O Level Computer Science: Hardware — Revision Notes

Condensed for the final weeks. For the full explanation, use the Hardware study guide.

3.1 The CPU and the FDE cycle

Category	Components
Units	ALU (arithmetic/logic operations), CU (controls sequencing)
Registers	PC (next instruction’s address), MAR (address being accessed), MDR (data/instruction transferred), CIR (instruction being decoded), ACC (result of ALU operations)
Buses	Address (where), data (what), control (signals)

Cycle order: instruction fetched from RAM via MAR/MDR into CIR → CU decodes it → ALU/CU execute it, result stored in ACC.

Performance factors: more cores, larger cache, higher clock speed — all improve performance, but be ready to explain how, not just list them. Instruction set = fixed list of machine-code commands a CPU can process. Embedded system = one dedicated function (e.g. a car’s engine-management unit) vs a general-purpose computer running many applications.

3.2 Input/output devices and sensors

Category	Named examples
Input	Barcode scanner, digital camera, keyboard, microphone, optical mouse, QR scanner, touch screen (resistive/capacitive/infra-red), 2D/3D scanner
Output	Actuator, DLP/LCD projector, inkjet/laser printer, LED/LCD screen, speaker, 3D printer
Sensor	Acoustic, accelerometer, flow, gas, humidity, infra-red, level, light, magnetic field, moisture, pH, pressure, proximity, temperature

Scenario questions reward matching a sensor to what it physically measures, not recognising it by name alone — build a sensor → measured-quantity table for revision, e.g. a moisture sensor in an automated plant-watering system, or a pH sensor in a swimming-pool monitoring system.

3.3 Data storage

Type	CPU accesses directly?	Volatile?
RAM (primary)	Yes	Yes
ROM (primary)	Yes	No
Magnetic (HDD)	No	No
Optical (CD/DVD/Blu-ray)	No	No
Solid-state (SSD/SD/USB)	No	No

Virtual memory: pages swapped between RAM and disk when physical RAM is insufficient. Cloud storage: advantage (remote access from anywhere) vs disadvantage (dependence on physical servers/storage hosted elsewhere, compared with direct local control).

3.4 Network hardware

• NIC required to access a network.
• MAC address — fixed at manufacture (manufacturer code + serial code), usually hexadecimal.
• IP address — allocated by the network, static or dynamic; know IPv4 vs IPv6 differences.
• Router — directs data to its destination, can assign IP addresses, connects a local network to the internet.

Builds directly on Topic 2 (Data transmission) — a router’s packet-directing role connects straight to that topic’s packet-switching content.

Worked example: tracing an instruction

FETCH:   address of next instruction copied from PC into MAR
         instruction copied from RAM into MDR, then into CIR
         PC incremented
DECODE:  CU interprets the instruction in CIR
EXECUTE: ALU performs the operation; result placed in ACC

Practise naming which register or bus is active at a specific named stage — exam questions rarely ask for the whole cycle recited from memory, but frequently ask “what happens to the MAR during fetch?” style questions.

This O Level shares content with IGCSE Computer Science

Cambridge explicitly builds O Level Computer Science (2210) on the same subject content as Cambridge IGCSE Computer Science (0478) – Topic 3’s structure, named devices, sensors and storage technologies are identical between the two qualifications. If your centre also offers 0478, resources for that qualification’s Hardware topic cover exactly the same content and can be used interchangeably for practice, though always confirm with your teacher which specific syllabus code and series applies to your own examination entry before relying on cross-qualification material for anything beyond general revision.

Exam traps

• Reciting the register list without being able to trace a single instruction through fetch, decode and execute.
• Naming a sensor without stating what it actually measures under scenario pressure.
• Treating RAM and secondary storage as interchangeable, rather than distinguishing direct CPU access (primary) from indirect (secondary).
• Confusing MAC address (fixed, hardware-level) with IP address (network-allocated, can change).
• Listing cloud storage’s benefit without a matched limitation.

Worked example: matching sensors to a scenario

A smart greenhouse system automatically opens vents, waters plants and adjusts shading. Identify a suitable sensor for each function.

Opens vents when too hot:        Temperature sensor
Waters plants when soil is dry:  Moisture sensor
Adjusts shading when too bright: Light sensor

For each, the answer needs the sensor named and the physical quantity it measures stated explicitly — “a sensor” alone, without naming which type, does not earn the mark even if the surrounding explanation of the automated response is otherwise correct.

RAM vs ROM: why both are needed together

A common exam question asks why a computer needs both RAM and ROM rather than just one. RAM is fast and directly accessible by the CPU, but volatile — its contents are lost when power is removed, so it cannot hold the instructions needed to start the computer up in the first place. ROM is also directly accessible but non-volatile, so it permanently stores the startup instructions (the bootstrap program) that run before an operating system has even been loaded into RAM. Once the operating system is loaded, RAM takes over as the main working memory because of its speed, while ROM’s contents remain untouched and unnecessary until the next power-on.

Self-test

1. What happens to the Program Counter during the fetch stage?
2. Name the three secondary storage technologies and one example device for each.
3. What does “embedded system” mean, with an example?
4. Give one advantage and one disadvantage of cloud storage.
5. What is the difference between a MAC address and an IP address?

Answers: 1. It holds the address of the next instruction to be fetched, and is incremented once the current instruction has been fetched. 2. Magnetic (HDD), optical (CD/DVD/Blu-ray), solid-state (SSD/SD card/USB drive). 3. A device built to perform one dedicated function, such as a car’s engine-management system, as opposed to a general-purpose computer that runs many different applications. 4. Advantage: remote accessibility from any location; disadvantage: dependence on physical servers/infrastructure hosted elsewhere rather than direct local control. 5. A MAC address is fixed at manufacture and identifies specific hardware; an IP address is allocated by the network, can be static or dynamic, and identifies a device’s location on that network.