock along with the data. Specifically, the timing of unidirectional data signals is referenced to a clock sourced by the same device that generates those signals, and not to a global clock Source-Synchronous clocking refers to...
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 (i.e. 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
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
-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
NUMAlink
NUMAlink is a system interconnect developed by SGI for use in its distributed shared memory ccNUMA computer systems. NUMAlink was originally developed by SGI for their Origin 2000 and Onyx2 systems...
) 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 delicate compression connectors, which should be handled with extreme care.