Instructions Per Cycle
Encyclopedia
In computer architecture
, instructions per clock (instruction per cycle or IPC) is a term used to describe one aspect of a processor
's performance: the average number of instructions executed for each clock cycle. It is the multiplicative inverse
of cycles per instruction
.
for a processor can be derived by multiplying the instructions per cycle and the clock speed (measured in cycles per second or Hertz
[Hz]) 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
. However, certain processor features tend to lead to designs that have higher-than-average IPC values; the presence of multiple arithmetic logic unit
s (an ALU is a processor subsystem that can perform elementary arithmetic and logical operations), and short pipelines. When comparing different instruction set
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.
and Intel Core 2), or from a low IPC and high clock speed (like the Intel Pentium 4
and to a lesser extent the AMD Bulldozer (microarchitecture). Both are valid processor designs, and the choice between the two is often dictated by history, engineering constraints, or marketing pressures.
system, the speed of other attached devices, the efficiency of the operating system
, and most importantly the high level design of the application software
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
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.
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
, instructions per clock (instruction per cycle or IPC) is a term used to describe one aspect of a processor
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...
's performance: the average number of instructions 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. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
of cycles per instruction
Cycles Per Instruction
In computer architecture, cycles per instruction is a term used to describe one aspect of a processor's performance: the number of clock cycles that happen when an instruction is being executed...
.
Calculation of IPC
The number of instructions per secondInstructions 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 typically lead to significantly lower IPS values...
for a processor can be derived by multiplying the instructions per cycle and the clock speed (measured in cycles per second or Hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
[Hz]) 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 term memory hierarchy is used in the theory of computation when discussing performance issues in computer architectural design, algorithm predictions, and the lower level programming constructs such as involving locality of reference. A 'memory hierarchy' in computer storage distinguishes each...
. However, certain processor features tend to lead to designs that have higher-than-average IPC values; the presence of multiple arithmetic logic unit
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.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, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
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 AthlonAthlon
Athlon is the brand name applied to a series of x86-compatible microprocessors 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's competing processors...
and Intel Core 2), or from a low IPC and high clock speed (like the Intel Pentium 4
Pentium 4
Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...
and to a lesser extent the AMD Bulldozer (microarchitecture). 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 storageDisk storage
Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating disks...
system, the speed of other attached devices, the efficiency of the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
, and most importantly the high level design of the application software
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
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 instructionCycles Per InstructionIn computer architecture, cycles per instruction is a term used to describe one aspect of a processor's performance: the number of clock cycles that happen when an instruction is being executed...
- Megahertz mythMegahertz MythThe megahertz myth, or less commonly the gigahertz myth, refers to the misconception of only using clock rate to compare the performance of different microprocessors...
- The benchmarkBenchmark (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.