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CPU Components

FoundationHigherAQA

Learn CPU Components for GCSE Computer Science with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA GCSE Computer Science (8525). The CPU contains the arithmetic logic unit, the control unit, registers and cache.

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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 CPU contains several components, each with a specific job, and the exam expects each to be named and explained.

The arithmetic logic unit performs all calculations and logical comparisons. The control unit directs the operation of the processor, decoding instructions and coordinating the other components. Cache is very fast memory inside the CPU holding frequently used instructions and data.

Registers are small, extremely fast storage locations within the CPU. The program counter holds the address of the next instruction. The memory address register holds the address currently being read from or written to. The memory data register holds the data itself. The accumulator stores the results of calculations.

Revision notes

ALU, control unit and cache

Arithmetic logic unit: performs all calculations and logical comparisons.

Control unit: directs the processor's operation, decoding instructions and coordinating the other components. Cache: very fast memory inside the CPU holding frequently used instructions and data.

The registers

Program counter: holds the address of the NEXT instruction to be fetched.

Memory address register (MAR): holds the address being read from or written to. Memory data register (MDR): holds the data or instruction itself. Accumulator: stores the results of calculations.

Distinguishing MAR from MDR

The MAR holds an ADDRESS — where in memory to look.

The MDR holds the DATA found there. Confusing the two is the most frequent error in fetch-execute questions, and the distinction is simply address versus contents.

Key points

  • The ALU performs calculations and comparisons.
  • The control unit directs the processor.
  • Cache is fast memory inside the CPU.
  • The program counter holds the next instruction address.
  • The MAR holds an address.
  • The MDR holds the data at that address.

Worked examples

Example 1

State the function of the arithmetic logic unit. [1 mark]

Working

It performs all calculations and logical comparisonsstate the function

Example 2

Explain the difference between the MAR and the MDR. [2 marks]

Working

The MAR holds the address in memory being read from or written todescribe the MAR
The MDR holds the actual data or instruction found at that addressdescribe the MDR

Example 3

State what the program counter holds. [2 marks]

Working

The address of the next instruction to be fetchedstate the contents
It is incremented after each fetch so the CPU moves through the program in orderexplain how it changes

Common mistakes

  • Confusing the MAR and MDR.

    MAR holds an address; MDR holds the data at that address.

  • Saying the program counter holds the current instruction.

    It holds the address of the NEXT one.

  • Saying the control unit performs calculations.

    That is the ALU's job.

  • Calling cache the same as RAM.

    Cache is much faster and sits inside the CPU.

Exam tips

  • Learn each component with its specific job.
  • Remember MAR is address, MDR is data.
  • Say the program counter holds the NEXT address.
  • Keep the ALU and control unit roles distinct.

Key terms

ALU
The unit performing calculations and comparisons.
Control unit
The component directing the processor's operation.
Program counter
A register holding the next instruction's address.
Accumulator
A register storing calculation results.

Related topics

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