GCSE · Computer Science · AQA · Spec 8525

Von Neumann architecture

Your phone is a camera one minute and a calculator the next, with no rewiring. So where does the program actually live? In memory, stored just like data.

Stored program · step by step

Follow a tiny program out of memory

Main memory has six locations here, numbered 0 to 5. Addresses 0 to 3 hold the program's instructions; addresses 4 and 5 hold its data. They are written as words and numbers so you can read them. Inside a real computer, every one of them is stored in binary.

Predict first: at step 3, will the CPU be reading an instruction or a piece of data from memory?

Phase: Fetch

Addr 0

▶ LOAD 4

Addr 1

ADD 5

Addr 2

OUTPUT

Addr 3

STOP

Addr 4

Addr 5

Address sent

On the bus

LOAD 4 (instruction)

CPU holds

—

Output

Step 1: The CPU sends address 0 to memory. Back along the bus comes an instruction: LOAD 4.

1 / 11

Press Next one step at a time. ▶ marks the memory location being used. Watch which address the CPU asks for, and whether an instruction or data comes back.

Step 1 of 11: The CPU sends address 0 to memory. Back along the bus comes an instruction: LOAD 4..

Computer Science · Structure

The parts of a von Neumann computer

Tap each part. The lines are buses: pathways that carry instructions and data between the parts.

Tap any part of the diagram to see what it does.

What do you think?

Where does a program live?

You close a game on a laptop and open a spreadsheet. Same CPU, same memory chips, same wires. Nobody opens the case. Yet the computer is now doing a completely different job.

While the spreadsheet runs, where are its instructions?

Built into the CPU. The CPU has its own circuits for each program.In memory, but in a separate memory from the data.In main memory, stored in binary alongside the data it works on.

How sure are you?

I'm sureI think soI'm guessing

Built into the CPU. The CPU has its own circuits for each program.

This is a common misconception

If that were true, switching to a new program would mean changing the hardware, and nobody touched a wire. The CPU carries out instructions; it does not store the program. The instructions sit in main memory, and the CPU fetches them from there.

In memory, but in a separate memory from the data.

This is a common misconception

You are half right: the instructions are in main memory. But in the von Neumann design there is only ONE main memory, and instructions and data share it. An instruction can sit at one address with a number right next door, just like STOP at address 3 and the number 7 at address 4 in the stepper above.

In main memory, stored in binary alongside the data it works on.

This matches the evidence

Exactly. The program is just more binary in memory, each instruction at its own address. That is the stored-program idea: load different instructions into memory and the same hardware runs a different program, with no rewiring.

Stored program: the instructions live in main memory next to the data. Change the instructions and you change what the computer does. The hardware stays exactly the same.

Taking turns

Reason it through

Why can't the CPU fetch an instruction and the data it needs at the same moment?

Link 1 of 4

First link · your turn

Where do the instructions and the data both live?

Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The program lives in memory, right next to the data it works on.

What you need to know

The main parts are the CPU, main memory, input devices and output devices, connected by buses.
Main memory stores the program's instructions and the data, together, in binary.
Every location in main memory has its own address, which is how the CPU finds what it needs.
The CPU fetches an instruction from memory, decodes it, executes it, then moves on to the next one.
Instructions and data share the same buses, so they travel one transfer at a time.

The big picture

In the von Neumann architecture a computer has a CPU, one main memory, input devices and output devices, joined by buses. Main memory holds both the program's instructions and the data they use, in binary, each at its own address. The CPU fetches instructions from memory one at a time, decodes them and executes them. Because instructions and data share one memory and the same buses, they are transferred one after the other.

Key points

1Stored program: a program is a set of instructions held in main memory, not wired into the CPU.

2Running a different program means loading different instructions into memory; the hardware stays the same.

3An address says WHERE something is in memory; the contents are WHAT is stored there.

4Executing an instruction can mean another trip to memory, for example to read the data it needs.

5One memory and shared buses mean an instruction and its data cannot be fetched at the same moment.

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Worked example

Problem

Main memory holds: address 10: LOAD 20, address 11: SUBTRACT 21, address 12: STORE 22, address 13: STOP, address 20: 9, address 21: 4, address 22: 0. LOAD copies a value into the CPU, SUBTRACT takes a value away from the one the CPU holds, and STORE writes the CPU's value into memory. The CPU starts at address 10. How many transfers happen between memory and the CPU, and what does address 22 hold at the end?

⚠ Watch out

Writing that the program is "stored in the CPU". The CPU carries out the instructions, but the program is stored in main memory, in the same memory as the data.

🧠

Memory hook

Picture a row of numbered lockers with one corridor to the CPU. Program and data share the lockers, and only one thing can walk down the corridor at a time. (Unlike real lockers, every one holds nothing but binary.)

✓

Check yourself

Without scrolling up: name the four main kinds of part, say where a running program is kept and what shares that space, and explain why an instruction and its data cannot arrive together.

Flashcards

(11)

What is the stored-program concept?

A program's instructions are stored in main memory alongside the data, instead of being built into the hardware.

Name the main parts of a von Neumann computer.

CPU, main memory, input devices and output devices, connected by buses.

What is a bus?

A pathway that carries instructions and data between the parts of the computer.

In what form are instructions and data held in main memory?

Binary.

What is a memory address?

The number that identifies one location in main memory.

Address or contents? "Location 4 holds 7"

4 is the address (where); 7 is the contents (what is stored there).

What are the three stages the CPU repeats for every instruction?

Fetch, decode, execute.

What happens in the decode stage?

The CPU works out what the instruction it has just fetched means it must do.

What has to change for a computer to run a different program?

Only the instructions in main memory. No rewiring.

Why can't an instruction and its data be fetched at the same time?

They share the same memory and the same buses, which carry one transfer at a time.

What do input and output devices do?

Input devices bring data in (e.g. keyboard); output devices send results out (e.g. screen).

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow a tiny program out of memory

The parts of a von Neumann computer

Where does a program live?

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