How computers work

By Charlotte Jones

The Central Processing Unit (CPU)

CPU Speed

A computer’s speed is heavily influenced by the CPU it uses. There are three main factors that affect how quickly a CPU can carry out instructions:

  • clock speed
  • cores
  • cache

Clock Speed

CPUs can only carry out one instruction at a time.

It might seem like CPUs can perform many instructions simultaneously, since it is possible for you to do homework, read instant messages and listen to music at the same time. However, the CPU is able to carry out instructions at such speed that it can seem like it is simultaneous.The speed at which the CPU can carry out instructions is called the clock speed. This is controlled by a clock. With every tick of the clock, the CPU fetches and executes one instruction. The clock speed is measured in cycles per second, and one cycle per second is known as 1 hertz. This means that a CPU with a clock speed of 2 gigahertz (GHz) can carry out two thousand million (or two billion) cycles per second.


A CPU is traditionally made up of a processor with a single core. Most modern CPUs have two, four or even more cores.

A CPU with two cores, called a dual core processor, is like having two processors in one.

A quad core processor has four cores and can carry out even more instructions in the same period of time. The main downside of using quad core processors is that they are more expensive to design and make, and they also use more power than single or dual core processors. Another disadvantage is that the instructions have to be split up to decide which core will execute them and the results have to be merged together again at the end, which slows the processor down a little.


A cache (pronounced ‘cash’) is a tiny block of memory built right onto the processor. The most commonly used instructions and data are stored in the cache so that they are close at hand. The bigger the cache is, the more quickly the commonly used instructions and data can be brought into the processor and used

The Fetch-Execute Cycle

The basics operation of a computer is call the 'fetch-execute' cycle. The computer fetches the instructions from its memory and then executes it. This is done repeatedly from when the computer is booted up to when it is shut down.

Fetching The Instruction

The first step the fetch-execute cycle carries out is fetching the instruction, which is either a program or data. The CPU fetches this from the main memory (the hard drive) and stores it in the CPU temporary memory, the immediate access store (the registers).

Once the instruction has been fetched, the CPU will need to understand the instruction to action it. This is called deconding.

Executing The Instruction

When the instruction has been decoded, the CPU can carry out the action that is needed. This is called expecuting the instruction. The CPU is designed to understand a set of instructions.