Describe the operation of magnetic, optical and solid-state storage with examples

Data Storage – IGCSE Computer Science (0478)

Learning objective

Describe the operation of magnetic, optical and solid‑state storage devices, give examples of each type, and explain how they differ from primary (RAM/ROM) storage.

1. Primary vs. secondary storage

• Primary storage – fast, volatile memory that the CPU can read or write directly while a program is running.
  • Random‑Access Memory (RAM): stores data and programmes temporarily; loses its contents when power is removed.
  • Read‑Only Memory (ROM): contains permanent instructions (e.g. BIOS); non‑volatile but normally cannot be rewritten by the user.
• Secondary storage – non‑volatile devices that retain data when the computer is switched off. They store data on a separate medium (magnetic, optical or solid‑state) rather than in the CPU’s registers.
• Why secondary storage is needed: primary storage is limited in capacity and loses data on power‑off, so larger, permanent media are required for programmes, documents, multimedia files and backups.

2. Data‑size units (bits, bytes and binary prefixes)

3. Magnetic storage

Data is stored by magnetising tiny regions (domains) on a rotating magnetic platter. The direction of the magnetic field (north‑south) represents a binary 0 or 1.

How it works

1. The platter is coated with a ferromagnetic material and divided into concentric tracks. Each track is split into sectors.
2. The write head creates a magnetic field that aligns the domains in the chosen direction, thereby writing a 0 or 1.
3. The read head senses the polarity of the domains as the platter spins past it and converts the changes into electrical signals.

Typical examples

• Internal hard‑disk drive (HDD) – the standard primary secondary storage in most desktops and laptops.
• External USB‑connected HDD – portable, plug‑and‑play solution.
• Floppy disk (legacy) – magnetic disc with a flexible jacket.
• Magnetic tape – used for large‑scale backup and archival.

4. Optical storage

Data is encoded as microscopic pits (low) and lands (high) on a reflective disc surface. A laser reads the disc by detecting changes in reflected light intensity.

How it works

1. The disc surface is divided into concentric tracks, each track split into sectors.
2. Laser‑write: a high‑precision laser heats a tiny spot; the material either melts to form a pit (binary 0) or remains a land (binary 1).
3. Laser‑read: the laser shines on the rotating disc and a photodiode measures the reflected light. A pit reflects less light than a land, producing a voltage change that is interpreted as binary data.

Typical examples

• CD‑ROM / CD‑R / CD‑RW – audio/video distribution and small data backups.
• DVD‑R / DVD‑RW – portable storage for larger media files.
• Blu‑ray Disc (BD‑R / BD‑RE) – high‑capacity optical storage for HD video and large data sets.

5. Solid‑state (flash) storage

Data is stored in semiconductor memory cells (usually NAND flash). Each cell contains a floating‑gate transistor that can trap electrons, representing a binary state.

How it works

1. Writing: electrons are forced onto (or removed from) the floating gate, changing the cell’s threshold voltage – this stores a 0 or 1.
2. Reading: the cell’s threshold voltage is measured; a high voltage indicates one state, a low voltage the other.
3. Erasing: cells are grouped into blocks**; an entire block must be erased before any cell in that block can be rewritten.
4. Wear‑leveling: algorithms distribute write/erase cycles evenly across the memory to extend lifespan.

Typical examples

• Internal solid‑state drive (SSD) – primary storage in modern PCs, laptops and servers.
• USB flash drive – portable device for data transfer.
• Memory cards (SD, micro‑SD) – used in cameras, smartphones and tablets.

Feature	Typical value	Impact on use
Capacity	128 GB – 8 TB	Fits both consumer and enterprise workloads.
Access time (random read)	≈ 0.1 ms	Extremely fast; improves system responsiveness.
Durability	Shock‑resistant, no moving parts	Ideal for mobile devices and laptops.
Write‑cycle limit	~10 000 – 100 000 cycles per block	Managed by wear‑leveling; affects long‑term lifespan.
Cost per GB	≈ $0.10 – $0.25	Higher than HDD but falling rapidly.

6. Data compression

Compression reduces the amount of storage required for a file by removing redundancy. Two main types are used in everyday computing.

Lossless compression

• Original data can be perfectly reconstructed.
• Common algorithms: ZIP, PNG, RLE (run‑length encoding).
• Best for text, spreadsheets, programmes and any data where loss of information is unacceptable.

Lossy compression

• Some original information is permanently discarded to achieve higher reduction.
• Common algorithms: JPEG (images), MP3/AAC (audio), H.264/HEVC (video).
• Used where a slight loss of quality is tolerable in exchange for much smaller file sizes.

Aspect	Lossless	Lossy
Typical reduction	2 – 3 ×	10 – 100 × (or more)
Data integrity	Exact original recovered	Some data permanently lost
Common uses	Documents, source code, archival images	Photos, music, streaming video
Examples of formats	ZIP, PNG, FLAC	JPEG, MP3, H.264

7. Comparison of magnetic, optical and solid‑state storage

Aspect	Magnetic	Optical	Solid‑state
Speed (random access)	5 – 10 ms (seek)	50 – 100 ms (seek)	≈ 0.1 ms
Typical capacity	500 GB – 20 TB	700 MB – 25 GB	128 GB – 8 TB
Durability	Shock‑sensitive; magnetic‑field sensitive	Scratch‑sensitive; magnetic‑immune	Shock‑resistant; no moving parts; limited write cycles
Cost per GB	Low	Medium‑high	Higher (but falling)
Typical uses	Primary secondary storage in PCs/servers, large backups, tape archives	Media distribution, occasional backups, archival copies	Operating‑system drives, portable storage, high‑performance applications

8. Quick revision questions

1. Explain why solid‑state drives have faster random access times than hard‑disk drives.
2. What role does the photodiode play in an optical‑disc drive?
3. Identify one advantage and one disadvantage of magnetic tape for data backup.
4. How does the “write‑erase‑write” requirement affect the lifespan of flash memory, and what technique is used to mitigate this?
5. Give one example of a lossless compression format and one of a lossy format, and state when each would be preferred.

9. Key formulas

Capacity of a rotating or disc‑based medium:

$$\text{Capacity} = \text{Number of tracks} \times \text{Sectors per track} \times \text{Bytes per sector}$$

Areal density for magnetic storage (bits per square inch):

$$D = \frac{N_{\text{bits}}}{A_{\text{area}}}$$

These formulas illustrate how increasing the number of tracks, reducing sector size, or improving areal density raises overall storage size.