A superscalar processor is a

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a

scalar processor Scalar processors are a class of computer processors that process only one data item at a time. Typical data items include integers and floating point numbers. Classification A scalar processor is classified as a single instruction, single da ...

, which can execute at most one single instruction per clock cycle, a superscalar processor can execute more than one instruction during a clock cycle by simultaneously dispatching multiple instructions to different

execution unit In computer engineering, an execution unit (E-unit or EU) is a part of the central processing unit (CPU) that performs the operations and calculations as instructed by the computer program. It may have its own internal control sequence unit (not ...

s on the processor. It therefore allows more

throughput Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel, such as Ethernet or packet radio, in a communication network. The data that these messages contain may be delivered ov ...

(the number of instructions that can be executed in a unit of time) than would otherwise be possible at a given

clock rate In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the ...

. Each execution unit is not a separate processor (or a core if the processor is a

multi-core processor A multi-core processor is a microprocessor on a single integrated circuit with two or more separate processing units, called cores, each of which reads and executes program instructions. The instructions are ordinary CPU instructions (suc ...

), but an execution resource within a single CPU such as an

arithmetic logic unit In computing, an arithmetic logic unit (ALU) is a combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on floating point numb ...

. In

Flynn's taxonomy Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1966 and extended in 1972. The classification system has stuck, and it has been used as a tool in design of modern processors and their functionalities ...

, a single-core superscalar processor is classified as an SISD processor (single instruction stream, single data stream), though a single-core superscalar processor that supports short vector operations could be classified as

SIMD Single instruction, multiple data (SIMD) is a type of parallel processing in Flynn's taxonomy. SIMD can be internal (part of the hardware design) and it can be directly accessible through an instruction set architecture (ISA), but it should ...

(single instruction stream, multiple data streams). A

multi-core A multi-core processor is a microprocessor on a single integrated circuit with two or more separate processing units, called cores, each of which reads and executes program instructions. The instructions are ordinary CPU instructions (such ...

superscalar processor is classified as an MIMD processor (multiple instruction streams, multiple data streams). While a superscalar CPU is typically also pipelined, superscalar and pipelining execution are considered different performance enhancement techniques. The former executes multiple instructions in parallel by using multiple execution units, whereas the latter executes multiple instructions in the same execution unit in parallel by dividing the execution unit into different phases. The superscalar technique is traditionally associated with several identifying characteristics (within a given CPU): * Instructions are issued from a sequential instruction stream * The CPU dynamically checks for data dependencies between instructions at run time (versus software checking at

compile time In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled. The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concept ...

) * The CPU can execute multiple instructions per clock cycle

History

Seymour Cray Seymour Roger Cray (September 28, 1925 – October 5, 1996
) was an American

CDC 6600 The CDC 6600 was the flagship of the 6000 series of mainframe computer systems manufactured by Control Data Corporation. Generally considered to be the first successful supercomputer, it outperformed the industry's prior recordholder, the IBM ...

from 1964 is often mentioned as the first superscalar design. The 1967 IBM System/360 Model 91 was another superscalar mainframe. The Motorola MC88100 (1988), the

Intel i960 Intel's i960 (or 80960) was a RISC-based microprocessor design that became popular during the early 1990s as an embedded microcontroller. It became a best-selling CPU in that segment, along with the competing AMD 29000. In spite of its succes ...

CA (1989) and the AMD 29000-series 29050 (1990) microprocessors were the first commercial single-chip superscalar microprocessors.

RISC In computer engineering, a reduced instruction set computer (RISC) is a computer designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a complex instruction set compu ...

microprocessors like these were the first to have superscalar execution, because RISC architectures free transistors and die area which can be used to include multiple execution units (this was why RISC designs were faster than CISC designs through the 1980s and into the 1990s). Except for CPUs used in low-power applications,

embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' ...

s, and battery-powered devices, essentially all general-purpose CPUs developed since about 1998 are superscalar. The P5

Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The original Pentium processor from which the brand took its name was first released on March 22, 1993. After that, the Pentium II and P ...

was the first superscalar x86 processor; the Nx586, P6 Pentium Pro and AMD K5 were among the first designs which decode x86-instructions asynchronously into dynamic

microcode In processor design, microcode (μcode) is a technique that interposes a layer of computer organization between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. Microcode is a la ...

-like '' micro-op'' sequences prior to actual execution on a superscalar

microarchitecture In computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as µarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular processor. A given ISA may be imp ...

; 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

; it also simplified speculative execution and allowed higher clock frequencies compared to designs such as the advanced Cyrix 6x86.

Scalar to superscalar

The simplest processors are scalar processors. 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 In computing, a vector processor or array processor is a central processing unit (CPU) that implements an instruction set where its instructions are designed to operate efficiently and effectively on large one-dimensional arrays of data called ...

operates simultaneously on many data items. An analogy is the difference between scalar and vector arithmetic. A superscalar processor is a mixture of the two. Each instruction processes one data item, but there are multiple execution units within each CPU thus multiple instructions can be processing separate data items concurrently. Superscalar CPU design emphasizes improving the instruction dispatcher accuracy, and allowing it to keep the multiple execution 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

ALU ALU, Alu or alu may refer to: Computing and science ;Computing *Arithmetic logic unit, a digital electronic circuit ;Biology * Alu sequence, a type of short stretch of DNA *'' Arthrobacter luteus'', a bacterium Organizations * Abraham Lincoln ...

s and a single

FPU FPU may stand for: Universities * Florida Polytechnic University, in Lakeland, Florida, United States * Franklin Pierce University, in New Hampshire, United States * Fresno Pacific University, in California, United States * Fukui Prefectural Univ ...

, a later 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 be no better than that of a simpler, cheaper design. A superscalar processor usually sustains an execution rate in excess of one instruction per machine cycle. But merely processing multiple instructions concurrently does not make an architecture superscalar, since pipelined,

multiprocessor Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There ar ...

architectures also achieve that, but with different methods. In a superscalar CPU the dispatcher reads instructions from memory and decides which ones can be run in parallel, dispatching each to one of the several execution 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.

Limitations

Available performance improvement from superscalar techniques is limited by three key areas: * The degree of intrinsic parallelism in the instruction stream (instructions requiring the same computational resources from the CPU) * The complexity and time cost of dependency checking logic and register renaming circuitry * The branch instruction processing 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 a = b + c; d = e + f can be run in parallel because none of the results depend on other calculations. However, the instructions a = b + c; b = e + f might not be runnable in parallel, depending on the order in which the instructions complete while they move through the units. Although 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. 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 execution units (e.g. ALUs), the burden of checking instruction dependencies grows rapidly, as does the complexity of register renaming circuitry to mitigate some dependencies. Collectively the power consumption, complexity and gate delay costs limit the achievable superscalar speedup. 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.

Alternatives

Collectively, these limits drive investigation into alternative architectural changes such as

very long instruction word Very long instruction word (VLIW) refers to instruction set architectures designed to exploit instruction level parallelism (ILP). Whereas conventional central processing units (CPU, processor) mostly allow programs to specify instructions to exe ...

(VLIW), explicitly parallel instruction computing (EPIC), simultaneous multithreading (SMT), and multi-core computing. With VLIW, the burdensome task of dependency checking by hardware logic at run time is removed and delegated to the

compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...

. Explicitly parallel instruction computing (EPIC) is like VLIW with extra cache prefetching instructions. Simultaneous multithreading (SMT) is a technique for improving the overall efficiency of superscalar processors. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures. Superscalar processors differ from

s in that the several execution units are not entire processors. A single processor is composed of finer-grained execution units such as the

integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...

multiplier, integer shifter,

, etc. There may be multiple versions of each execution 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 processing unit (called "core"). It also differs from a pipelined processor, where the multiple instructions can concurrently be in various stages of execution, assembly-line fashion. 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 Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

capability.

References

Mike Johnson James Michael Johnson (born January 30, 1972) is an American lawyer and politician serving as the List of speakers of the United States House of Representatives, 56th speaker of the United States House of Representatives since October 25, 20 ...

, ''Superscalar Microprocessor Design'', Prentice-Hall, 1991, * Sorin Cotofana, Stamatis Vassiliadis, "On the Design Complexity of the Issue Logic of Superscalar Machines", EUROMICRO 1998: 10277-10284 * Steven McGeady, "The i960CA SuperScalar Implementation of the 80960 Architecture", IEEE 1990, pp. 232–240 * 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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