Systems architecture: clock

The idea

The clock is a component that generates regular pulses to synchronise the operations of the CPU. Each pulse marks a step, and the fetch–decode–execute cycle advances in time with these signals so that components act in the correct order. A faster clock means more cycles per second, so more instructions can potentially be processed in a given time, which is one factor affecting CPU performance. For example, a processor running at a higher clock speed completes more cycles each second than a slower one. Exam questions often ask what the clock does or how clock speed affects performance, so link the clock to synchronisation and to the number of cycles per second rather than to storage.

What earns the marks

• The clock generates regular pulses/signals that synchronise the CPU's operations.
• Clock speed is measured in cycles per second (hertz), and a higher clock speed means more cycles per second.
• A higher clock speed can allow more instructions to be processed in a given time, improving performance.
• The clock does not store data or perform calculations; it provides timing signals.

Common mistakes and how to put them right

• Saying the clock stores instructions or data.

  Put it right by explaining that the clock only provides timing signals and does not hold data.

• Confusing clock speed with the amount of memory.

  Put it right by stating that clock speed is the number of cycles per second, not a measure of storage capacity.

• Assuming a faster clock always guarantees faster overall performance.

  Put it right by noting that clock speed is one factor among others, such as cache size and number of cores.

Exam tips

• State that the clock synchronises operations and times the fetch–decode–execute cycle.
• Use the unit hertz (cycles per second) when describing clock speed.
• When asked about performance, mention clock speed as one factor rather than the only factor.