Hardware vs software

Every computer system is built from two things working together:

• Hardware — the physical parts you can touch: CPU, memory, hard drive, keyboard, screen.
• Software — the programs (instructions) that run on the hardware. Software splits into two kinds:

• System software — runs and manages the computer itself: the operating system (manages hardware, memory, files, processes, users) and utility programs (housekeeping tools such as backup, defragmentation, antivirus and compression).
• Application software — programs the user runs to do a task, e.g. a web browser, word processor or game.

Definition to memorise: hardware = the physical components; software = the programs that tell the hardware what to do.

Which memory is volatile?

The CPU & von Neumann architecture

The CPU (Central Processing Unit) is the "brain" of the computer — it fetches, decodes and executes program instructions. Most computers use the von Neumann architecture, whose key idea is the stored program concept: both the instructions and the data are held together in the same main memory.

Inside the CPU:

• ALU (Arithmetic Logic Unit) — does the arithmetic (add, subtract) and logic (comparisons, AND/OR/NOT).
• Control Unit (CU) — directs the flow of data and controls the fetch–decode–execute cycle.
• Registers — tiny, extremely fast storage locations inside the CPU used during processing.

The fetch–decode–execute cycle

The CPU runs a program by repeating three steps, over and over, billions of times a second:

• Fetch — the next instruction is copied from main memory into the CPU.
• Decode — the Control Unit works out what the instruction means.
• Execute — the instruction is carried out (e.g. the ALU does a calculation).

Special registers keep the cycle running:

• Program Counter (PC) — holds the address of the next instruction to be fetched.
• MAR (Memory Address Register) — holds the address in memory to be read from or written to.
• MDR (Memory Data Register) — holds the data or instruction just fetched from (or to be written to) memory.
• Accumulator — holds the result of calculations done by the ALU.

Tip: the PC points to the next instruction; the Accumulator stores results. Don't mix them up — that's a common exam trap.

Stored program concept

Name that register

What makes a CPU faster?

Three factors decide how much work a CPU can do:

• Clock speed — how many cycles the CPU does each second, measured in hertz (Hz). A higher clock speed means more instructions per second (e.g. 3 GHz = 3 billion cycles per second).
• Number of cores — each core can process instructions independently, so more cores means more instructions can be handled at the same time (e.g. dual-core, quad-core).
• Cache size — a larger cache stores more frequently-used data/instructions close to the CPU, so the CPU waits less for slower RAM.

Remember: more clock speed, more cores and a bigger cache all tend to improve performance — but they trade off against cost and power use.

RAM, ROM and cache

These are the fast primary memory the CPU works with directly:

• RAM (Random Access Memory) — volatile: holds the programs and data currently running. Its contents are lost when the power is off.
• ROM (Read Only Memory) — non-volatile: holds the boot / start-up instructions the computer needs when it is first switched on. Its contents are kept without power.
• Cache — a small, very fast memory that sits closest to the CPU and stores frequently-used data so the CPU doesn't have to wait for slower RAM.

Key contrast: RAM is volatile (loses data without power); ROM is non-volatile (keeps boot instructions); cache is small, fast and closest to the CPU.

RAM, ROM or cache?

Tap a description, then tap the memory type it belongs to.

Secondary storage

RAM is lost when the power goes off, so we also need secondary storage — non-volatile, permanent storage that keeps files and programs when the computer is switched off. AQA names three types:

• Magnetic — e.g. a hard disk drive (HDD): data stored magnetically on spinning platters. High capacity and cheap per gigabyte.
• Optical — e.g. CD, DVD, Blu-ray: data read/written with a laser. Cheap and portable, but lower capacity.
• Solid state — e.g. SSD / flash / USB memory sticks: no moving parts, very fast and robust.

Why we need it: secondary storage is non-volatile, so it keeps data permanently even when the power is off — unlike RAM.

Embedded systems

An embedded system is a computer built into a larger device to perform a single dedicated task. Unlike a general-purpose PC, it does one fixed job and cannot easily run other programs.

• The controller in a washing machine (runs the wash cycles).
• Engine management systems in a car.
• Central heating thermostats, microwave ovens, digital cameras and traffic lights.

Definition: an embedded system is a computer built into a larger device to carry out a specific, dedicated function.

Logic gates: NOT, AND, OR

Deep inside the CPU, everything is built from logic gates — tiny circuits that take binary inputs (0 or 1) and give a single binary output. AQA needs three:

• NOT — reverses the input: output is 1 only when the input is 0.
• AND — output is 1 only when both inputs are 1.
• OR — output is 1 when at least one input is 1.

Boolean logic: you write these as NOT A, A AND B, A OR B. Combining gates lets a computer make decisions and do arithmetic from nothing but 0s and 1s.

Your turn — the AND gate

Your turn — the OR gate

Your turn — the NOT gate

What is an embedded system?

Match term to meaning

Tap a description on the left, then its matching term on the right.

The big ideas to know

You've covered all of AQA 3.4 — hardware & software, the CPU, memory, storage, embedded systems and logic gates. Press Finish to see your score.