Von Neumann Architecture
What Is It?
The Von Neumann Architecture is the design used by almost all modern computers. The key idea is the stored-program concept: both program instructions and data are stored together in the same memory (RAM), and the CPU fetches them from there.
Key Principle
Stored-program concept, the program is stored in memory alongside data, and the CPU fetches, decodes and executes each instruction in sequence.
Stored-Program vs Fixed-Program
| Type | How the Program Is Stored | Example |
|---|---|---|
| Fixed-program computer | Program is wired into the hardware | Early calculators |
| Stored-program computer | Program is loaded into memory at runtime | Modern PCs, laptops, phones |
A fixed-program computer can only ever do one thing. A stored-program computer can run any software loaded into its memory.
The Von Neumann Model, Components
┌──────────────────────────────────────────┐
│ CPU │
│ ┌──────────┐ ┌──────────────────┐ │
│ │ CU │ │ ALU │ │
│ │(Control │ │(Arithmetic Logic │ │
│ │ Unit) │ │ Unit) │ │
│ └──────────┘ └──────────────────┘ │
│ ┌──────────────────────────────────┐ │
│ │ Registers: PC · MAR · MDR · ACC │ │
│ └──────────────────────────────────┘ │
│ ┌──────────┐ │
│ │ Cache │ │
│ └──────────┘ │
└────────────┬─────────────────────────────┘
│ Buses (Address / Data / Control)
▼
┌─────────┐
│ RAM │ (stores both instructions AND data)
└─────────┘
All components are covered in CPU Components, CPU Registers and System Buses.
Why It Matters
- The stored-program concept means a computer is general-purpose, it can run any program
- The CPU accesses instructions and data through the same memory bus (a potential bottleneck, called the Von Neumann bottleneck)
- Cache memory was developed partly to reduce this bottleneck
Exam Questions
Q: What is the fundamental principle of the Von Neumann architecture?
The stored-program concept, both instructions and data are stored together in the same memory, and the CPU fetches and executes them sequentially.
Q: How does a stored-program computer differ from a fixed-program computer?
A stored-program computer can run any program loaded into memory, while a fixed-program computer has its program wired into the hardware and can only perform one task.
Exam Tip
Distinguish clearly between the architecture (the design principle) and the specific components (CU, ALU, registers). The architecture is the idea; the components are the implementation.