Computer Architecture

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Cambridge IGCSE (CIE) Computer Science

Computer Architecture

The Big Idea: A computer works because its CPU constantly grabs an instruction from memory, figures out what that instruction means, and then does it — over and over, billions of times a second.

📋 Summary — Everything In This Chapter

The CPU is the "brain" of the computer — its job is to fetch, decode, and execute instructions.
A microprocessor is the physical chip that houses the CPU (and often other parts too).
The Von Neumann architecture describes how the CPU, memory, and I/O devices are connected and work together.
The CPU has 5 key registers: PC, MAR, MDR, CIR, and ACC — each with one specific job.
Data moves between components along three buses: Address, Data, and Control.
The Fetch-Decode-Execute cycle is the repeating process that actually runs every program.
CPU performance depends on three characteristics: clock speed, cache size, and number of cores.
An instruction set is the CPU's "dictionary" of commands it understands, written in machine code.
Embedded systems are small, dedicated computers built into larger devices (like washing machines).

1. The CPU & Microprocessors

1What actually is the CPU?

The Central Processing Unit (CPU) is the component inside every computer, games console, laptop, and phone that does the actual "thinking." Its one job — its — is to fetch an instruction, decode it (work out what it means), and execute it (actually do it).

🍳 Analogy: Think of the CPU like a chef following a recipe card. The chef fetches the next line of the recipe (reads it), decodes it (works out what "fold in the eggs" actually means), and then executes it (actually folds in the eggs). Then moves to the next line. That's it — that's the whole loop, just repeated non-stop.

Every device follows the same input → process → output pattern:

Step	Example
Input	A keyboard is used to input a number
Process	If the instruction is ADD, the inputted value is added to an existing value
Output	The result of the calculation is shown to the user on the monitor

Exam Keyword Rule

Always say the CPU "fetches, decodes, and executes" instructions.

Writing "the CPU processes data" is too vague and won't get you the mark — examiners are matching your answer against a mark scheme that expects these exact three words.

Practice Question

In one sentence, state the purpose of the CPU.

2CPU vs. Microprocessor — don't mix these up

This is one of the most commonly confused pairs in the whole topic, so let's be really precise about it.

A microprocessor is a type of integrated circuit contained on a single chip. It's the actual physical hardware you could pick up and hold. It the CPU, and often other components too, such as:

Memory controllers
I/O interfaces

🧠 The Key Distinction

The CPU is a component (it's the logical part that does fetch-decode-execute). The microprocessor is the chip — the physical container that the CPU (and sometimes other parts) sits inside. So the CPU is a microprocessor, not the same thing as it.

Practice Question

Explain the difference between a CPU and a microprocessor.

2. Von Neumann Architecture

c

Register	Acronym

d

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Also in the full note

3. The Fetch-Decode-Execute Cycle (FDE)
4. Characteristics of the CPU
5. CPU Instruction Sets
6. Embedded Systems
🧠 What to Memorise
✅ Concepts Checklist
🎯 Exam Tips & Common Mistakes
aArithmetic Logic Unit (ALU)

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Computer Architecture