Von Neumann Architecture | Computer for GCSE/IGCSE - Year 11 PDF Download

The components in a CPU, in a computer that has a Von Neumann architecture

• In the 1940s, John Von Neumann pioneered the concept of the stored program computer.
• The Von Neumann computer architecture, utilized by most modern computers, is founded on this concept.
  • Central to the stored program concept and Von Neumann architecture is the storage of both data and instructions in the same memory (RAM) as binary.
  • Another hallmark of Von Neumann architecture is the central processing unit (CPU) fetching instructions from memory and executing them sequentially.
  • Subsequently, the CPU stores the results back into memory. 

Components of the Central Processing Unit

• The primary function of the CPU is to carry out instructions and handle data.
• The CPU comprises two key units: the Control Unit (CU) and the Arithmetic Logic Unit (ALU).
  • The Control Unit manages data flow within the CPU and issues control signals to various components for operations like reading, writing, and arithmetic calculations.
  • It interprets instructions, controls operation timing (clock speed), and decodes instructions into opcode and operand.
  • The ALU executes arithmetic operations like addition and subtraction, as well as logical operations including comparison.
  • Interim results are stored in the ALU's built-in register before being transferred to the Memory Data Register (MDR).
• The CPU also features several registers, small memory locations that temporarily store data necessary for instruction execution.
• Special-purpose registers have specific functions in executing instructions efficiently.
  

Buses

• Computers use buses to connect components within the CPU and the wider computer system. Buses are like highways made of wires that allow electronic signals and data to travel.
• The system bus, consisting of the data bus, control bus, and address bus, serves as the main communication pathway in a computer.
  • The data bus is bidirectional, facilitating the transmission of data between the CPU and memory or input/output controllers in both directions.
  • The address bus carries addresses from the CPU to memory in a unidirectional manner, ensuring addresses flow only from the CPU to memory.
  • The control bus conveys control signals from the control unit to various components like memory or input/output controllers. It is bidirectional in nature.

A diagram showing how the different buses connect the components in a computer system

The Fetch, Decode, Execute Cycle

The Central Processing Unit (CPU) carries out instructions through the Fetch Decode Execute cycle.

• The CPU retrieves an instruction from memory.
• The instruction is subsequently decoded by the Control Unit into an opcode and an operand.
• Following decoding, the instruction is executed, and the cycle iterates with the next instruction in the sequence.