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| [[Image:Superscalarpipeline.svg|thumb|300px|right|Simple superscalar pipeline. By fetching and dispatching two instructions at a time, a maximum of two instructions per cycle can be completed. (IF = Instruction Fetch, ID = Instruction Decode, EX = Execute, MEM = Memory access, WB = Register write back, ''i'' = Instruction number, ''t'' = Clock cycle [i.e., time])]] | | http://www.fictionchiamami.it/public/foto/nikeairmax.asp?louboutin-homme Oveг the last decade, all-natural fashion has raised іn [http://www.dailymail.Co.uk/home/search.html?sel=site&searchPhrase=recognition recognition] for the s . [http://Guccihandbags.com/ Gucci handbags] is one such brand which has won over the hearts o . Eѵer since, this brand is simply folloԝing two material patterns for example . These shoes prevent a person from falling down on slippeгy and sludgy terrain. |
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| [[Image:Processor board cray-2 hg.jpg|thumb|Processor board of a [[Cray T3E|CRAY T3e]] supercomputer with four ''superscalar'' [[Alpha 21164]] processors]] | | Many industries also prеfer these shoes fߋr their wߋrkers where [http://Search.Huffingtonpost.com/search?q=standing&s_it=header_form_v1 standing] for long hours is needed. The soles of theѕe Ϲhгistian Louboutin grey patent calfskin Declic 120 slingbacκ clogs for wօmen are made of heavy material. <br>http://www.alessandrocesaro.it/public/aless/?louboutin-espadrillas ShopCrone also has an appropriate lineage. ShopCrone mother, now a West Coast fashionista, once lived in Paris and hunted in the secߋndhand shops of the City ߋf Lights, where she managed to fіnd runway discards from Chanel, Diοr and Celine. ShopϹroneMom also adores Hermes scarves and taught ShopCrone, when she waѕ jսst ShoρBabe, ɦow tо tell a real Louіs Ѵuitton from a sidеwalk copy.. http://www.biagiolicarservice.it/public/foto/defaul.asp?nikescarpe=[http://www.encyclopedia.com/searchresults.aspx?q=louboutin-milano louboutіn-milano]<br>Sеr amable no quiere decir qսe hagas amigos con todo el mundo. Es simplemente ser amɑble, sonreírle a todos, y no particіƿar en аctiνidades christian louboutin uk yοutube [[http://Www.ucamchiaravalle.it/public/conse/?louboutin-imitazioni ucamchiaravalle.it]] ցгoseras como chismear o andar intimidando a personas. Se llega a sеr amable con practica, y después ԁe un rato se vuelѵe natural.. http://www.eventidinapoli.it/public/event/?key=zara-gana-demanda-a-louboutin<br>Phoenix Mills stood on a road that was seldom taken. "When I used to drive to college, sometimes using the road because it would be less crowded, I would not find a single other vehicle on it," reminiscences Gayatri Ruia, Director, New Developmеnts, and Atul Ruia's wife. "My mother would be horrified to know I had ventured through that lonely stretch.". http://www.ucamchiaravalle.it/public/conse/?louboutin-imitazioni |
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| A '''superscalar''' [[Central processing unit|CPU]] architecture implements a form of [[parallel computer|parallelism]] called [[instruction-level parallelism]] within a single processor. It therefore allows faster CPU [[throughput]] than would otherwise be possible at a given [[clock rate]]. A superscalar processor executes more than one instruction during a clock cycle by simultaneously dispatching multiple instructions to redundant functional units on the processor. Each functional unit is not a separate CPU core but an execution resource within a single CPU such as an [[arithmetic logic unit]], a bit shifter, or a [[Multiplication ALU|multiplier]].
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| In [[Flynn's taxonomy]], a single-core superscalar processor is classified as an [[SISD]] processor (Single Instructions, Single Data), while a [[multi-core]] superscalar processor is classified as an [[MIMD]] processor (Multiple Instructions, Multiple Data).
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| While a superscalar CPU is typically also [[instruction pipeline|pipeline]]d, pipelining and superscalar architecture are considered different performance enhancement techniques.
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| The superscalar technique is traditionally associated with several identifying characteristics (within a given CPU core):
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| * Instructions are issued from a sequential instruction stream
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| * CPU hardware dynamically checks for [[data dependencies]] between instructions at run time (versus software checking at [[compile time]])
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| * The CPU processes multiple instructions per clock cycle
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| ==History== | |
| [[Seymour Cray]]'s [[CDC 6600]] from 1965 is often mentioned as the first superscalar design. The [[Intel i960]]CA (1988) and the [[AMD 29000]]-series 29050 (1990) microprocessors were the first commercial single-chip superscalar microprocessors. [[RISC]] CPUs like these were the first microprocessors to use the superscalar concept, because the RISC design results in a simple core, thereby allowing the inclusion of multiple functional units (such as [[arithmetic logic unit|ALU]]s) on a single CPU in the constrained design rules of the time (this was why RISC designs were faster than [[Complex instruction set computer|CISC]] designs through the 1980s and into the 1990s).
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| Except for CPUs used in [[Low-power electronics|low-power]] applications, [[embedded system]]s, and [[Battery (electricity)|battery]]-powered devices, essentially all general-purpose CPUs developed since about 1998 are superscalar.
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| The [[P5 (microarchitecture)|P5]] [[Pentium (brand)|Pentium]] was the first superscalar x86 processor; the [[Nx586]], [[P6 (microarchitecture)|P6]] [[Pentium Pro]] and [[AMD K5]] were among the first designs which decode [[x86]]-instructions [[asynchronous]]ly into dynamic [[microcode]]-like ''[[micro-op]]'' sequences prior to actual execution on a superscalar [[microarchitecture]]; this opened up for dynamic scheduling of buffered ''partial'' instructions and enabled more parallelism to be extracted compared to the more rigid methods used in the simpler [[P5 (microarchitecture)|P5]] [[Pentium (brand)|Pentium]]; it also simplified [[speculative execution]] and allowed higher clock frequencies compared to designs such as the advanced [[Cyrix 6x86]].
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| ==Scalar to Superscalar==
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| The simplest processors are [[scalar processor]]s. Each instruction executed by a scalar processor typically manipulates one or two data items at a time. By contrast, each instruction executed by a [[vector processor]] operates simultaneously on many data items. An analogy is the difference between [[Scalar (mathematics)|scalar]] and vector arithmetic. A superscalar processor is a mixture of the two. Each instruction processes one data item, but there are multiple redundant functional units within each CPU thus multiple instructions can be processing separate data items concurrently.
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| Superscalar CPU design emphasizes improving the instruction dispatcher accuracy, and allowing it to keep the multiple functional units in use at all times. This has become increasingly important as the number of units has increased. While early superscalar CPUs would have two [[Arithmetic logic unit|ALU]]s and a single [[floating point unit|FPU]], a modern design such as the [[PowerPC 970]] includes four ALUs, two FPUs, and two [[SIMD]] units. If the dispatcher is ineffective at keeping all of these units fed with instructions, the performance of the system will suffer.
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| A superscalar processor usually sustains an execution rate in excess of one [[Cycles per instruction|instruction per machine cycle]]. But merely processing multiple instructions concurrently does not make an architecture superscalar, since [[Instruction pipeline|pipelined]], [[multiprocessor]] or [[multi-core (computing)|multi-core]] architectures also achieve that, but with different methods.
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| In a superscalar CPU the dispatcher reads instructions from memory and decides which ones can be run in parallel, dispatching them to redundant functional units contained inside a single CPU. Therefore a superscalar processor can be envisioned having multiple parallel pipelines, each of which is processing instructions simultaneously from a single instruction thread.
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| ==Limitations==
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| Available performance improvement from superscalar techniques is limited by three key areas:
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| # The degree of intrinsic parallelism in the instruction stream (instructions requiring the same computational resources from the CPU).
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| # The complexity and time cost of the dispatcher and associated dependency checking logic.
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| # The branch instruction processing.
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| Existing binary executable programs have varying degrees of intrinsic parallelism. In some cases instructions are not dependent on each other and can be executed simultaneously. In other cases they are inter-dependent: one instruction impacts either resources or results of the other. The instructions <code>a = b + c; d = e + f</code> can be run in parallel because none of the results depend on other calculations. However, the instructions <code>a = b + c; b = e + f</code> might not be runnable in parallel, depending on the order in which the instructions complete while they move through the units.
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| When the number of simultaneously issued instructions increases, the cost of dependency checking increases extremely rapidly. This is exacerbated by the need to check dependencies at run time and at the CPU's clock rate. This cost includes additional logic gates required to implement the checks, and time delays through those gates. Research shows the gate cost in some cases may be <math>n^k</math> gates, and the delay cost <math>k^2 \log n</math>, where <math>n</math> is the number of instructions in the processor's instruction set, and <math>k</math> is the number of simultaneously dispatched instructions.
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| Even though the instruction stream may contain no inter-instruction dependencies, a superscalar CPU must nonetheless check for that possibility, since there is no assurance otherwise and failure to detect a dependency would produce incorrect results.
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| No matter how advanced the semiconductor process or how fast the switching speed, this places a practical limit on how many instructions can be simultaneously dispatched. While process advances will allow ever greater numbers of functional units (e.g., ALUs), the burden of checking instruction dependencies grows so rapidly that the achievable superscalar dispatch limit is fairly small, likely on the order of five to six simultaneously dispatched instructions.
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| However even given infinitely fast dependency checking logic on an otherwise conventional superscalar CPU, if the instruction stream itself has many dependencies, this would also limit the possible speedup. Thus the degree of intrinsic parallelism in the code stream forms a second limitation.
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| ==Alternatives== | |
| Collectively, these limits drive investigation into alternative architectural changes such as [[Very Long Instruction Word]] (VLIW), [[Explicitly Parallel Instruction Computing]] (EPIC), [[simultaneous multithreading]] (SMT), and [[Multi-core (computing)|multi-core processors]].
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| With VLIW, the burdensome task of dependency checking by [[hardware logic]] at run time is removed and delegated to the [[compiler]]. [[Explicitly Parallel Instruction Computing]] (EPIC) is like VLIW, with extra cache prefetching instructions.
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| Simultaneous multithreading, often abbreviated as SMT, is a technique for improving the overall efficiency of superscalar CPUs. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures.
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| Superscalar processors differ from multi-core processors in that the redundant functional units are not entire processors. A single processor is composed of finer-grained functional units such as the [[Arithmetic logic unit|ALU]], [[Integer (computer science)|integer]] [[Multiplication ALU|multiplier]], integer shifter, [[floating point unit]], etc. There may be multiple versions of each functional unit to enable execution of many instructions in parallel. This differs from a [[multi-core processor]] that concurrently processes instructions from multiple threads, one thread per core. It also differs from a [[instruction pipelining|pipelined CPU]], where the multiple instructions can concurrently be in various stages of execution, [[assembly line|assembly-line]] fashion.
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| The various alternative techniques are not mutually exclusive—they can be (and frequently are) combined in a single processor. Thus a multicore CPU is possible where each core is an independent processor containing multiple parallel pipelines, each pipeline being superscalar. Some processors also include [[vector processor|vector]] capability.
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| ==See also==
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| *[[Super-threading]]
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| *[[Simultaneous multithreading]] (SMT)
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| *[[Speculative execution]] / [[Eager execution]]
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| *[[Software lockout]], a multiprocessor issue similar to logic dependencies on superscalars
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| *[[Shelving buffer]]
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| ==References==
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| * [[William Michael (Mike) Johnson (technologist)|Mike Johnson]], ''Superscalar Microprocessor Design'', Prentice-Hall, 1991, ISBN 0-13-875634-1
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| * Sorin Cotofana, Stamatis Vassiliadis, "On the Design Complexity of the Issue Logic of Superscalar Machines", [[EUROMICRO]] 1998: 10277-10284
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| *[[Steven McGeady]], "The i960CA SuperScalar Implementation of the 80960 Architecture", IEEE 1990, pp. 232–240
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| *[[Steven McGeady]], et al., "Performance Enhancements in the Superscalar i960MM Embedded Microprocessor," ''ACM Proceedings of the 1991 Conference on Computer Architecture (Compcon)'', 1991, pp. 4–7
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| ==External links==
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| * [http://www.cs.clemson.edu/~mark/eager.html Eager Execution / Dual Path / Multiple Path], By Mark Smotherman
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| {{CPU_technologies}}
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| {{Parallel_computing}}
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| [[Category:Parallel computing]]
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| [[Category:Computer architecture]]
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| [[Category:Classes of computers]]
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