Scalable Coherent Interface

<hardware, protocol>

(SCI) The ANSI/IEEE 1596-1992 standard that defines a point-to-point interface and a set of packet protocols. The SCI protocols use packets with a 16-byte header and 16, 64, or 256 data bytes. Each packet is protected by a 16-bit CRC code.

The standard defines 1 Gbit/second serial fiber-optic links and 1 Gbyte/second parallel copper links. SCI has two unidirectional links that operate concurrently.

The SCI protocols support shared memory by encapsulating bus requests and responses into SCI request and response packets. Packet-based handshake protocols guarantee reliable data delivery. A set of cache coherence protocols are defined to maintain cache coherence in a shared memory system.

Message passing is supported by a compatible subset of the SCI protocols. This protocol subset does not invoke SCI cache coherency protocols.

SCI uses 64-bit addressing and the most significant 16 bits are used for addressing up to 64K nodes.


Last updated: 1999-03-22

Scalable Processor ARChitecture


(SPARC) An instruction set architecture designed by Sun Microsystems for their own use in 1985. Sun was a maker of 680x0-based Unix workstations. Research versions of RISC processors had promised a major step forward in speed but existing manufacturers were slow to introduce a RISC type processor, so Sun went ahead and developed its own, based on the University of California at Berkley's RISC I and RISC II 1980-2. In keeping with their open philosophy, they licenced it to other companies, rather than manufacture it themselves. The evolution and standardisation of SPARC is now directed by the non-profit consortium SPARC International, Inc.

SPARC was not the first RISC processor. The AMD 29000 came before it, as did the MIPS R2000 (based on Stanford's design) and Hewlett-Packard Precision Architecture CPU, among others. The SPARC design was radical at the time, even omitting multiple cycle multiply and divide instructions (like a few others), while most RISC CPUs are more conventional.

SPARC implementations usually contain 128 or 144 registers, (CISC designs typically had 16 or less). At each time 32 registers are available - 8 are global, the rest are allocated in a "window" from a stack of registers. The window is moved 16 registers down the stack during a function call, so that the upper and lower 8 registers are shared between functions, to pass and return values, and 8 are local. The window is moved up on return, so registers are loaded or saved only at the top or bottom of the register stack. This allows functions to be called in as little as 1 cycle. Like some other RISC processors, reading global register zero always returns zero and writing it has no effect. SPARC is pipelined for performance, and like previous processors, a dedicated condition code register holds comparison results.

SPARC is "scalable" mainly because the register stack can be expanded (up to 512, or 32 windows), to reduce loads and saves between functions, or scaled down to reduce interrupt or context switch time, when the entire register set has to be saved. Function calls are usually much more frequent, so the large register set is usually a plus.

SPARC is not a chip, but a specification, and so there are various implementations of it. It has undergone revisions, and now has multiply and divide instructions. Most versions are 32 bits, but there are designs for 64-bit and superscalar versions. SPARC was submitted to the IEEE society to be considered for the P1754 microprocessor standard.

SPARC(R) is a registered trademark of SPARC International, Inc. in the United States and other countries.

[The SPARC Architecture Manual, v8, ISBN 0-13-825001-4].

Last updated: 1994-11-01

Scalable Sampling Rate

<compression, standard, algorithm>

(SSR) See, e.g., MPEG-4 AAC SSR.

Last updated: 2001-12-08

Scalable Vector Graphics

<graphics, web>

A W3C standard for vector graphics, based on XML.

Last updated: 2001-02-06

Nearby terms:

Scalable Coherent InterfaceScalable Processor ARChitecture

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