Clock rate
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The clock rate is the fundamental rate in cycles per second (measured in hertz) for the frequency of the clock in any synchronous circuit. For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave pulse [typically] for digital electronics applications. In this context the use of the word, speed (physical movement), should not be confused with frequency or its corresponding clock rate. Thus, the term "clock speed" is a misnomer.
A single clock cycle (typically shorter than a nanosecond in modern non-embedded microprocessors) toggles between a logical zero and a logical one state. Historically, the logical zero state of a clock cycle persists longer than a logical one state due to thermal and electrical specification constraints.
CPU manufacturers typically charge premium prices for CPUs that operate at higher clock rates. 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 relatively lower price.[1] [2] Those looking to "overclock" a CPU to its maximum would be well-advised to purchase the highest clock rate sold for that CPU, since it has been tested at the highest standards for that CPU. However when going for a good price to performance ratio when buying a CPU it often pays off to get a lower clocked version of a CPU which can be "overclocked" the furthest compared to other CPUs from that same CPU family, percentage-wise.
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[edit] Limits to clock rate
The clock rate of a CPU is normally determined by the frequency of an oscillator crystal. The first commercial PC, the Altair 8800 (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 Pentium chip ran at 100 MHz (100 million cycles/second), and in 2002, an Intel Pentium 4 model was introduced as the first CPU with a clock rate of 3 GHz (three billion cycles/second corresponding to ~3.3 10-10seconds per cycle).
With any particular CPU, replacing the crystal with another crystal that oscillates half the frequency ("underclocking") will generally make the CPU run at half the performance. It will also make the CPU produce roughly half as much waste heat.
Some people try to increase performance of a CPU by replacing the oscillator crystal with a higher frequency crystal ("overclocking").[1] However, those people will soon hit one or another of these 2 limits on clock rate:
- After each clock pulse, the signal lines inside the CPU need time to settle to their new state. If the next clock pulse comes in too soon, while the signals are still settling (before every signal line has finished transitioning from 0 to 1, or from 1 to 0), 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).
- 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 lots of transitions, higher clock rates produce more heat. If electricity is converted to heat faster than a particular computer cooling system can get rid of it, then the transistors may get hot enough to be destroyed.
Engineers continue to find new ways to design CPUs that settle a little quicker 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, although no reversible computers have yet been implemented.
People also continue to find new ways to design CPUs such that, although they may run at the same or a lower clock rate as older CPUs, get more instructions completed per clock cycle. (See also Moore's Law).
[edit] Comparing
The clock rate of a computer is only useful for providing comparisons between computer chips in the same processor family. An IBM PC with an Intel 486 CPU running at 50 MHz will be about twice as fast as one with the same CPU, memory and display 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 with different functionality. Furthermore, there are many other factors to consider when comparing the performance of entire computers, like the clock rate of the computer's front side bus (FSB), the clock rate of the RAM, the width in bits of the CPU's bus and the amount of Level 1, Level 2 and Level 3 cache.
Clock rates should not be used when comparing different computers or different processor families. Rather, some software benchmark should be used. Clock rates can be very misleading since the amount of work different computer chips can do in one cycle varies. For example, RISC CPUs tend to have simpler instructions than CISC CPUs (but higher clock rates), and 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 quality of the computer regardless of clock rate.
[edit] History
In 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 8100/110 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 superiority in clock rate, however, was meaningless since the PowerPC and Pentium CPU architectures were completely different.
After 2000, Intel's competitor, AMD, 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" 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 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 sets, and the development of multi-core processors, rather than clock rate increases (which have been constrained by CPU power dissipation issues).
[edit] References
- ^ "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
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
