GCSE · Computer Science · Edexcel · Spec 1CP2
Von Neumann architecture
Your computer isn't clever. It grabs one instruction, works out what it means, does it — then repeats, billions of times a second. Let's follow one instruction round.
Computer Science · Fetch–decode–execute
Follow one instruction round the loop
RAM holds a tiny program. Address 20 holds the instruction LOAD 25, which means “copy the number stored at address 25 into the accumulator”. Address 25 holds the number 14. Your cast: PC = where the next instruction is · MAR = the address we're about to visit · MDR = whatever just arrived from memory · CIR = the instruction being worked on · ACC = the accumulator, where data being processed ends up.
Before you press Next: when the instruction arrives from memory, which register do you think it lands in first?
Phase: Ready
PC
20
MAR
—
MDR
—
CIR
—
ACC
—
Address bus
—
Data bus
—
Control bus
—
Control unit
—
Output
Step 1: The program is already loaded into main memory (RAM). The program counter (PC) holds 20: the address of the next instruction to fetch. Nothing has moved yet.
Press Next to move one step at a time. A dash means that store isn't in use at this step — a bus sitting idle, or a register this walk-through hasn't needed yet.
Computer Science · Structure
Meet the machine that runs the loop
The dashed box is the CPU — the core of the computer, which receives instructions and executes them. Tap any part to see its job. The lines are buses: pathways that carry addresses, data and control signals between the parts.
Tap any part of the diagram to see what it does.
Computer Science · Registers and buses
Address or contents?
Pick an item, then choose the category it belongs in: does it hold or carry an address, the contents of memory (data or an instruction), or control signals?
Still to sort
An address (0)
Where something is in memory
Where the line is: An address says WHERE to look. It is never the thing stored there.
Data or an instruction (0)
What is stored in memory
Where the line is: This is WHAT was found at an address — a value or an instruction — not the address itself.
Control signals (0)
Keeping everything in order and on time
It's easy to mix up WHERE something is with WHAT is stored there. Sort each register and bus to fix that line in your head.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
One small loop — fetch, decode, execute — repeated billions of times a second.
What you need to know
- Von Neumann architecture: the program's instructions are stored in memory alongside the data, both in binary.
- The architecture: a CPU, main memory (RAM), connections for input/output devices and secondary storage, joined by buses.
- Inside the CPU: the control unit decodes instructions and controls the other parts; the ALU does arithmetic and logic; registers are tiny, fast stores; the clock sets the pace.
- Registers: PC = address of the next instruction; MAR = address being accessed; MDR = data or instruction just fetched; CIR = instruction being decoded and executed; ACC = data being processed and results.
- Buses: the address bus carries addresses; the data bus carries data and instructions; the control bus carries control signals.
- Fetch: PC to MAR → address along the address bus → instruction back along the data bus to the MDR → MDR to CIR → PC + 1. Then decode (control unit) and execute.
The big picture
Modern computers are built on the von Neumann design. The program is stored in memory alongside the data, both in binary. The CPU runs it with one loop: fetch the next instruction from memory, decode it in the control unit, execute it — then go round again for the next one. Registers (the PC, MAR, MDR, CIR and accumulator) are tiny, fast stores inside the CPU, each with its own job, and three buses carry addresses, data and control signals between the components.
Key points
Worked example
Problem
The program counter holds 47. Describe what happens during the fetch stage of the next instruction, naming each register and bus involved, and state what the PC holds at the end of the fetch.
⚠ Watch out
Swapping the MAR and the MDR (or the address bus and the data bus). The MAR and the address bus deal with WHERE something is — an address. The MDR and the data bus deal with WHAT is there — the data or instruction itself.
Memory hook
Point, ask, catch, hold, decode, do: the PC POINTS at the next instruction, the MAR ASKS memory for it, the MDR CATCHES what comes back, the CIR HOLDS it, the control unit DECODES it — and then the CPU DOES it. Then round again.
Check yourself
Try it without looking: the PC holds 90. Which register gets 90 first, which bus takes it to memory, where does the instruction arrive, and what does the PC end on?
Flashcards
(15)What is computer architecture?
What is the stored program concept?
Why was the stored-program computer more versatile than a hardwired machine?
Name the four main parts of the von Neumann architecture.
What does the control unit do?
What does the ALU do?
What does the CPU's clock do, and what does 2.5 GHz mean?
What is a register?
What does the program counter (PC) hold?
MAR vs MDR — what does each hold?
What do the CIR and the accumulator hold?
What does each bus carry — address, data and control?
What is RAM's job, and what does 'volatile' mean?
What happens in the decode stage?
What happens in the execute stage of an instruction that loads data?
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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