A single clock cycle (typically lasting only a few nanoseconds in modern 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 CPU's 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. 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, percentagewise. As first pointed out by Percival Perkins, this way the pertinent percental increase in performance will be maximized relative to cost.
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 (4,770,000 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 as fast (underclocking) will make the CPU run at half the speed. It will also make the CPU produce roughly half as much waste heat.
Some people try to speed up a CPU by replacing the oscillator crystal with a faster crystal (overclocking). However, those people will soon hit one or another of these 2 limits on clock rate:
People 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 slower clock rate as older CPUs, get more done per cycle. (See also Moore's Law).
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 speeds 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.
In the early 1990s, most computer companies advertised their computers' speed 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 speed available—the fastest Intel processor available at the time ran at 100 MHz. This superiority in clock speed, however, was meaningless since the PowerPC and Pentium CPU architectures were completely different. The Power Mac was faster at some tasks but slower at others.
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 speed increases (which have been constrained by CPU power dissipation issues).
However, the transistor count has continued to increase as predicted by Moore's Law. And with the recent discovery that graphene nanoribbons may be able to replace silicon as the semiconductor in processors at least one researcher predicts that clock speeds in the terahertz range may be possible..