System Architecture (Cambridge (CIE) A Level Computer Science) : Revision Note

Von Neumann Model

What is the Von Neumann model?

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

• The Von Neumann model consists of:

  • Central processing unit (CPU)

  • Able to access memory directly

  • Memory that can store programs as well as data (registers)

  • Stored programs that contain instructions that can be executed in order

Registers

• Extremely small, extremely fast memory located in the CPU

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

• Includes both:

  • General purpose registers

  • Special purpose registers.

• A general purpose register can store any data the CPU is currently working on

  • It is flexible and can be used for various operations

Special purpose registers

What is a special purpose register?

• A special purpose register has a dedicated role within the operation of the CPU

• There are a number of special purpose registers that control or track data

• Examples of special purpose registers includes:

  • The Program Counter (PC)

  • The Memory Address Register (MAR)

  • The Memory Data Register (MDR)

  • The Accumulator (ACC)

  • Current Instruction Register (CIR)

  • Index register (IX)

  • Status register (SR)

• For each of the registers you must know 

  • The name of the register

  • Its acronym

  • The purpose of the register