Von Neumann & Harvard Architecture (OCR A Level Computer Science)

Revision Note

Author

Expertise

Maths

A computer architecture describes how it uses the components and instructions to make the computer function
There are 2 common types of computer architecture:
- Von Neumann architecture
- Harvard architecture

Architecture	Memory Organisation	Bus	Address Space	Control Units	Usage
Von Neumann	Unified Memory	Single	Shared	Single Control Unit	Most modern computers, microcontrollers
Harvard	Separated Memory	Separate	Distinct	Separate Control Units	Specialized embedded systems

Layout of Von Neumann Architecture

Von Neumann's architecture includes:

Control Unit (CU)
- The control unit controls the operation of the processor and its components
- It retrieves instructions stored in memory, decodes or interprets them, and then executes them
- The CU generates timing signals and controls the other units of the computer
Arithmetic Logic Unit (ALU)
- The ALU carries out all the arithmetic and logic operations in the computer such as:
  - Addition
  - Subtraction
  - Multiplication
  - Division
  - Comparisons, etc.
Special registers within the CPU
A single set of buses to connect the CPU to memory and Input/Output
- These are communication systems that transfer data between the components inside a computer, or between computers
- There are 3 types of buses:
  - The data bus carries data
  - The address bus carries the addresses where data must be picked up or stored
  - The control bus carries signals relating to the control and coordination of all the activities within the computer

Memory (RAM)
- The memory unit stores both data and instructions for processing
- These are stored in the same format
- The memory is divided into cells, each of which can be accessed by their address
- The memory is a linear or sequential array of bytes

Layout of Harvard Architecture

Harvard architecture includes:

Separate Instruction and Data Memory
- In Harvard architecture, the system has separate memory units for storing data and instructions
- This separation provides greater speed and efficiency as the system can fetch data and instructions simultaneously from separate buses, without one interfering with the other
Separate Instruction and Data Buses
- Harvard architecture uses separate buses for data and fetching instructions, meaning that data transfers do not interfere with instruction fetches
- This can lead to better overall system performance
Control Unit (CU)
- The control unit controls the operation of the processor and its components
- It retrieves instructions stored in memory, decodes or interprets them, and then executes them
- The CU generates timing signals and controls the other units of the computer
Arithmetic Logic Unit (ALU)
- The ALU carries out all the arithmetic and logic operations in the computer such as:
  - Addition
  - Subtraction
  - Multiplication
  - Division
  - Comparisons, etc.

There are several things that Harvard architecture has or can do which can improve performance over Von Neumann architecture:

Feature	Explanation
Two separate areas of memory	One for instructions and one for data Instructions and data can be accessed concurrently
Different sets of buses	One for instructions and one for data Instructions and data can be accessed concurrently
Pipelining	Whilst an instruction is being executed the next can be decoded and the subsequent one fetched
Use of Cache	A small amount of high performance memory which is next to the CPU It stores frequently used data and instructions
Virtual cores / Hyper-threading	This is where a physical core can act as two virtual cores
Multiple Cores	Each core acts as a separate processing unit
Onboard Graphics	Built-in circuitry for graphics processing
Performance boosting mode	The clock speed can be temporarily increased for a performance boost
Out of Order Execution	Instructions can be executed before earlier ones if they are ready
Super Scalar	Multiple instructions can be executed simultaneously

You will not be asked about specific aspects of “contemporary processor architecture” apart from those on this page. You may be asked to show an awareness of how contemporary processors differ from a pure Von Neumann architecture in more open questions

to absolutely everything:

the (exam) results speak for themselves:

Did this page help you?