The clock rate, or clock speed, of a computer is the rate at which a central processing unit (CPU) is able to perform basic functions. It is normally measured in megahertz, or millions of cycles per second, or even gigahertz, which are billions of cycles per second. The clock rate is useful for comparing the speed of chips made by the same company, but is not a reliable way to compare different types of computers because many other factors can determine the speed of a computer. Instead, computers should be compared based on how quickly they perform specific tasks such as loading graphics or performing calculations.
All computers have clocks that emit regular electrical pulses to help the computer synchronize the timing of tasks it is asked to perform. The CPU requires a certain number of clock cycles, or pulses from the computer's clock, to execute a command. The number of cycles a computer can achieve in a second determines its clock rate. The first PCs created by IBM in the early 1980s had a clock rate of around 4.77 MHz, which is about 5 million cycles per second. By 2010, the clock rate of the average computer could be measured in GHz.
Though clock rate most commonly refers to the speed of the CPU, other devices in a computer also have a clock. Expansion buses, which allow the computer to be modified using printed circuit boards, are governed by internal clocks. Random access memory (RAM), the read and write memory used to perform work while the computer is running, uses a clock as well.
The CPU can only process as quickly as the other components in the computer allow. If the expansion bus or the RAM are slower than the central processing unit, the computer will not run as quickly as the CPU allows. In addition, some processors may perform different tasks at different rates. For example, one may be able to divide two numbers in 12 cycles while another takes 20 to perform the same task. For this reason, clock rate is not a reliable way to compare computers.
Benchmarks are a more reliable source of comparison. Several organizations offer reliable and monitored industry standard benchmarks. These include Standard Performance Evaluation Corporation (SPEC), the Transaction Processing Performance Council (TPC), and the Embedded Microprocessor Benchmark Corsortium (EEMBC). These companies run computers through a series of closely controlled tests to determine how quickly each one performs specific tasks. The numbers are published free of charge on organization websites.