Describe the role and main parts of the CPU, the fetch-execute cycle, and how clock speed and cores affect performance

What this dot point is asking

SEAB wants you to describe the role and main parts of the CPU, the fetch-execute cycle by which it runs every instruction, and how clock speed and the number of cores affect performance. The central idea is that the CPU is the worker of the computer, repeatedly fetching, decoding and executing instructions, and that doing this faster or in parallel makes the computer quicker.

The answer

The role of the CPU

The CPU (Central Processing Unit), or processor, is the part of the computer that carries out instructions. It fetches each instruction of a program from memory and executes it, processing data and controlling the rest of the machine. It is often called the brain of the computer.

Main parts of the CPU

At O-Level you should know the CPU contains:

• The control unit (CU), which directs the operation of the processor and tells other parts what to do.
• The arithmetic logic unit (ALU), which performs calculations and logical comparisons.
• Registers, which are tiny, very fast stores inside the CPU for holding instructions and data being worked on right now.

The fetch-execute cycle

A program is a list of instructions in memory. The CPU runs them using the fetch-execute cycle, repeated over and over:

1. Fetch: the next instruction is copied from main memory into the CPU.
2. Decode: the control unit works out what the instruction means.
3. Execute: the CPU carries it out (for example the ALU does a calculation, or data is moved).

Fetch -> Decode -> Execute -> Fetch -> Decode -> Execute -> ...

The cycle repeats billions of times per second.

Clock speed

The clock speed measures how many cycles the CPU performs each second, in hertz. A speed of $3\ \text{GHz}$ is about $3$ billion cycles per second. A higher clock speed means more instructions processed per second, so the computer is faster (other things being equal).

Cores

A core is a processing unit within the CPU that can run its own fetch-execute cycle. A CPU with several cores can work on several instructions or tasks at the same time (in parallel), doing more work overall, especially for software designed to use multiple cores.

Examples in context

Example 1. Opening an app. Tapping an app sends its instructions to memory, and the CPU fetches, decodes and executes them in turn to draw the screen and respond to taps. A phone with a faster CPU runs this cycle quicker, so the app opens and responds more smoothly.

Example 2. Editing video. Video editing splits work across many CPU cores, so each core processes part of the footage at once. This is why a computer with more cores renders video faster than one with the same clock speed but fewer cores, showing the benefit of parallel processing.

Try this

Q1. Name the three stages of the fetch-execute cycle. [3 marks]

• Cue. Fetch, decode, execute.

Q2. State what clock speed measures. [2 marks]

• Cue. The number of cycles the CPU performs per second, measured in hertz (for example GHz, billions per second).

Q3. Explain how more cores can make a computer faster. [2 marks]

• Cue. Each core can run its own fetch-execute cycle, so several instructions or tasks are processed in parallel, doing more work at once.