Clock rate
The clock rate typically refers to the frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 that a CPU is running at.(It uses the SI unit '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....


For example, a crystal oscillator
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...

 frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry (AD Converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

) into a corresponding square wave pulse [typically] pr sampling rate
Sampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...

 for digital electronics applications. In this context the use of the word, speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...

 (physical movement), should not be confused with frequency or its corresponding clock rate. Thus, the term "clock speed" is a misnomer
A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...


A single clock cycle
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...

 (typically shorter than a nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....

 in modern non-embedded
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

s) toggles between a logical zero and a logical one state .

CPU manufacturers typically charge premium prices for CPUs that operate at higher clock rates, a practice called binning
Product binning
In semiconductor device fabrication, product binning is the process of sorting manufactured products based on tested levels of performance. Large variances in performance are condensed into a smaller number of marketed designations...

. For a given CPU, the clock rates are determined at the end of the manufacturing process through actual testing of each CPU. CPUs that are tested as complying with a given set of standards may be labeled with a higher clock rate, e.g., 1.50 GHz, while those that fail the standards of the higher clock rate yet pass the standards of a lesser clock rate may be labeled with the lesser clock rate, e.g., 1.33 GHz, and sold at a lower price.

The clock of a CPU is normally determined by the frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of an oscillator crystal
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...

. The first commercial PC, the Altair 8800
Altair 8800
The MITS Altair 8800 was a microcomputer design from 1975 based on the Intel 8080 CPU and sold by mail order through advertisements in Popular Electronics, Radio-Electronics and other hobbyist magazines. The designers hoped to sell only a few hundred build-it-yourself kits to hobbyists, and were...

 (by MITS), used an Intel 8080 CPU with a clock rate of 2 MHz (2 million cycles/second). The original IBM PC (c. 1981) had a clock rate of 4.77 MHz (4,772,727 cycles/second). In 1995, Intel's
Intel Corporation
Intel Corporation is an American multinational semiconductor chip maker corporation headquartered in Santa Clara, California, United States and the world's largest semiconductor chip maker, based on revenue. It is the inventor of the x86 series of microprocessors, the processors found in most...

P5 (microarchitecture)
The original Pentium microprocessor was introduced on March 22, 1993. Its microarchitecture, deemed P5, was Intel's fifth-generation and first superscalar x86 microarchitecture. As a direct extension of the 80486 architecture, it included dual integer pipelines, a faster FPU, wider data bus,...

 Pentium chip ran at 100 MHz (100 million cycles/second), and in 2002, an 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...

 model was introduced as the first CPU with a clock rate of 3 GHz (three billion cycles/second corresponding to ~0.3 10−9seconds per cycle).

With any particular CPU, replacing the crystal with another crystal that oscillates half the frequency ("underclocking
Underclocking, also known as downclocking, is the practice of modifying a synchronous circuit's timing settings to run at a lower clock rate than it was specified to operate at. It may be said to be the computer equivalent of driving a car below the speed limit...

") will generally make the CPU run at half the performance. It will also make the CPU produce roughly half as much waste heat. Conversely, some people try to increase performance of a CPU by replacing the oscillator crystal with a higher frequency crystal ("overclocking
Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...

"). However, the amount of overclocking is limited by the time for the CPU to settle after each pulse, and by the extra heat created.

After each clock pulse, the signal lines inside the CPU need time to settle to their new state. That is, every signal line must finish transitioning from 0 to 1, or from 1 to 0. If the next clock pulse comes before that, the results will be incorrect. Chip manufacturers publish a "maximum clock rate" specification, and they test chips before selling them to make sure they meet that specification, even when executing the most complicated instructions with the data patterns that take the longest to settle (testing at the temperature and voltage that runs the lowest performance).

Also, some energy is wasted as heat (mostly inside the driving transistors) whenever a signal line makes a transition from the 0 to the 1 state or vice versa. When executing complicated instructions that cause many transitions, higher clock rates produce more heat. If electricity is converted to heat faster than a particular computer cooling
Computer cooling
Computer cooling is required to remove the waste heat produced by computer components, to keep components within their safe operating temperature limits.Various cooling methods help to improve processor performance or reduce the noise of cooling fans....

 system can cool it, then the transistors may get hot enough to be destroyed.

Engineers continue to find new ways to design CPUs that settle a little more quickly or use slightly less energy per transition, pushing back those limits, producing new CPUs that can run at slightly higher clock rates. The ultimate limits to energy per transition are explored in reversible computing
Reversible computing
Reversible computing is a model of computing where the computational process to some extent is reversible, i.e., time-invertible. A necessary condition for reversibility of a computational model is that the transition function mapping states to their successors at a given later time should be...

, although no reversible computers have yet been implemented. Engineers have struggled to design CPUs that run much faster than about 3.5 GHz due to thermodynamic limits in current semiconductor process technologies and other limitations. The highest clock speed microprocessor ever sold commercially to date is found inside IBM's zEnterprise 196 mainframe, introduced in July, 2010. The z196's cores run continuously at 5.2 GHz.

Engineers also continue to find new ways to design CPUs so that, although they may run at the same or a lower clock rate as older CPUs, they complete more instructions per clock cycle.


The clock rate of a CPU is most useful for providing comparisons between CPUs in the same family. The clock rate is only one of several factors that can influence performance when comparing processors in different families. For example, an IBM PC with an Intel 80486
Intel 80486
The Intel 80486 microprocessor was a higher performance follow up on the Intel 80386. Introduced in 1989, it was the first tightly pipelined x86 design as well as the first x86 chip to use more than a million transistors, due to a large on-chip cache and an integrated floating point unit...

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...

 running at 50 MHz will be about twice as fast (internally only) as one with the same CPU and memory running at 25 MHz, while the same will not be true for MIPS R4000 running at the same clock rate as the two are different processors that implement different architectures and microarchitectures. There are many other factors to consider when comparing the performance of CPUs, like the clock rate and width of the CPU's data bus, the latency of the memory, and the cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...


Clock rates alone should not be used when comparing different CPUs families. Software 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 more useful. Clock rates can sometimes be misleading since the amount of work different CPUs can do in one cycle varies. For example, superscalar
A superscalar CPU architecture implements a form of 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...

 processors can execute more than one instruction per cycle (on average), yet it is not uncommon for them to do "less" in a clock cycle. In addition, subscalar CPUs or use of parallelism can also affect the performance of the computer regardless of clock rate.


For most of the early history of microcomputers, clock rate was not a differentiating factor between models. Each CPU type was typically clocked at a standard rate - 1 MHz for 6502-based architectures like the Commodore 64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

 and Apple II series
Apple II series
The Apple II series is a set of 8-bit home computers, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977 with the original Apple II...

, 4.77 MHz for Z-80 computers and the first generation of Intel 8086 as used in the original IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

, 8 MHz for early Motorola 68000 machines such as the Macintosh 128k
Macintosh 128K
The Macintosh 128K machine, released as the "Apple Macintosh", was the original Apple Macintosh personal computer. Its beige case contained a monitor and came with a keyboard and mouse. An indentation in the top of the case made it easier for the computer to be lifted and carried. It had a selling...

 and Amiga 1000
Amiga 1000
The A1000, or Commodore Amiga 1000, was Commodore's initial Amiga personal computer, introduced on July 23, 1985 at the Lincoln Center in New York City....

. Since these processor generations followed each other quickly and generally did not compete between themselves (except for the Z-80 and 8086, which shared the same clock rate), manufacturers tended not to emphasize clock rate in their marketing material.

Computer buyers first became aware of clock speed when new generations of PC compatibles started to appear with "Turbo" clock rates faster than 4.77 MHz. On some of these computers the speed was selectable by a front panel switch from the faster speed down to the then-standard 4.77 MHz. This was used for games, which had no timing routines of their own then, or for compatibility with software that couldn't operate at the faster speed. When the 80286 was released in 1982 at a standard clock rate of 6 MHz, followed by the 80386 in 1985, running at 12 MHz, computer manufacturers seized on the clock rate as an easy way to promote the faster, more expensive CPUs to potential buyers. They were helped by Intel, which was able to increase the rate of the 286 to 25 MHz over that processor's lifetime, and the 386 was clocked up to 40 MHz by the time it was superseded by the 80486.

By the early 1990s, most computer companies advertised their computers' performance chiefly by referring to their CPUs' clock rates. This led to various marketing games, such as Apple Computer's decision to create and market the Power Macintosh 8100
Power Macintosh 8100
The Power Macintosh 8100 is a personal computer that is a part of Apple Computer's Power Macintosh series of Macintosh computers...

 with a clock rate of 110 MHz so that Apple could advertise that its computer had the fastest clock rate available—the fastest Intel processor available at the time ran at 100 MHz. This slight superiority in clock rate was meaningless, however, since the PowerPC 601 and Pentium implemented different instruction set architectures and had different microarchitecture
In computer engineering, microarchitecture , also called computer organization, is the way a given instruction set architecture is implemented on a processor. A given ISA may be implemented with different microarchitectures. Implementations might vary due to different goals of a given design or...


After 2000, Intel's competitor, Advanced Micro Devices
Advanced Micro Devices
Advanced Micro Devices, Inc. or AMD is an American multinational semiconductor company based in Sunnyvale, California, that develops computer processors and related technologies for commercial and consumer markets...

, started using model numbers instead of clock rates to market its CPUs because of the lower CPU clocks when compared to Intel. Continuing this trend it attempted to dispel the "megahertz myth
Megahertz Myth
The megahertz myth, or less commonly the gigahertz myth, refers to the misconception of only using clock rate to compare the performance of different microprocessors...

" which it claimed did not tell the whole story of the power of its CPUs. In 2004, Intel announced it would do the same, probably because of consumer confusion over its Pentium M
Pentium M
The Pentium M brand refers to a family of mobile single-core x86 microprocessors introduced in March 2003 , and forming a part of the Intel Carmel notebook platform under the then new Centrino brand...

 mobile CPU, which reportedly ran at about half the clock rate of the roughly equivalent Pentium 4 CPU. As of 2007, performance improvements have continued to come through innovations in pipelining, 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, and the development of multi-core processors, rather than clock rate increases (which have been constrained by CPU power dissipation issues).

See also

  • Crystal oscillator frequencies
    Crystal oscillator frequencies
    Crystal oscillators can be manufactured for oscillation over a wide range of frequencies, from a few kilohertz up to several hundred megahertz. Many applications call for a crystal oscillator frequency conveniently related to some other desired frequency, so hundreds of standard crystal frequencies...

  • Double data rate
    Double data rate
    In computing, a computer bus operating with double data rate transfers data on both the rising and falling edges of the clock signal. This is also known as double pumped, dual-pumped, and double transition....

  • Quad Data Rate
    Quad Data Rate
    Quad data rate is a communication signaling technique wherein data are transmitted at four points in the clock cycle: on the rising and falling edges, and at two intermediate points between them. The intermediate points are defined by a 2nd clock that is 90° out of phase from the first...

  • Pulse wave
    Pulse wave
    A pulse wave or pulse train is a kind of non-sinusoidal waveform that is similar to a square wave, but does not have the symmetrical shape associated with a perfect square wave. It is a term common to synthesizer programming, and is a typical waveform available on many synths. The exact shape of...

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