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{{Short description|Frequency at which a CPU chip or core is operating}}
{{Redirect|Clocking|the practice of tampering with odometers to read less mileage|Odometer fraud}}
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<!-- [[File:Microprocessor clock speed, OWID.svg|thumb
[[File:CPU_clock_speed_and_Core_count_Graph.png|thumb|upright=1.8|Microprocessor clock speed measures the number of pulses per second generated by an oscillator that sets the tempo for the processor. It is measured in hertz (pulses per second).]]
In [[computing]], the '''clock rate''' or '''clock speed''' typically refers to the [[frequency]] at which the [[clock generator]] of a [[Microprocessor|processor]] can generate [[Clock signal|pulses]], which are used to [[Synchronization (computer science)|synchronize]] the operations of its components,<ref>{{FOLDOC|new=yes|Clock}}</ref> and is used as an indicator of the processor's speed. It is measured in the [[International System of Units|SI]] unit of frequency [[hertz]] (Hz).
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==Determining factors==
=== Binning ===
[[File:Clock signal and clock rate.png|thumb|upright=1.8|Representation of a clock signal and clock rate]]
Manufacturers of modern processors typically charge higher prices for processors that operate at higher clock rates, a practice called [[Product binning|binning]]. For a given CPU, the clock rates are determined at the end of the manufacturing process through
===Engineering===
The clock rate of a CPU is normally determined by the [[frequency]] of an [[Crystal oscillator|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,
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.
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==Historical milestones and current records==
The first fully mechanical analog computer, the [[Z1 (computer)|Z1]], operated
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 Personal Computer|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 (DEC) exceeded 100 MHz with [[RISC]] techniques in the PA-7100 and AXP 21064 [[DEC Alpha]] respectively. In 1995, [[Intel Corporation|Intel's]] [[P5 (microarchitecture)|P5]] [[Pentium (brand)|Pentium]] chip ran at 100 MHz (100 million cycles per second). On March 6, 2000, [[Advanced Micro Devices|AMD]] demonstrated passing the 1 GHz milestone a few days ahead of Intel shipping 1 GHz in systems. 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 ~ 0.33 [[nanosecond]]s per cycle). Since then, the clock rate of production processors has increased more slowly, with performance improvements coming from other design changes.
Set in 2011, the [[Guinness World Record]] for the highest CPU clock rate is 8.42938 GHz with an [[Overclocking|overclocked]] AMD FX-8150 [[Bulldozer (microarchitecture)|Bulldozer]]-based chip in an [[Liquid helium|LHe]]/[[Liquid nitrogen|LN2]] cryobath, 5 GHz [[Air cooling|on air]].<ref>{{Cite web|url=https://www.guinnessworldrecords.com/world-records/98281-highest-clock-frequency-achieved-by-a-silicon-processor|title = Highest clock frequency achieved by a silicon processor}}</ref><ref>{{cite web |first=Marco |last=Chiappetta |date=23 September 2011 |url=http://hothardware.com/News/AMD-Breaks-Frequency-Record-with-Upcoming-FX-Processor/ |title=AMD Breaks 8 GHz Overclock with Upcoming FX Processor, Sets World Record with AMD FX 8350 |publisher=HotHardware |access-date=2012-04-28 |archive-date=2015-03-10 |archive-url=https://web.archive.org/web/20150310020437/http://hothardware.com/News/AMD-Breaks-Frequency-Record-with-Upcoming-FX-Processor |url-status=dead }}</ref> This is surpassed by the [[CPU-Z]] [[overclocking]] record for the highest CPU clock rate at 8.79433 GHz with an AMD FX-8350 [[Piledriver (microarchitecture)|Piledriver]]-based chip bathed in [[Liquid nitrogen|LN2]], achieved in November 2012.<ref>{{Cite web |title=CPU-Z Validator – World Records |url=https://valid.x86.fr/records.html
|url=https://www.digitaltrends.com/computing/overclockers-surpass-elusive-9ghz-new-world-record/ |work=digitaltrends |location= |access-date=20 January 2023}}</ref>
The highest [[Dynamic frequency scaling|base clock]] rate on a production processor is the [[
== 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
{{Cite web |last=Frank |first=Michael |title=The Reversible and Quantum Computing Group (Revcomp) |url=https://www.cise.ufl.edu/research/revcomp/ |access-date=2024-03-17 |website=www.cise.ufl.edu}}
</ref><ref>
{{Cite web |last=Swaine |first=Michael |date=2004 |title=Backward to the Future |url=http://www.drdobbs.com/backward-to-the-future/184405563 |access-date=2024-03-17 |website=Dr. Dobb's}}
</ref><ref>
Michael P. Frank. [http://www.zettaflops.org/PES/frank.html "Reversible Computing: A Requirement for Extreme Supercomputing"].▼
▲[http://www.zettaflops.org/PES/frank.html "Reversible Computing: A Requirement for Extreme Supercomputing"].
</ref><ref>
Matthew Arthur Morrison. [http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=6278&context=etd "Theory, Synthesis, and Application of Adiabatic and Reversible Logic Circuits For Security Applications"]. 2014.▼
▲[http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=6278&context=etd "Theory, Synthesis, and Application of Adiabatic and Reversible Logic Circuits For Security Applications"].
</ref>
Engineers also continue to find new ways to design CPUs so that they complete more instructions per clock cycle, thus achieving a lower [[cycles per instruction|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.
== Comparing ==
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