ReviseRush logoREVISERUSH

Cambridge IGCSE GCSE Computer Science

Cambridge: Hardware, CPU and logic — Cambridge IGCSE GCSE Computer Science revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 8 minutes to read.

Von Neumann architecture

  • The CPU contains the control unit, arithmetic logic unit, registers and buses. Key registers include the program counter (PC), memory address register (MAR), memory data register (MDR), current instruction register (CIR) and accumulator (ACC).
  • The fetch-decode-execute cycle: the PC holds the address of the next instruction, which is copied to the MAR; the instruction is fetched into the MDR then the CIR; the PC increments; the instruction is decoded and executed, often using the ACC.
  • Buses: the address bus is unidirectional and carries addresses, the data bus is bidirectional and carries data and instructions, and the control bus carries signals such as read and write.

Performance, memory and storage

  • Performance improves with a higher clock speed, more cores, and larger or faster cache memory, though software must be written to use multiple cores effectively.
  • RAM is volatile primary storage holding running programs and data; ROM is non-volatile and holds start-up instructions. Virtual memory uses secondary storage when RAM is full, causing slowdown from disk access.
  • Secondary storage types: magnetic (moving parts, cheap per GB), optical (laser reads pits and lands) and solid state (no moving parts, fast, limited write cycles). Embedded systems are dedicated computers built into devices.

Key terms

Control unit
CPU component that decodes instructions and coordinates activity.
ALU
Arithmetic logic unit performing calculations and logical comparisons.
Program counter
Register holding the address of the next instruction.
Accumulator
Register storing intermediate results of calculations.
Cache
Small fast memory holding frequently used instructions and data.
Volatile
Memory that loses its contents when power is removed.

Common exam traps

  • Saying the MAR stores data — it stores the address; the MDR stores the data or instruction.
  • Claiming more cores always doubles speed; software must support parallel execution.
  • Confusing RAM (volatile, working memory) with ROM (non-volatile, start-up).

Practice questions with answers

  1. 1. Which register holds the address of the next instruction?

    • • MDR
    • • ACC
    • • PC
    • • CIR

    Answer: PC

    The program counter tracks execution position.

  2. 2. State the three stages of the machine cycle.

    Answer: Fetch, decode, execute

    Repeated continuously while the computer runs.

  3. 3. The data bus is:

    • • Unidirectional
    • • Bidirectional
    • • Only for addresses
    • • Only for control signals

    Answer: Bidirectional

    Data flows both to and from memory.

  4. 4. Explain the role of the ALU.

    Answer: It performs arithmetic calculations and logical comparisons, using the accumulator for results

    It works with the control unit during execution.

  5. 5. Increasing cache size improves performance because:

    • • It stores files permanently
    • • Frequently used data is accessed faster than from RAM
    • • It replaces the CPU
    • • It increases clock speed

    Answer: Frequently used data is accessed faster than from RAM

    Fewer slow RAM accesses are needed.

Keep going in the app

This topic has 47 questions in total, plus flashcards, cram mode and Grade 9 Push challenges.

Open the full topic Quick-fire quiz

More free Computer Science revision

ReviseRush is an independent educational revision aid. It is not affiliated with, endorsed by or connected to AQA, Edexcel, OCR, WJEC, Pearson or any exam board. All notes and questions are original. Always check content against your official course and specification. No grade outcome is guaranteed.