4.1 Computer hardware and architecture — Edexcel 4CP0

Hardware vs Software

Hardware is the physical components of a computer system — parts you can touch. Software is the programs and data that run on hardware. Hardware without software is useless, and vice versa.

The Von Neumann Architecture

Most modern computers are based on the von Neumann architecture (proposed by John von Neumann in 1945). Its key principle is that programs and data are stored together in the same memory, and the CPU fetches and executes instructions sequentially.

The main components are:

CPU (Central Processing Unit) — the "brain" that processes instructions
Main memory (RAM/ROM) — stores current programs and data
Input devices — allow data to be entered (keyboard, mouse, microphone)
Output devices — display or output results (monitor, printer, speakers)
Storage — secondary storage for long-term data (HDD, SSD)
Buses — pathways that connect components (data bus, address bus, control bus)

The CPU

The CPU contains the following key components:

Component	Role
ALU (Arithmetic Logic Unit)	Performs arithmetic (+, -, *, /) and logical (AND, OR, NOT) operations
Control Unit (CU)	Directs the operation of the processor — fetches, decodes, and executes instructions; coordinates all other components
Registers	Tiny, extremely fast storage locations inside the CPU used during instruction processing
Cache	Very fast memory inside or near the CPU that stores frequently used instructions and data to speed up processing

Memory

Type	Full name	Description
RAM	Random Access Memory	Volatile — loses data when power is off; holds the OS, running programs and current data
ROM	Read-Only Memory	Non-volatile — retains data without power; stores the BIOS/firmware for booting

The System Bus

The system bus connects the CPU, memory and I/O devices. It consists of three buses:

Data bus — carries the actual data being transferred; bidirectional
Address bus — carries the memory address of data; unidirectional (CPU → memory)
Control bus — carries control signals (read, write, interrupt); bidirectional

The IPO Model

All computer systems follow the IPO model: Input → Process → Output. Data is entered, the CPU processes it, and a result is produced.

Input — data received from an input device or file (e.g. keystrokes, sensor readings)
Process — the CPU manipulates the data according to a program's instructions
Output — the result is sent to an output device or stored (e.g. screen display, saved file)

Storage can be considered a fourth component: data may be stored and retrieved as part of the processing cycle.

Types of Processing

Type	Description	Example
Sequential processing	Instructions executed one after another in order, on a single processor core	A simple script running each line in turn
Parallel processing	Multiple processors or cores work simultaneously on different parts of a task, then combine results	Rendering different sections of a 3D image at the same time
Multi-agent systems	Multiple independent agents (programs), each with their own goals, work together to solve a problem	A self-driving car with separate agents handling steering, braking, and obstacle detection

📝 Exam Tip: A common exam question asks you to name the components of the von Neumann architecture and state what each does. Focus on CPU, RAM, ROM, input/output devices, and secondary storage. Know the difference between RAM (volatile, temporary) and ROM (non-volatile, permanent).

⚠️ Common Mistakes

Saying RAM is "permanent storage" — RAM is volatile; it loses data when power is off
Confusing the ALU with the CU — ALU does the calculations; CU coordinates everything
Saying the address bus is bidirectional — the address bus carries addresses one way: CPU to memory only

🎥

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Q1. State two differences between RAM and ROM. [2 marks]

✅ Mark Scheme:

Award 1 mark per difference (any two): RAM is volatile / ROM is non-volatile; RAM contents are lost when power is removed / ROM retains its contents; RAM is read and write / ROM is read-only; RAM stores the current running programs and data / ROM stores the boot firmware/BIOS.

Q2. Describe the role of the Control Unit (CU) in a CPU. [2 marks]

✅ Mark Scheme:

Award 1 mark: the CU directs/coordinates the operation of the processor; award 1 mark: it fetches instructions from memory, decodes them, and controls the execution of each instruction by sending signals to other components.

Q3. Explain the purpose of the data bus and the address bus. [2 marks]

✅ Mark Scheme:

Award 1 mark: the data bus carries the actual data being transferred between CPU and memory/devices; award 1 mark: the address bus carries the memory addresses that specify where data is to be read from or written to.

Q4. State the main principle of the von Neumann architecture. [1 mark]

✅ Mark Scheme:

Award 1 mark: programs (instructions) and data are stored in the same memory / in the same format in main memory, and the CPU fetches and executes instructions sequentially from memory.

Q5. A computer has 8 GiB of RAM. Explain what this means and why RAM size affects performance. [3 marks]

✅ Mark Scheme:

Award 1 mark: 8 GiB means 8 gibibytes of volatile main memory; award 1 mark: RAM stores the currently running programs and data the CPU is actively using; award 1 mark: more RAM means more programs can run simultaneously without the CPU having to swap data to secondary storage (virtual memory), which is much slower — so more RAM improves performance.

Computer Hardware Quiz

5 questions · Edexcel 4CP0 specific

out of 5 correct

Click card to reveal definition

CPU

Central Processing Unit — the component that processes instructions and controls the other components.

ALU

Arithmetic Logic Unit — part of the CPU that performs arithmetic and logical operations.

Control Unit (CU)

Part of the CPU that fetches, decodes, and controls the execution of instructions.

RAM

Random Access Memory — volatile main memory that stores current programs and data; lost when power off.

ROM

Read-Only Memory — non-volatile memory that stores firmware/BIOS; retained without power.

Cache

Very fast memory inside/near the CPU that stores frequently used data to speed up processing.

Von Neumann architecture

Computer design where programs and data share the same memory; CPU fetches and executes instructions sequentially.

Data bus

A set of wires that carries data between CPU, memory, and I/O devices (bidirectional).

10:00

Timed test · 4 questions · Edexcel mark scheme

Computer Hardware — Mini Test10:00

Question 1 · Short Answer

Explain the difference between volatile and non-volatile memory. Give one example of each.

Question 2 · Multiple Choice

Which component of the CPU performs arithmetic operations?

Question 3 · Short Answer

State the purpose of the address bus in a computer system.

Question 4 · Short Answer

Describe one way that increasing cache size could improve the performance of a CPU.

Check mark schemes below

Q1: Volatile vs non-volatile memory.

Your answer:

Mark scheme: Volatile — loses data when power is removed (e.g. RAM); non-volatile — retains data without power (e.g. ROM). 1 mark per correct point, up to 2 marks.

Q2: CPU arithmetic operations.

Your selection:

Correct answer: B — ALU (Arithmetic Logic Unit).

Q3: Purpose of address bus.

Your answer:

Mark scheme: Carries the memory address of the data to be read from or written to memory. (1 mark)

Q4: Cache and CPU performance.

Your answer:

Mark scheme: Larger cache can store more frequently used instructions/data; fewer accesses needed to slower RAM; reduces the time the CPU spends waiting for data. (1 mark for a valid explanation)