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The CPU and Main Memory

FoundationHigherAQA

Master The CPU and Main Memory for GCSE Computer Science with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers. The CPU fetches and executes the instructions held in main memory.

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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA specification. Every worksheet comes with a full mark scheme.

Foundation

WorksheetFoundation · PDFMark SchemeFoundation · PDF

Higher

WorksheetHigher · PDFMark SchemeHigher · PDF

Topic overview

The central processing unit is the component that executes program instructions, and main memory holds the instructions and data it is working on.

The CPU fetches each instruction from main memory, decodes it to work out what it means, then executes it. This repeats continuously while the computer is running.

Main memory, usually RAM, is directly accessible by the CPU and holds the programs and data currently in use. It is volatile, meaning its contents are lost when power is removed. Secondary storage is not directly accessible by the CPU — data must be copied into main memory before it can be used, which is why loading a program takes time.

Revision notes

What the CPU does

The CPU executes program instructions.

It fetches each instruction from main memory, decodes it to determine what is required, then executes it. This cycle repeats continuously while the computer runs.

Main memory

Main memory holds the programs and data currently in use.

It is directly accessible by the CPU, which is what makes it fast enough to work from. RAM is volatile — its contents are lost when power is removed.

Why secondary storage is different

Secondary storage is not directly accessible by the CPU.

Data must first be copied into main memory before the CPU can use it, which is why opening a large program takes noticeable time. Secondary storage is non-volatile, so it retains data without power.

Key points

  • The CPU executes program instructions.
  • It fetches, decodes and executes each instruction.
  • Main memory holds programs and data in use.
  • Main memory is directly accessible by the CPU.
  • RAM is volatile and loses data without power.
  • Secondary storage must be copied to memory first.

Worked examples

Example 1

State the three stages the CPU repeats when running a program. [3 marks]

Working

Fetch the instruction from main memoryname the first stage
Decode it to work out what is requiredname the second stage
Execute the instructionname the third stage

Example 2

Explain why data must be copied from secondary storage into main memory. [2 marks]

Working

The CPU can only directly access main memory, not secondary storagestate the limitation
so the data must be copied into main memory before the CPU can work with itexplain the consequence

Example 3

Explain what is meant by saying RAM is volatile. [2 marks]

Working

Its contents are lost when the power is removeddefine volatile
which is why work must be saved to non-volatile secondary storage before shutting downexplain the implication

Common mistakes

  • Saying the CPU accesses secondary storage directly.

    Data must be copied into main memory first.

  • Confusing volatile and non-volatile.

    Volatile loses data without power; non-volatile retains it.

  • Giving only two stages of the cycle.

    Fetch, decode and execute are all required.

  • Saying main memory stores everything permanently.

    RAM is volatile; permanent storage is secondary.

Exam tips

  • Learn fetch, decode, execute in order.
  • State that only main memory is directly accessible.
  • Define volatile precisely.
  • Link volatility to the need to save work.

Key terms

CPU
The component executing program instructions.
Main memory
Memory directly accessible by the CPU.
Volatile
Losing contents when power is removed.
Secondary storage
Non-volatile storage not directly accessible by the CPU.

Related topics

Written and reviewed against the current AQA specification. Spotted an error? Let us know.