Understand the purpose and components of the CPU, the fetch-execute cycle, and the factors that affect CPU performance.

What this dot point is asking

AQA wants you to describe the purpose of the CPU and its main components, explain the fetch-execute cycle in order, and explain the factors that affect CPU performance.

Purpose and components of the CPU

The components work as a team: the control unit fetches and interprets instructions and tells the ALU what to do; the ALU does the arithmetic and logic; the registers hold the small amounts of data each step needs because they are far faster to reach than RAM.

The fetch-execute cycle

Factors affecting performance

These factors interact. A high clock speed only helps if the CPU is not waiting on memory, which is why cache matters; and extra cores only help when the work can be split into parts that run at the same time. The clock speed in hertz means cycles per second, so a 3 GHz processor performs three billion cycles per second.

How the components cooperate during the cycle

It helps to see the components and the cycle together. During fetch, the control unit reads the address of the next instruction and copies that instruction from RAM into the CPU, storing it temporarily in a register. During decode, the control unit interprets the bit pattern to work out the operation and operands. During execute, the control unit directs the work: if it is a calculation, the values are sent to the ALU, which adds, subtracts or compares them and puts the result in a register; if it is a data move, values are copied between registers and memory. Registers are central throughout because they are the only stores fast enough to keep up with the cycle.

Clock speed, cores and cache together

The three performance factors improve the cycle in different ways. A higher clock speed runs more fetch-execute cycles per second, so more instructions complete each second. More cores let several cycles run at once on separate instruction streams, multiplying throughput when work can be split. More cache keeps the instructions and data a core needs close by, so fewer cycles are wasted waiting for slow RAM. A balanced processor improves all three, because a fast clock is held back if the CPU constantly waits for memory, and extra cores help only when the workload can be divided.

Try this

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

• Cue. Fetch, decode, execute.

Q2. State one factor that affects CPU performance and explain its effect. [2 marks]

• Cue. Higher clock speed means more cycles per second, so more instructions are executed per second.