Von Neumann Architecture

This lesson covers the Von Neumann architecture, including key registers and the stored program concept, as required by OCR J277 Section 1.1.1. Understanding this architecture is fundamental to explaining how modern computers work.

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The Stored Program Concept

Most modern computers are based on a design proposed by mathematician John von Neumann in 1945. The central idea is the stored program concept:

• Instructions (programs) and data are stored together in the same main memory (RAM).
• Instructions are fetched from memory one at a time and executed in sequence.
• The CPU uses a single set of buses to communicate with memory.

This was a revolutionary idea at the time. Before Von Neumann's proposal, computers had to be physically rewired to change the program they ran. With the stored program concept, simply loading different instructions into memory allows the same hardware to perform completely different tasks.

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Structure of Von Neumann Architecture

The Von Neumann architecture consists of these main components:

Component	Purpose
CPU	Processes instructions and data
Main memory (RAM)	Stores both instructions and data
System bus	Carries data, addresses, and control signals between CPU and memory
Input/Output (I/O)	Allows communication with external devices

Inside the CPU, the key components are:

• Arithmetic Logic Unit (ALU) — performs calculations and logical comparisons
• Control Unit (CU) — manages the FDE cycle and sends control signals
• Registers — small, fast storage locations for data the CPU is currently using

The following diagram shows how these components are connected:

graph TD
    subgraph CPU
        CU["Control Unit\n(CU)"]
        ALU["Arithmetic Logic Unit\n(ALU)"]
        REG["Registers\nPC | MAR | MDR | ACC"]
    end
    MEM["Main Memory\n(RAM)"]
    IO["Input/Output\nDevices"]
    CPU --- |"Data Bus"| MEM
    CPU --- |"Address Bus"| MEM
    CPU --- |"Control Bus"| MEM
    CPU --- IO
    CU --- ALU
    CU --- REG
    ALU --- REG

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Key Registers in Von Neumann Architecture

You must know these four registers for OCR J277:

Register	Full Name	Purpose
MAR	Memory Address Register	Holds the address of the memory location about to be read from or written to
MDR	Memory Data Register	Holds the data that has been fetched from memory, or data about to be written to memory
PC	Program Counter	Holds the address of the next instruction to be fetched
ACC	Accumulator	Stores the result of calculations performed by the ALU