# Clock rate

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The clock rate typically refers to the frequency at which the clock circuit of a processor can generate pulses, which are used to synchronize the operations (such as adding two numbers or transfering a value from one register to another) [1] of its components, [2] and is used as an indicator of the processor's speed. It is measured in clock cycles per second or its equivalent, the SI unit hertz (Hz). The clock rate of the first generation of computers was measured in hertz or kilohertz (kHz), the first personal computers (PCs) to arrive throughout the 1970s and 1980s had clock rates measured in megahertz (MHz), and in the 21st century the speed of modern CPUs is commonly advertised in gigahertz (GHz). This metric is most useful when comparing processors within the same family, holding constant other features that may affect performance. Video card and CPU manufacturers commonly select their highest performing units from a manufacturing batch and set their maximum clock rate higher, fetching a higher price.[ citation needed ]

Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.

A clock generator is an electronic oscillator (circuit) that produces a timing signal for use in synchronizing a circuit's operation. The signal can range from a simple symmetrical square wave to more complex arrangements. The basic parts that all clock generators share are a resonant circuit and an amplifier.

A microprocessor is a computer processor that incorporates the functions of a central processing unit on a single integrated circuit (IC), or at most a few integrated circuits. The microprocessor is a multipurpose, clock driven, register based, digital integrated circuit that accepts binary data as input, processes it according to instructions stored in its memory and provides results as output. Microprocessors contain both combinational logic and sequential digital logic. Microprocessors operate on numbers and symbols represented in the binary number system.

## Determining factors

### Binning

Manufacturers of modern processors typically charge premium prices for processors that operate at higher clock rates, a practice called binning. For a given CPU, the clock rates are determined at the end of the manufacturing process through actual testing of each processor. 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). Processors successfully tested for compliance with a given set of standards may be labeled with a higher clock rate, e.g., 3.50 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., 3.3 GHz, and sold at a lower price. [3]

Product binning is the categorizing of finished products based on their characteristics. In order to undergo binning, manufactured products require testing, usually performed by machines in bulk. Binning allows large variances in performance to be condensed into a smaller number of marketed designations. This ensures coherency in the marketplace, with tiers of performance clearly delineated. The immediate consequence of this practice is that, for liability reasons, products sold under a certain designation must meet that designation at a minimum. Individual products may still exceed advertised performance. Different bins usually have different model numbers and prices.

### Engineering

The clock rate of a CPU is normally determined by the frequency of an oscillator crystal. Typically a crystal oscillator produces a fixed sine wave—the frequency reference signal. Electronic circuitry translates that into a square wave at the same frequency for digital electronics applications (or, in using a CPU multiplier, some fixed multiple of the crystal reference frequency). The clock distribution network inside the CPU carries that clock signal to all the parts that need it. An A/D Converter has a "clock" pin driven by a similar system to set the sampling rate. With any particular CPU, replacing the crystal with another crystal that oscillates at half the frequency ("underclocking") will generally make the CPU run at half the performance and reduce waste heat produced by the CPU. Conversely, some people try to increase performance of a CPU by replacing the oscillator crystal with a higher frequency crystal ("overclocking"). [4] However, the amount of overclocking is limited by the time for the CPU to settle after each pulse, and by the extra heat created.

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 precise frequency. This frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits incorporating them became known as crystal oscillators, but other piezoelectric materials including polycrystalline ceramics are used in similar circuits.

A sine wave or sinusoid is a mathematical curve that describes a smooth periodic oscillation. A sine wave is a continuous wave. It is named after the function sine, of which it is the graph. It occurs often in pure and applied mathematics, as well as physics, engineering, signal processing and many other fields. Its most basic form as a function of time (t) is:

A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. Although not realizable in physical systems, the transition between minimum and maximum is instantaneous for an ideal square wave.

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. In the process of transitioning, some energy is wasted as heat (mostly inside the driving transistors). When executing complicated instructions that cause many transitions, the higher the clock rate the more heat produced. Transistors may be damaged by excessive heat.

There is also a lower limit of the clock rate, unless a fully static core is used.

Static core generally refers to a microprocessor (MPU) entirely implemented in static logic. A static core MPU may be halted by stopping the system clock oscillator that is driving it, maintaining its state and resume processing at the point where it was stopped when the clock signal is restarted, as long as power continues to be applied. Static core MPUs are fabricated in the CMOS process and hence consume very little power when the clock is stopped, making them useful in designs in which the MPU remains in standby mode until needed and minimal loading of the power source is desirable during standby.

## Historical milestones and current records

The first electromechanical general purpose computer, the Z3, operated at a frequency of about 5–10 Hz. The first electronic general purpose computer, the ENIAC, used a 100 kHz clock in its cycling unit. As each instruction took 20 cycles, it had an instruction rate of 5 kHz.

The Z3 was a German electromechanical computer designed by Konrad Zuse. It was the world's first working programmable, fully automatic digital computer. The Z3 was built with 2,600 relays, implementing a 22-bit word length that operated at a clock frequency of about 4–5 Hz. Program code was stored on punched film. Initial values were entered manually.

ENIAC was the first electronic general-purpose computer. It was Turing-complete, digital and able to solve "a large class of numerical problems" through reprogramming.

The first commercial PC, the Altair 8800 (by MITS), used an Intel 8080 CPU with a clock rate of 2 MHz (2 million cycles per second). The original IBM PC (c. 1981) had a clock rate of 4.77 MHz (4,772,727 cycles per second). In 1992, both Hewlett-Packard and Digital Equipment Corporation broke the difficult 100 MHz limit with RISC techniques in the PA-7100 and AXP 21064 DEC Alpha respectively. In 1995, Intel's P5 Pentium chip ran at 100 MHz (100 million cycles per second). On March 6, 2000, AMD reached the 1 GHz milestone a few months ahead of Intel. In 2002, an Intel Pentium 4 model was introduced as the first CPU with a clock rate of 3 GHz (three billion cycles per second corresponding to ~3.3×10−10seconds or 0.33 nanoseconds per cycle). Since then, the clock rate of production processors has increased much more slowly, with performance improvements coming from other design changes.

The Altair 8800 is a microcomputer designed in 1974 by MITS and based on the Intel 8080 CPU. Interest grew quickly after it was featured on the cover of the January 1975 issue of Popular Electronics, and was sold by mail order through advertisements there, in Radio-Electronics, and in other hobbyist magazines. The designers hoped to sell a few hundred build-it-yourself kits to hobbyists, and were surprised when they sold thousands in the first month. The Altair also appealed to individuals and businesses that just wanted a computer and purchased the assembled version. The Altair is widely recognized as the spark that ignited the microcomputer revolution as the first commercially successful personal computer. The computer bus designed for the Altair was to become a de facto standard in the form of the S-100 bus, and the first programming language for the machine was Microsoft's founding product, Altair BASIC.

The IBM Personal Computer, commonly known as the IBM PC, is the original version of the IBM PC compatible hardware platform. It is IBM model number 5150 and was introduced on August 12, 1981. It was created by a team of engineers and designers under the direction of Philip Don Estridge of the IBM Entry Systems Division in Boca Raton, Florida.

Alpha, originally known as Alpha AXP, is a 64-bit reduced instruction set computing (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC), designed to replace their 32-bit VAX complex instruction set computer (CISC) ISA. Alpha was implemented in microprocessors originally developed and fabricated by DEC. These microprocessors were most prominently used in a variety of DEC workstations and servers, which eventually formed the basis for almost all of their mid-to-upper-scale lineup. Several third-party vendors also produced Alpha systems, including PC form factor motherboards.

As of 2014, the Guinness World Record for the highest CPU clock rate is an overclocked, 8.723 GHz AMD Piledriver-based FX-8370 chip. It surpassed the previous record achieved in 2011, an 8.429 GHz AMD FX-8150 Bulldozer-based chip. [5]

As of mid-2013, the highest clock rate on a production processor is the IBM zEC12, clocked at 5.5 GHz, which was released in August 2012.

## Research

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.

The first fully reversible CPU, the Pendulum, was implemented using standard CMOS transistors in the late 1990s at MIT. [6] [7] [8] [9]

Engineers also continue to find new ways to design CPUs so that they complete more instructions per clock cycle, thus achieving a lower CPI (cycles or clock cycles per instruction) count, although they may run at the same or a lower clock rate as older CPUs. This is achieved through architectural techniques such as instruction pipelining and out-of-order execution which attempts to exploit instruction level parallelism in the code.

IBM is working on 100Ghz CPU. In 2010, IBM demonstrated a graphene based transistor that can execute 100 billion cycles per second [10] .

## Comparing

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 CPU 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. Further, a "cumulative clock rate" measure is sometimes assumed by taking the total cores and multiplying by the total clock rate (e.g. dual core 2.8 GHz being considered processor cumulative 5.6 GHz). There are many other factors to consider when comparing the performance of CPUs, like the width of the CPU's data bus, the latency of the memory, and the cache architecture.

The clock rate alone is generally considered to be an inaccurate measure of performance when comparing different CPUs families. Software benchmarks 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 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.

## Related Research Articles

Athlon is the brand name applied to a series of x86-compatible microprocessors designed and manufactured by Advanced Micro Devices (AMD). The original Athlon was the first seventh-generation x86 processor and was the first desktop processor to reach speeds of one gigahertz (GHz). It made its debut on June 23, 1999.

The Cyrix 6x86 is a sixth-generation, 32-bit x86 microprocessor designed by Cyrix and manufactured by IBM and SGS-Thomson. It was originally released in 1996.

The Intel 80486, also known as the i486 or 486, is the successor model of 32-bit x86 microprocessor to the Intel 80386. Introduced in 1989, the 80486 improved on the performance of the 80386DX thanks to on-die L1 cache and floating-point unit, as well as an improved, five-stage tightly-coupled pipelined design. It was the first x86 chip to use more than a million transistors. It represents the fourth generation of binary compatible CPUs since the original 8086 of 1978.

Instructions per second (IPS) is a measure of a computer's processor speed. For CISC computers different instructions take different amounts of time, so the value measured depends on the instruction mix; even for comparing processors in the same family the IPS measurement can be problematic. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches and no cache contention, whereas realistic workloads typically lead to significantly lower IPS values. Memory hierarchy also greatly affects processor performance, an issue barely considered in IPS calculations. Because of these problems, synthetic benchmarks such as Dhrystone are now generally used to estimate computer performance in commonly used applications, and raw IPS has fallen into disuse.

Celeron is a brand name given by Intel to a number of different low-end IA-32 and x86-64 computer microprocessor models targeted at low-cost personal computers.

Pentium 4 is a brand by Intel for an entire series of single-core CPUs for desktops, laptops and entry-level servers. The processors were shipped from November 20, 2000, until August 8, 2008.

The Pentium III brand refers to Intel's 32-bit x86 desktop and mobile microprocessors based on the sixth-generation P6 microarchitecture introduced on February 26, 1999. The brand's initial processors were very similar to the earlier Pentium II-branded microprocessors. The most notable differences were the addition of the SSE instruction set, and the introduction of a controversial serial number embedded in the chip during the manufacturing process.

In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly operating voltage is also increased to maintain a component's operational stability at accelerated speeds. Semiconductor devices operated at higher frequencies and voltages increase power consumption and heat. An overclocked device may be unreliable or fail completely if the additional heat load is not removed or power delivery components cannot meet increased power demands. Many device warranties state that overclocking and/or over-specification voids any warranty.

A front-side bus (FSB) is a computer communication interface (bus) that was often used in Intel-chip-based computers during the 1990s and 2000s. The competing EV6 bus served the same function for AMD CPUs. Both typically carry data between the central processing unit (CPU) and a memory controller hub, known as the northbridge.

Central processing unit power dissipation or CPU power dissipation is the process in which central processing units (CPUs) consume electrical energy, and dissipate this energy in the form of heat due to the resistance in the electronic circuits.

Underclocking, also known as downclocking, is modifying a computer or electronic circuit's timing settings to run at a lower clock rate than is specified. Underclocking is used to reduce a computer's power consumption, increase battery life, reduce heat emission, and it may also increase the system's stability and compatibility. Underclocking may be implemented by the factory, but many computers and components may be underclocked by the end user.

The K6-III, code-named "Sharptooth", is an x86 microprocessor manufactured by AMD, released on February 22, 1999, with 400 and 450 MHz models. It was the last Socket 7 desktop processor. For an extremely short time after its release, the fastest available desktop processor from Intel was the Pentium II 450 MHz. However, the K6-III also competed against the Pentium III "Katmai" line, released just days later on February 26. "Katmai" CPUs reached speeds of 500 MHz, slightly faster than the K6-III 450 MHz. K6-III performance was significantly improved over the K6-2 due to the addition of an on-die L2 cache running at full clock speed. When equipped with a 1MB L3 cache on the motherboard the 400 and 450 MHz K6-IIIs is claimed by Ars Technica to often outperform the more expensive Pentium III "Katmai" 450- and 500-MHz models, respectively.

A northbridge or host bridge is one of the two chips in the core logic chipset architecture on a PC motherboard, the other being the southbridge. Unlike the southbridge, northbridge is connected directly to the CPU via the front-side bus (FSB) and is thus responsible for tasks that require the highest performance. The northbridge, also known as Memory Controller Hub, is usually paired with a southbridge. In systems where they are included, these two chips manage communications between the CPU and other parts of the motherboard, and constitute the core logic chipset of the PC motherboard.

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. While clock rates are a valid way of comparing the performance of different speeds of the same model and type of processor, other factors such as an amount of execution units, pipeline depth, cache hierarchy, branch prediction, and instruction sets can greatly affect the performance when considering different processors. For example, one processor may take two clock cycles to add two numbers and another clock cycle to multiply by a third number, whereas another processor may do the same calculation in two clock cycles. Comparisons between different types of processors are difficult because performance varies depending on the type of task. A benchmark is a more thorough way of measuring and comparing computer performance.

In computing, the clock multiplier sets the ratio of an internal CPU clock rate to the externally supplied clock. A CPU with a 10x multiplier will thus see 10 internal cycles for every external clock cycle. For example, a system with an external clock of 100 MHz and a 36x clock multiplier will have an internal CPU clock of 3.6 GHz. The external address and data buses of the CPU also use the external clock as a fundamental timing base; however, they could also employ a (small) multiple of this base frequency to transfer data faster.

AMD Turion is the brand name AMD applies to its x86-64 low-power consumption (mobile) processors codenamed K8L. The Turion 64 and Turion 64 X2/Ultra processors compete with Intel's mobile processors, initially the Pentium M and the Intel Core and Intel Core 2 processors.

Dynamic frequency scaling is a technique in computer architecture whereby the frequency of a microprocessor can be automatically adjusted "on the fly" depending on the actual needs, to conserve power and reduce the amount of heat generated by the chip. Dynamic frequency scaling helps preserve battery on mobile devices and decrease cooling cost and noise on quiet computing settings, or can be useful as a security measure for overheated systems. Dynamic frequency scaling is used in all ranges of computing systems, ranging from mobile systems to data centers to reduce the power at the times of low workload.

Phenom II is a family of AMD's multi-core 45 nm processors using the AMD K10 microarchitecture, succeeding the original Phenom. Advanced Micro Devices released the Socket AM2+ version of Phenom II in December 2008, while Socket AM3 versions with DDR3 support, along with an initial batch of triple- and quad-core processors were released on February 9, 2009. Dual-processor systems require Socket F+ for the Quad FX platform. The next-generation Phenom II X6 was released on April 27, 2010.

AMD FX is a series of high-end AMD microprocessors for personal computers debuted in 2011, claimed as AMD's first native 8-core desktop processor. The line was introduced with the Bulldozer microarchitecture at launch, and was then succeeded by its derivative Piledriver in 2012.

## References

1. "Overclocking" early processors was as simple – and as limited – as changing the discrete clock crystal ... The advent of adjustable clock generators has allowed "overclocking" to be done without changing parts such as the clock crystal."-- Overclocking Guide Part 1: Risks, Choices and Benefits : Who Overclocks? by Thomas Soderstrom
2. Chiappetta, Marco (23 September 2011). "AMD Breaks 8 GHz Overclock with Upcoming FX Processor, Sets World Record. The record has been surpassed with 8794 mhz of overclocking with AMD FX 8350". HotHardware. Retrieved 2012-04-28.
3. Michael Swaine. "Backward to the Future". Dr. Dobb's Journal. 2004.
4. Michael P. Frank. "Reversible Computing: A Requirement for Extreme Supercomputing".
5. Matthew Arthur Morrison. "Theory, Synthesis, and Application of Adiabatic and Reversible Logic Circuits For Security Applications". 2014.
6. "IBM Details World's Fastest Graphene Transistor". PCWorld. 2010-02-05. Retrieved 2019-04-23.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.