1.2 The main components of computer systems

2026 Syllabus Objectives

  • Central Processing Unit (CPU): The role of the CPU in processing instructions entered into the computer in order to produce an output
  • Internal memory: Characteristics of ROM and RAM; Differences between ROM and RAM
  • Input and output devices: Characteristics of input and output devices; Differences between input and output devices
  • Backing storage: Characteristics of backing storage; Differences between backing storage and internal memory

Central Processing Unit (CPU)

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Role of the CPU 🔑

The Central Processing Unit (CPU) is the part of the computer which interprets and executes the commands from the computer hardware and software. The CPU acts as the "brain" of the computer system.

Key functions of the CPU:

  • Takes an input from input devices
  • Processes the data according to instructions
  • Produces an output for the user

CPU Evolution: From Discrete Components to Microprocessors

Historically, CPUs were made up of discrete components and numerous small integrated circuits, which were combined together on one or more circuit boards. However, due to modern manufacturing techniques, the term microprocessor is now used instead of CPU.

A microprocessor is a single integrated circuit which is at the heart of most PCs and is also found in many household devices and equipment where some control or monitoring is needed (e.g., the engine management system in a car).

The Fetch-Execute Cycle ⚡

The CPU runs through the Fetch-Execute Cycle billions of times per second to make a computer work. This cycle consists of three main stages:

1. Fetch Stage

  • The next instruction or data is fetched from the computer's memory (RAM)
  • The instruction or data is brought back to the CPU

2. Decode Stage

  • The CPU needs to work out what is required from the instruction
  • This could be a range of tasks depending on what the instruction or data included

3. Execute Stage

  • The CPU carries out the instruction that was fetched
  • Examples of actions at this stage:
    • Performing a calculation
    • Storing a result or data back in main memory (RAM)
    • Going to main memory to fetch data from a different location

Important: The CPU continuously repeats this Fetch-Decode-Execute cycle to process all instructions and produce outputs.

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