From Wikipedia, the free encyclopedia
The clock rate typically refers to the
at which a chip like a
(CPU), one core of a , is running and is used as an indicator of the 's speed. It is measured in clock cycles per second or its equivalent, the
(Hz). The clock rate of the first generation of computers was measured in hertz or kilohertz (kHz), but 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 impact performance.
and CPU manufacturers commonly select their highest performing units from a manufacturing batch and set their maximum clock rate higher, fetching a higher price.
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Manufacturers of modern processors typically charge premium prices for processors that operate at higher clock rates, a practice called . 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., 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., 1.3 GHz, and sold at a lower price.
The clock rate of a CPU is normally determined by the
of an . Typically a
produces a fixed —the frequency reference signal. Electronic circuitry translates that into a
at the same frequency for digital electronics applications (or, in using a , some fixed multiple of the crystal reference frequency). The
inside the CPU carries that
to all the parts that need it. An
has a "clock" pin driven by a similar system to set the . With any particular CPU, replacing the crystal with another crystal that oscillates half the frequency ("") 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 (""). 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. 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.
The first electromechanical general purpose computer, the
operated at a frequency of about 5–10 Hz. The first electronic general purpose computer, the , used a 100 kHz clock in its cycling unit. As each instruction took 20 cycles, it had an instruction rate of 5 kHz.
The first commercial PC, the
(by MITS), used an Intel 8080 CPU with a clock rate of 2 MHz (2 million cycles per second). The original
(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
techniques in the PA-7100 and AXP 21064
respectively. In 1995,
chip ran at 100 MHz (100 million cycles per second). On March 6, 2000,
reached the 1 GHz milestone a few months ahead of Intel. In 2002, an Intel
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. A nanosecond is the time for electric signal to travel a distance of about 30 cm, coming close to the distances a signal travels in the computer.
As of 2011, the
for fastest CPU is by AMD with a
based FX chip "" to 8.805 GHz, trumping the maximum recorded 8.670 GHz performance of their next generation FX "Piledriver" chips.
As of mid-2013, the highest clock rate on a production processor is the , clocked at 5.5 GHz, which was released in August of 2012.
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 , although no reversible computers have yet been implemented.
The first fully reversible CPU, the Pendulum, was implemented using standard CMOS transistors in the late 1990s at MIT.
Engineers continue to find new ways to design CPUs so that they complete more instructions per clock cycle (achieving a lower
count), although it may run at the same or a lower clock rate as older CPUs. This is achieved through architectural techniques such as
which attempts to exploit
in the code.
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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
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 width of the CPU's , the latency of the memory, and the
architecture.
The clock rate alone is generally considered to be an inaccurate measure of performance when comparing different CPUs families. Software
are more useful. Clock rates can sometimes be misleading since the amount of work different CPUs can do in one cycle varies. For example,
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.
"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."--
by Thomas Soderstrom
Chiappetta, Marco (23 September 2011). . HotHardware.
Michael Frank. .
Michael Swaine. . Dr. Dobb's Journal. 2004.
Michael P. Frank. .
Matthew Arthur Morrison. . 2014.
This article is based on material taken from the
prior to 1 November 2008 and incorporated under the "relicensing" terms of the , version 1.3 or later.
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