Von Neumann Architecture (Cambridge (CIE) IGCSE Computer Science)

Revision Note

James Woodhouse

Written by: James Woodhouse

Reviewed by: Lucy Kirkham

Von Neumann Architecture

What is the Von Neumann architecture?

  • The Von Neumann Architecture is a design of the CPU which was proposed by Mathematician John Von Neumann in the 1940s, which most general-purpose computers are built upon

  • The Von Neumann Architecture outlines how the computer memory, input/output devices and processor all work together 

The Von-Neumann-architecture
The Von-Neumann architecture
  • The Von Neumann architecture consists of:

    • Control unit (CU)

    • Arithmetic logic unit (ALU)

    • Registers

    • Buses

What is the function of each component?

Arithmetic logic unit (ALU)

  • Performs arithmetic operations

  • Performs logical decisions

    • IF X > 5 THEN
            DO ……….

Control unit (CU)

  • Coordinates how data moves around the CPU by sending a signal to control the movement of the data 

  • Decodes the instructions fetched from memory

Registers

  • Extremely small, extremely fast memory located in the CPU

  • Hold small amounts of data needed as part of the fetch-execute cycle

  • Each register has its own specific purpose

  • It consists of 5 main registers

    • The Program Counter (PC)

    • The Memory Address Register (MAR)

    • The Memory Data Register (MDR)

    • The Accumulator (ACC)

    • Current Instruction Register (CIR)

  • For each of the registers you must know 

    • The name of the register

    • Its acronym

    • The purpose of the register

Name

Acronym

Purpose

Program Counter

PC

  • Holds the memory address of the next instructions to be executed 

  • Increments by 1 as the fetch-decode-execute cycle runs

Memory Address  Register

MAR

  • Holds the memory address of where data or instructions are to be fetched from memory

Memory Data Register

MDR

  • Stores the data or instruction which has been fetched from memory

Current Instruction Register

CIR

  • Stores the instruction the CPU is currently decoding or executing

Accumulator

ACC

  • Stores the results of any calculations that have taken place in the Arithmetic Logic Unit (ALU)

Buses 

  • Components within the CPU and wider computer system are connected by buses

  • A bus is a set of parallel wires through which data/signals are transmitted from one component to another

  • There are 3 types of bus:

    • Address - unidirectional, carries location data (addresses), data is written to/read from

    • Data - bidirectional, carries data or instructions

    • Control - bidirectional, carries commands and control signals to tell components when they should be receiving reads or writes etc..

3-1-computer-architecture--von-neumann-architecture-1

Examiner Tips and Tricks

If asked to describe the purpose of the PC, MAR or the MDR make sure you explain how the data is being fetched or written to memory

Worked Example

Describe the role of the control unit, the control bus, the data bus and the address bus when fetching an instruction from memory [4]

Answer

  • The address of memory (holding instruction) is placed on the address bus (1)

  • The control unit sends a signal (1) on the control bus (to start a read operation) (1)

  • The instruction is/the contents of the memory are placed on the data bus (1)

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James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.

Lucy Kirkham

Author: Lucy Kirkham

Expertise: Head of STEM

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.