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In computer architecture
Computer architecture

Computer architecture in computer engineering is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit performs internally an...
, Instructions Per Clock (Instruction Per Cycle or IPC) is a term used to describe one aspect of a processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
's performance: the average number of instruction
Instruction (computer science)

In computer science, an instruction is a single operation of a central processing unit defined by an instruction set architecture. In a broader sense, an "instruction" may be any representation of an element of an executable program, such as a bytecode....
s executed for each clock cycle. It is the multiplicative inverse
Multiplicative inverse

In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1⁄x or x −1, is a number which when multiplied by x yields the multiplicative identity, 1....
 of Cycles Per Instruction
Cycles Per Instruction

In computer architecture, Cycles per instruction is a term used to describe one aspect of a central processing unit performance: the number of clock cycles that happen when an Instruction is being executed....
.

number of instructions per second
Instructions per second

Instructions per second is a measure of a computer's processor speed. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches, whereas realistic workloads consist of a mix of instructions and applications, some of which take longer to execute than others....
 for a processor can be derived by multiplying the instructions per cycle and the clock speed (measured in cycles per second or Hz
Hertz

The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....
) of the processor in question.






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In computer architecture
Computer architecture

Computer architecture in computer engineering is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit performs internally an...
, Instructions Per Clock (Instruction Per Cycle or IPC) is a term used to describe one aspect of a processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
's performance: the average number of instruction
Instruction (computer science)

In computer science, an instruction is a single operation of a central processing unit defined by an instruction set architecture. In a broader sense, an "instruction" may be any representation of an element of an executable program, such as a bytecode....
s executed for each clock cycle. It is the multiplicative inverse
Multiplicative inverse

In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1⁄x or x −1, is a number which when multiplied by x yields the multiplicative identity, 1....
 of Cycles Per Instruction
Cycles Per Instruction

In computer architecture, Cycles per instruction is a term used to describe one aspect of a central processing unit performance: the number of clock cycles that happen when an Instruction is being executed....
.

Explanation


Calculation of IPC

The number of instructions per second
Instructions per second

Instructions per second is a measure of a computer's processor speed. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches, whereas realistic workloads consist of a mix of instructions and applications, some of which take longer to execute than others....
 for a processor can be derived by multiplying the instructions per cycle and the clock speed (measured in cycles per second or Hz
Hertz

The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....
) of the processor in question. The number of instructions per second is an approximate indicator of the likely performance of the processor.

The number of instructions executed per clock is not a constant for a given processor; it depends on how the particular software being run interacts with the processor, and indeed the entire machine, particularly the memory hierarchy
Memory hierarchy

The hierarchical arrangement of computer storage in current computer architectures is called the memory hierarchy. It is designed to take advantage of memory locality in computer programs....
. However, certain processor features tend to lead to designs that have higher-than-average IPC values; the presence of multiple ALU
Arithmetic logic unit

In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logicaloperations. The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers....
s (an ALU is a processor subsystem that can perform elementary arithmetic and logical operations), and short pipelines. When comparing different instruction set
Instruction set

An instruction set is a list of all the instruction , and all their variations, that a processor can execute.Instructions include:* Arithmetic such as add and subtract...
s, a simpler instruction set may lead to a higher IPC figure than an implementation of a more complex instruction set using the same chip technology; however, the more complex instruction set may be able to achieve more useful work with fewer instructions.

Factors governing IPC


A given level of instructions per second can be achieved with a high IPC and a low clock speed (like the AMD Athlon
Athlon

Athlon is the brand name applied to a series of different x86 Central processing unit designed and manufactured by Advanced Micro Devices. The original Athlon was the first seventh-generation x86 processor and, in a first, retained the initial performance lead it had over Intel Corporation's competing processors for a significant period of t...
, Intel Core 2, HP
Hewlett-Packard

The Hewlett-Packard Company , commonly referred to as HP, is a technology corporation headquartered in Palo Alto, California, United States....
 PA-RISC, or Sun Microsystems
Sun Microsystems

Sun Microsystems, Inc. is a multinational corporation vendor of computers, computer components, computer software, and information technology services, founded on February 24, 1982....
 SPARC
SPARC

SPARC is a Reduced Instruction Set Computer microprocessor instruction set Computer architecture originally designed in 1985 by Sun Microsystems....
), or from a low IPC and high clock speed (like the Intel Pentium 4
Pentium 4

The Pentium 4 brand refers to Intel's line of single-core mainstream Desktop computer and laptop central processing units introduced on November 20, 2000 ....
 or DEC Alpha
DEC Alpha

Alpha, originally known as Alpha AXP, was a 64-bit reduced instruction set computer instruction set architecture developed by Digital Equipment Corporation , designed to replace the 32-bit VAX complex instruction set computer ISA and its implementations....
).

High-IPC, low-clock speed processor designs are traditionally called "brainiac" designs, while low-IPC, high-clock speed are often called "speed-demons
Speed-demons

Speed demon is a term which sometimes has been used to describe a CPU design which favors high clock frequencies over high Instructions Per Cycle....
". Both are valid processor designs, and the choice between the two is often dictated by history, engineering constraints, or marketing pressures.

Computer speed


The useful work that can be done with any computer depends on many factors besides the processor speed. These factors include the processor architecture, the internal layout of the machine, the speed of the disk storage
Disk storage

Disk storage is a general category of a computer storage mechanisms, in which data is recorded on planar, round and rotating surfaces . A disk drive is a peripheral device used to record and retrieve information....
 system, the speed of other attached devices, the efficiency of the operating system
Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
, and most importantly the high level design of the application software
Application software

Application software is any tool that functions and is operated by means of a computer, with the purpose of supporting or improving the software user 's work....
 in use.

For users and purchasers of a computer system, Instructions Per Clock is not a particularly useful indication of the performance of their system. For an accurate measure of performance relevant to them, application benchmark
Benchmark (computing)

In computing, a benchmark is the act of running a computer program, a set of programs, or other operations, in order to assess the relative performance of an object, normally by running a number of standard tests and trials against it....
s are much more useful. Awareness of its existence is useful, in that it provides an easy-to-grasp example of why clock speed is not the only factor relevant to computer performance.

See also

  • Cycles Per Instruction
    Cycles Per Instruction

    In computer architecture, Cycles per instruction is a term used to describe one aspect of a central processing unit performance: the number of clock cycles that happen when an Instruction is being executed....
  • The benchmark
    Benchmark (computing)

    In computing, a benchmark is the act of running a computer program, a set of programs, or other operations, in order to assess the relative performance of an object, normally by running a number of standard tests and trials against it....
     article provides a useful introduction to computer performance measurement for those readers interested in the topic.