Cambridge IGCSE Computer Science (0478) · lesson 16 of 35 · 30 min
The Von Neumann Architecture
Almost all modern computers are based on the Von Neumann architecture, proposed by John von Neumann in 1945. Its key principle is the stored program concept: both program instructions and data are stored in the same memory, encoded as binary numbers. The CPU fetches instructions from memory and executes them sequentially.
The main components are:
- CPU (Central Processing Unit) — processes instructions.
- Main memory (RAM) — stores current program and data.
- Input/Output devices — communicate with the outside world.
- Buses — carry data, addresses, and control signals between components.
The Fetch-Execute Cycle
The CPU continuously repeats the fetch-execute cycle (also called the fetch-decode-execute cycle):
- Fetch: The Program Counter (PC) holds the address of the next instruction. The CPU copies this address to the Memory Address Register (MAR), then fetches the instruction from RAM into the Memory Data Register (MDR), then copies it to the Current Instruction Register (CIR). The PC is incremented.
- Decode: The Control Unit (CU) decodes the instruction in the CIR to determine what operation to perform and what operands are needed.
- Execute: The instruction is carried out — arithmetic/logic by the ALU, memory access via MAR/MDR, or a jump updates the PC.
The Three Buses
What does the Program Counter (PC) store?
- The current instruction
- The address of the next instruction
- The result of the last calculation
- The current data value
Answer: The address of the next instruction
The PC holds the memory address of the next instruction to fetch.
Which bus carries memory addresses from the CPU to RAM?
- Data bus
- Control bus
- Address bus
- System bus
Answer: Address bus
The address bus carries memory addresses and is unidirectional (CPU → memory).
The stored program concept means:
- Programs are stored on disk only
- Instructions and data are stored together in RAM as binary
- Programs cannot be changed
- Each program has its own CPU
- Answer: 1
Von Neumann’s key insight: instructions and data share the same memory, both as binary.
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