Lecture on von Neumann Architecture

Introduction

• Final video in a series on computer system architecture.
• Focus on von Neumann architecture.
• Early computers had fixed programs; von Neumann introduced stored-program concept.

Von Neumann Architecture

• Described by mathematician and physicist Jon von Neumann in 1945.
• Key Characteristics:
  • Central Processing Unit (CPU) with a single control unit.
  • Arithmetic Logic Unit (ALU).
  • On-board cache: small, high-speed memory.
  • Internal clock: synchronizes components.
  • Fetches, decodes, and executes instructions.
  • Programs are a sequence of instructions stored in main memory.

Memory and Instructions

• Each instruction stored in memory has a specific address.
• The processor fetches instructions from memory addresses.
• Special-purpose registers used:
  • Program Counter: Holds memory address of next instruction.
  • Memory Address Register (MAR): Holds address for data to be fetched/stored.
  • Memory Data Register (MDR): Holds data fetched from or written to memory.
  • Accumulator: Holds results of calculations from ALU.

Fetch-Execute Cycle

1. Fetch:
   • Program counter checked for next instruction address.
   • Address used by MAR to fetch instruction into MDR.
   • Program counter incremented.
2. Decode:
   • Control unit decodes instruction.
   • Determines action to be taken.
3. Execute:
   • Action based on instruction type (e.g., fetch data, jump instructions, write data).

Registers Summary

• Program counter: Address of next instruction.
• MAR: Address for data fetch/store.
• MDR: Data fetched/ready to write.
• Accumulator: Calculation results.

Advanced Fetch-Execute Cycle Example

• Process of executing instructions in memory locations 1-4:
  • Cycle 1: Load value from address 5 into MDR, copy to cache and accumulator.
  • Cycle 2: Add value from address 6 to accumulator value, store result in cache and accumulator.
  • Cycle 3: Store accumulator value into address 6, overwrite previous data.
  • Cycle 4: Execute end of program.
• Program execution example: Sum 12 + 23 = 35 stored in main memory.

Conclusion

• von Neumann architecture is integral to modern computing.
• Combines control, logic, and memory operations in a seamless cycle.
• Offers foundational knowledge for further studies in computer science.