The Von Neumann model and the stored program

By the end of this lesson

By the end you can explain the Von Neumann model, say what the stored program concept means, and name the main parts of the machine and how they connect.

The program is in memory while it runs

When you open an app, nothing is teleported into the processor. A copy of the program's instructions is loaded into main memory, in binary, alongside any data the program uses.

The processor then works through those instructions one at a time.

This is the stored program concept: instructions and data are stored together in main memory, in the same binary form, and the processor fetches and executes them in turn.

Definition

The Von Neumann model

A design for a computer in which both the program (its instructions) and the data it works on are held in the same main memory, and the computer repeatedly fetches an instruction, then executes it.

The four parts of the machine

Every Von Neumann machine can be split into four big pieces:

Part	Job
Central Processing Unit (CPU)	Fetches and executes instructions; does the arithmetic and the deciding
Main memory	Holds the program's instructions and the data, while they are in use
Input devices	Get data and commands into the computer (keyboard, sensor)
Output devices	Get results out (screen, speaker)

Inside the CPU sit two sub-units you will meet next: the Control Unit (CU) and the Arithmetic Logic Unit (ALU). The CPU talks to memory and the devices over buses — bundles of wires that carry addresses, data and control signals.

The kitchen analogy

Think of the CPU as a chef, main memory as the worktop right in front of them, and a recipe book open on that worktop. The chef does not memorise the whole book — they read one step, do it, read the next.

Where the analogy stops working: a chef chooses what to read. A CPU has no opinions; the Program Counter (a register you will meet shortly) decides which instruction comes next, every single time.

Worked example: A three-instruction program in memory

Memory holds this tiny program, one instruction per cell, followed by the data:

Address	Contents	Meaning
100	`LOAD 5`	Put the value 5 into the accumulator
101	`ADD 200`	Add the value at address 200
102	`STORE 300`	Write the result to address 300
200	`12`	The data

Walk through what the machine does.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Fill in the blanksQuestion 3 · Apply

Answer the 3 questions above to continue.