Systems architecture: clock speed

The idea

Clock speed is the rate at which the CPU's internal clock pulses, controlling how many fetch–decode–execute cycles the processor completes each second. It is measured in hertz (Hz), with modern processors often running at gigahertz (GHz) rates; 1 GHz equals 1 × 10⁹ cycles per second. A higher clock speed allows more cycles per second, so in principle more instructions are processed in a given time and performance improves. The benefit is not unlimited: slow main memory, heat generation, power consumption and cooling can all prevent extra cycles from producing extra completed work. In a multiple-choice question you must pick the option that correctly identifies clock speed as cycles per second measured in hertz, and reject distractors that confuse it with core count, cache size or memory capacity.

What earns the marks

• Identifies clock speed as the number of cycles the CPU completes per second.
• Recognises hertz (Hz) or gigahertz (GHz) as the correct unit of clock speed.
• Distinguishes clock speed from the number of cores, cache size and memory capacity.
• Explains that a higher clock speed can increase performance by allowing more instructions to be processed per second.

Common mistakes and how to put them right

• Selecting an option that describes clock speed as the number of cores.

  Put it right: , cores are separate processing units while clock speed is cycles per second.

• Choosing an option that gives the unit as bytes or gigabytes.

  Put it right: , clock speed is measured in hertz.

• Assuming a higher clock speed always guarantees faster performance.

  Put it right: , memory speed, heat and power can limit the gain.

Exam tips

• Read each option carefully and eliminate any that give the wrong unit, such as bytes instead of hertz.
• Remember that 1 GHz equals 1 × 10⁹ cycles per second when comparing values.
• If two options seem similar, choose the one that names cycles per second and hertz explicitly.