Launching your learning experience…
Detailed notes on Hardware for Cambridge IGCSE Computer Science, covering key concepts, explanations, examples, and exam-focused revision points.
How computers remember things. Primary memory (RAM, ROM) for now-stuff; secondary storage (HDD, SSD, optical, tape) for keeping things; off-line and cloud storage for backup. The physical mechanisms behind each. Virtual memory and the speed/capacity/cost trade-offs.
Mapped to the Cambridge IGCSE 0478 syllabus (2026-2028).
Fast, small, expensive at the top → slow, large, cheap at the bottom.
Computers use multiple kinds of storage at once because no single technology gives all of: speed, capacity, durability AND low cost.
| Tier | Examples | Speed | Capacity | Volatile? |
|---|---|---|---|---|
| Registers | CPU registers | Fastest | Bytes | Yes |
| Cache | L1, L2, L3 cache | Very fast | KB-MB | Yes |
| Primary | RAM, ROM | Fast | GB | RAM yes; ROM no |
| Secondary | SSD, HDD | Medium-slow | TB | No |
| Off-line | USB, optical, tape | Slow | Varies | No |
| Cloud | Remote servers | Network-bound | Effectively unlimited | No |
The CPU only directly accesses primary memory. To work on a file, it has to be loaded from secondary storage into RAM first.
Cambridge tip. Mark scheme expects the distinction: PRIMARY = directly accessible by CPU (RAM, ROM); SECONDARY = NOT directly accessible — must be loaded into RAM first.
RAM holds running things; ROM holds boot firmware.
RAM (Random Access Memory). The 'working memory' of the computer.
ROM (Read-Only Memory). Permanent, low-level firmware storage.
| RAM | ROM | |
|---|---|---|
| Volatile? | Yes | No |
| Read/Write | Both | Read only |
| Purpose | Running programs/data | Boot firmware |
| Capacity | Larger (GBs) | Smaller (MBs) |
Cambridge tip. Mark scheme almost always expects all three differences (volatility, R/W, purpose). Examiner reports flag answers that just say 'RAM is faster' — RAM and ROM serve completely different roles and can't really be compared on speed alone.
Magnetic vs solid-state vs optical — three families, different trade-offs.
HDD (Hard Disk Drive) — magnetic.
SSD (Solid-State Drive) — flash memory.
Optical storage — CDs, DVDs, Blu-ray.
Magnetic tape.
Cambridge tip. When comparing HDD and SSD, hit BOTH the technology (magnetic vs solid-state, moving vs no-moving parts) AND the trade-offs (price, speed, durability).
When RAM fills up, the OS pretends part of the SSD/HDD is also RAM.
When physical RAM fills up, the operating system uses an area of secondary storage as if it were extra RAM. This is called virtual memory (also 'swap' on Linux, 'page file' on Windows).
How it works.
Why use it. Lets you run more programs / larger programs than physical RAM alone would allow. The OS provides each program with the illusion of a large, continuous memory space — even on machines with very limited RAM.
Trade-off. Secondary storage is HUNDREDS of times slower than RAM. If virtual memory is being used heavily, the system slows dramatically — this is called thrashing. The fix: more physical RAM.
Cambridge tip. Mark scheme rewards the two halves: WHAT it is (using disk as if it were RAM) and WHY (so more programs can run when RAM is full). Don't forget the cost (slower than RAM).
Files on someone else's servers, accessed over the internet.
Cloud storage keeps files on remote servers managed by a provider (Google Drive, OneDrive, iCloud, Dropbox, AWS S3) and accessed over the internet.
Pros.
Cons.
Cambridge tip. Top-band evaluation answers note that cloud storage IS still physical hardware — it's just on someone else's servers. Don't say 'cloud means no hardware' — that's wrong.
Verbatim phrases and definitions Cambridge mark schemes credit.
Storage appears on every Paper 1. Most-tested questions: RAM vs ROM (4-6 marks), HDD vs SSD (4-6 marks), virtual memory (4-6 marks), choosing the right storage for a scenario (6-8 marks). Examiner reports flag confusion of RAM with secondary storage as the single most-common error.
Sources: Cambridge IGCSE Computer Science 0478 syllabus (2026-2028); 0478 Examiner Reports 2022-2024; 0478/12 Oct/Nov 2024 question paper and mark scheme. Last reviewed 2026-05-09.
Step-by-step solutions to past-paper-style questions on data storage, written exactly the way a tutor would explain them at the board.
Question
Tick (✓) ONE box in each row to show whether each storage device is magnetic, optical or solid-state. Device A: DVD. Device B: USB flash drive. (2 marks)
Step-by-step solution
Step 1
DVD = OPTICAL (1 mark). A DVD is read and written using a laser that detects pits and lands on a reflective disc surface — this is the defining feature of optical media.
Step 2
USB flash drive = SOLID-STATE (1 mark). A USB flash drive uses flash memory chips (floating-gate transistors) with no moving parts — solid-state. It is NOT magnetic and has no laser, so it is neither magnetic nor optical.
Answer
DVD → Optical. USB flash drive → Solid-state.
Examiner tip
One mark per correct classification. The most common error is calling a USB drive 'magnetic' because it is removable like a tape — the technology, not the form factor, decides the category.
Question
State ONE difference between RAM and ROM. (2 marks)
Step-by-step solution
Step 1
Volatility (1 mark for RAM, 1 mark for ROM — must contrast). RAM is VOLATILE: its contents are lost when power is removed. ROM is NON-VOLATILE: its contents are kept when the computer is switched off.
Step 2
An acceptable alternative difference. RAM can be both read from AND written to, whereas ROM is generally read-only. Either valid contrasting pair earns the marks — but you must state BOTH sides of the difference, not just one.
Answer
RAM is volatile (loses contents when power is off); ROM is non-volatile (keeps contents when power is off).
Examiner tip
A one-sided statement ('RAM is volatile') scores 1 at most — examiners require the contrasting property of ROM to award the second mark.
Question
Describe how a hard disk drive (HDD) reads and stores data. (4 marks)
Step-by-step solution
Step 1
Physical structure (1 mark). An HDD contains one or more rigid platters coated with a magnetic material; the platters spin at high speed (e.g. 5400/7200 rpm).
Step 2
Writing data (1–2 marks). A read/write head mounted on a moving arm passes over the surface and magnetises tiny regions of the coating. The DIRECTION of magnetisation represents binary 1s and 0s.
Step 3
Reading data (1 mark). To read, the head detects the magnetic field/polarity of each region as the platter spins beneath it and converts the pattern back into the binary data.
Step 4
Organisation (1 mark). Data is stored in concentric tracks divided into sectors; the head moves in/out and the platter rotates to position the correct sector under the head.
Answer
Spinning magnetic-coated platters; a read/write head magnetises regions (direction = 1/0) to write, and detects the magnetic polarity to read; data is laid out in tracks and sectors.
Examiner tip
Award marks for the OPERATION (magnetic field changed/detected, head, spinning platter), not for restating that 'it stores data on a disk'. Mentioning tracks/sectors is the easy extra mark candidates miss.
Question
Describe how data is stored on and read from an optical disc such as a DVD. (4 marks)
Step-by-step solution
Step 1
Storing data (1–2 marks). Data is represented as a spiral track of PITS (burned/marked areas) and LANDS (flat areas) on the reflective surface of the disc. On a writable disc, a laser burns marks into a dye/recording layer.
Step 2
Reading data (1–2 marks). A laser beam is shone at the spinning disc; pits and lands reflect the laser differently (the transitions between them) and a sensor detects the changes in reflected light, converting them into binary 1s and 0s.
Step 3
Capacity tiers (1 mark). CD < DVD < Blu-ray. Blu-ray uses a shorter-wavelength BLUE laser, so it can read smaller pits packed more densely, giving much higher capacity (~25 GB+ per layer) than a DVD (~4.7 GB) or CD (~700 MB).
Answer
Data is a spiral of pits and lands; a laser is shone at the spinning disc and a sensor reads the differently reflected light as 1s and 0s. Blu-ray's shorter blue-wavelength laser packs data more densely, so it holds far more than DVD or CD.
Examiner tip
Marks are for 'laser', 'pits and lands', 'reflected light detected/sensor', 'binary'. A frequent slip is saying the laser 'scans the colours' — it detects differences in REFLECTION, not colour.
Question
A photographer carries a laptop into the field and needs fast access to large image files. Compare an HDD and an SSD for this laptop and justify which you would recommend. (6 marks)
Step-by-step solution
Step 1
HDD — how it works + drawback (1–2 marks). An HDD uses spinning magnetic platters and a moving read/write head — it has mechanical moving parts, so it is slower to access data and more easily damaged by knocks/shock when carried.
Step 2
SSD — how it works + benefit (1–2 marks). An SSD uses flash memory (floating-gate transistors) with NO moving parts, so it gives faster read/write, lower power consumption, less heat/noise and far better resistance to physical shock.
Step 3
The trade-off (1 mark). HDDs are cheaper per GB and available in larger capacities; SSDs cost more per GB and flash cells have a limited number of write cycles (mitigated by wear levelling).
Step 4
Justified recommendation (1 mark). Recommend the SSD: the photographer needs FAST access to large files and the laptop is moved/knocked in the field, so the SSD's speed, low power (longer battery) and shock-resistance outweigh its higher cost per GB.
Answer
Recommend the SSD. It uses flash memory (no moving parts) so it is faster, uses less power and survives knocks far better than a mechanical, magnetic-platter HDD — exactly what a mobile photographer handling large files needs, despite the higher cost per GB.
Examiner tip
Top band needs BOTH technologies described at operation level AND a justified choice tied to the scenario (speed + portability/durability). A recommendation with no link to the scenario caps the marks.
Question
Explain what virtual memory is, how it works, and why it is used. (6 marks)
Step-by-step solution
Step 1
What it is (1–2 marks). Virtual memory uses an area of SECONDARY storage (e.g. on the SSD or HDD), called the swap space/page file, as if it were extra RAM.
Step 2
How it works (1–2 marks). When RAM is (nearly) full, the OS moves pages of data/instructions that are not currently being used OUT of RAM into the swap area on disk, freeing RAM for active processes. When a swapped-out page is needed again, it is paged back IN, swapping something else out.
Step 3
Why used (1 mark). It lets the computer run more / larger programs at once than the physical RAM would otherwise allow, instead of failing with an 'out of memory' error.
Step 4
Trade-off (1 mark). Secondary storage is much slower than RAM, so heavy swapping slows the system ('disk thrashing'); it does NOT increase the amount of physical RAM, it only simulates it.
Answer
Virtual memory uses a swap area on secondary storage as 'extra RAM': the OS pages inactive data out of RAM to disk and back in when needed, so more/larger programs can run than physical RAM allows. It is slower than real RAM because disk access is slower, and it does not add physical RAM.
Examiner tip
Examiners want the swapping/paging MECHANISM, not just 'it gives more memory'. The classic lost mark is claiming virtual memory 'increases RAM' — it simulates extra RAM using disk space.
High-scoring sample answers for data storage on the Cambridge IGCSE 0478 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name one solid-state storage device. [1]
Model answer
A USB flash drive (also acceptable: SSD / solid-state drive, or a memory card / SD card).
Why this scores
1 mark (AO1) for any valid solid-state device. An HDD, magnetic tape, CD, DVD or Blu-ray would score 0 — they are magnetic or optical, not solid-state.
State what is meant by the term volatile and name one type of memory that is volatile. [2]
Model answer
Volatile means the contents/data are lost when the power is switched off / removed (1). An example of volatile memory is RAM (1).
Why this scores
1 mark (AO1) for the definition (loses contents when power removed), 1 mark (AO1) for a correct example (RAM). ROM, HDD or SSD as the example scores 0 — they are non-volatile.
A user buys a Blu-ray disc instead of a DVD. Identify the type of storage these discs are and give two reasons why a Blu-ray disc can hold more data than a DVD. [3]
Model answer
Both Blu-ray and DVD are optical storage (1). Blu-ray can hold more data because: it uses a shorter-wavelength blue laser, so smaller pits can be read and the data can be packed more densely (1); and the tracks/pits are placed closer together / more data layers can be used, increasing capacity (~25 GB+ per layer vs ~4.7 GB for a single-layer DVD) (1).
Why this scores
1 mark (AO1) for identifying optical, 2 marks (AO2) for two valid reasons. Examiners want the OPERATION link (blue laser → smaller pits → denser data), not just 'Blu-ray is newer/better'.
Give two differences between how a magnetic hard disk and an optical disc store data, and state one advantage of using the optical disc. [4]
Model answer
Differences (any two, 1 mark each): a magnetic HDD stores data by magnetising regions of a spinning platter (direction of magnetisation = 1/0), whereas an optical disc stores data as pits and lands on a reflective surface (1); an HDD is read/written by a read/write head that detects/changes the magnetic field, whereas an optical disc is read/written by a laser that detects differences in reflected light (1); an HDD typically has much larger capacity than a single optical disc (1). Advantage of the optical disc (1): it is cheaper to mass-produce / lightweight and easily portable / cheap for distributing software or media (any one).
Why this scores
2 marks (AO1/AO2) for two genuine operational differences (magnetic field vs laser/reflection), 1 mark for a second valid difference, 1 mark for a relevant advantage. Saying both 'store data on a disc' is not a difference and scores 0.
A tablet uses solid-state storage. Describe how a solid-state storage device stores and reads data. [5]
Model answer
A solid-state device uses flash memory chips and has no moving parts (1). Data is stored in cells made from floating-gate transistors (1). To write data, an electrical charge is forced onto/trapped in the floating gate of a cell, and the presence or absence of trapped charge represents binary 1s and 0s (1). The charge stays in place even when power is removed, so the storage is non-volatile (1). To read data, the device senses whether each cell is holding charge (its state) and converts the pattern of charged/uncharged cells back into the binary data (1). (Also creditable: cells have a limited number of write cycles, managed by wear levelling.)
Why this scores
5 marks across AO1/AO2: flash memory/no moving parts, transistor cells, charge trapped to write, charge = 1/0 and is non-volatile, sensing the charge to read. A top answer DESCRIBES the operation (charge in transistors) — 'it stores data on a chip' earns little.
A school needs to back up several terabytes of data each night and keep the backups in long-term, low-cost storage. It is also installing new computers that must boot quickly. Recommend a suitable storage technology for each need and justify your choices. [6]
Model answer
For the nightly backups: magnetic tape (or alternatively a high-capacity magnetic HDD). Magnetic tape stores data magnetically on a long reel and offers very large capacity at very low cost per GB (1), which suits backing up several terabytes cheaply (1). Tape access is sequential/slow, but that is acceptable because backups are written/read in bulk overnight rather than accessed randomly, and tapes are easily stored off-site for archiving (1).
For the new computers' boot/OS drive: a solid-state drive (SSD). An SSD uses flash memory with no moving parts, giving very fast read access (1), so the operating system loads and the computers boot quickly (1). It also uses less power, runs cooler/quieter and is more shock-resistant than an HDD; the higher cost per GB is acceptable because the OS drive does not need huge capacity (1).
Why this scores
6 marks (AO2/AO3): up to 3 for a justified backup choice (tape/HDD — high capacity, low cost per GB, sequential acceptable) and up to 3 for a justified boot-drive choice (SSD — fast/no moving parts → quick boot). Marks need the choice LINKED to the scenario; an unjustified one-word answer caps low.
Definitions to memorise and the exact keywords mark schemes credit for data storage answers — sharpened from recent examiner reports for the 2026 0478 sitting.
Memory directly accessible by the CPU — RAM and ROM. Fast, small capacity, generally volatile (RAM) or read-only (ROM).
Non-volatile storage NOT directly accessible by the CPU; data must be loaded into RAM first. Examples: HDD, SSD, USB flash drive, DVD.
Removable storage media used for backup, archiving or transfer (USB drives, external HDDs, optical discs, magnetic tape).
Volatile primary memory holding data and instructions currently in use. Contents are lost when power is removed.
Non-volatile primary memory holding fixed instructions (typically the BIOS / boot firmware).
Storage that records data by magnetising tiny regions of a coated surface; the DIRECTION of magnetisation represents the 1s and 0s. A read/write head changes (write) or detects (read) the magnetic field. Examples: HDD, magnetic tape.
Magnetic secondary storage. Binary data is stored as the direction of magnetisation on spinning platters; a moving read/write head writes and reads the data in tracks and sectors. Large capacity, lower cost per GB, mechanical and slower than SSD.
Flash-memory secondary storage with no moving parts. Data is stored by trapping charge in floating-gate transistors (control gates change the charge to write; the cell's state is sensed to read). Faster, lower power, more shock-resistant; higher cost per GB and limited write cycles per cell (managed by wear levelling).
Storage using flash memory (no moving parts) that stores 1s and 0s as trapped electrical charge in transistors. Fast, durable, low power. Examples: SSD, USB flash drive, memory card.
Storage that reads/writes data with a LASER on a reflective disc. Binary data is a spiral of pits and lands whose differing reflection is detected by a sensor. Examples: CD, DVD, Blu-ray. Removable and cheap to mass-produce, but lower capacity and easily scratched.
The bumps (pits) and flat areas (lands) along the spiral track of an optical disc. The CHANGES between pits and lands alter how the laser is reflected, and the sensor reads these changes as binary 1s and 0s.
Sequential-access magnetic storage used for high-capacity backups and archives. Very cheap per GB; slow to access individual files because access is linear (the tape must wind to the right position).
Using an area of secondary storage (swap space / page file) as if it were extra RAM by paging data between RAM and disk. Lets the OS run more programs than physical RAM allows; slower than real RAM and does not add physical RAM.
Storage hosted on remote servers and accessed over the internet. Backed up across multiple locations; user pays for capacity and bandwidth and needs an internet connection to access it.
Storage that LOSES its contents when power is removed. RAM is volatile; ROM, HDD, SSD are non-volatile.
The traps other students keep falling into on data storage questions — taken from recent Cambridge IGCSE 0478 examiner reports and mark schemes — and how to avoid them.
0478 Examiner Reports 2022-2024
Why it happens
Confusing primary memory with secondary storage.
How to avoid it
RAM only holds programs/data CURRENTLY IN USE. Files live in SECONDARY storage (HDD/SSD) and are loaded into RAM when opened.
Why it happens
Both are 'drives' and SSDs sometimes use the same form factor.
How to avoid it
SSDs use FLASH MEMORY — solid-state, no moving parts, charge in transistors. Only HDDs use magnetic platters. Always name the storage technology.
0478 Examiner Reports 2022-2024
Why it happens
Discs look shiny/rainbow-coloured, so colour seems to carry the data.
How to avoid it
The laser detects differences in REFLECTION caused by pits and lands, not colour. Marks are for 'laser', 'pits and lands' and 'reflected light detected'.
0478 Examiner Reports 2022-2024
Why it happens
Students name the device but never describe the operation.
How to avoid it
Describe the MECHANISM: magnetic = head magnetises regions (direction = 1/0); optical = laser + pits/lands + reflection; solid-state = charge trapped in transistors. Examiners reward the operation, not the noun.
Why it happens
Virtual memory acts like extra RAM.
How to avoid it
It SIMULATES extra RAM by paging data to disk space — physical RAM is unchanged. Performance is bottlenecked by the much slower secondary storage.
Why it happens
Cloud feels intangible.
How to avoid it
Cloud storage IS physical hardware — it's just on someone else's servers. The user trades local hardware for a network connection and a subscription.
The things students keep getting wrong in this sub-topic, answered.