The CPU is often considered the ‘brain’ of the computer because it does all of the processing of the data and instructions cpu-braininside the computer system.

There are different factors that can affect the performance of the CPU such as clock speed, number of cores and cache size (which we will look at in a later section).

You may hear the term ‘CPU Architecture’ and this is the way that a CPU is built that will help all of the parts communicate with each other; the two main types of architecture are known as ‘Von Neumann’ and ‘Harvard’ (for your GCSE you just need to know about Von Neumann).

Let’s take a look at the main parts of the CPU…



The Control Unit is in overall control of the CPU. It has one main job which is to run the program instructions carried out by following the fetch-decode-execute cycle. In addition,  it controls the flow of data inside the CPU (to registers, ALU, cache) and outside of the CPU (to memory and input/output devices).



The ALU is responsible for doing all the calculations; this includes addition, subtraction, comparisons, multiplication and division, as well as logic operations and binary shifts. The ALU uses registers to store the temporary results of calculations.


The cache is very fast memory inside of the CPU; it’s slower than registers but faster than RAM. It stores the most recent and most often used data so the CPU can access it quickly when it is needed next, otherwise it will get it from the RAM. The cache has a low capacity and they are very expensive compared to RAM; however, there are different levels of cache.

  • L1 is the quickest and has the lowest capacity
  • L2 is slower than L1 but can hold more than L1
  • L3 is slower than L2 but can hold more than L2


The clock sends out a signal that switches between zero and one; this synchronises when instructions will be carried out (like a metronome!). The number of clock cycles (or ticks) per second is called the clock speed.


Buses are a collection of wires that are used to transmit data between the CPU components and other parts of the computer system. A processor may use separate buses for data, instructions and memory locations.

*AQA GCSE only.