Data Storage

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 (Random Access Memory). The 'working memory' of the computer.

• Volatile — contents are lost when power is removed.
• Read AND write — can be modified by the OS and applications.
• Holds the operating system, running programs and the data they're using.
• More RAM = more programs / larger files can run simultaneously.

ROM (Read-Only Memory). Permanent, low-level firmware storage.

• Non-volatile — keeps its contents without power.
• Read-only under normal use (special modes can update some types).
• Holds the BIOS / UEFI — instructions that run when you press the power button to bring the system to life.

	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.

HDD (Hard Disk Drive) — magnetic.

• Spinning platters coated in magnetic material.
• Read/write head floats nanometres above the surface and changes magnetisation to write, reads it to read.
• Strengths: very high capacity (multi-terabyte), cheap per GB, well-understood.
• Weaknesses: mechanical (moving parts wear, sensitive to shock), slower than SSD, noisier, higher power.

SSD (Solid-State Drive) — flash memory.

• Flash-memory chips with no moving parts.
• Data stored by trapping electrons in floating-gate transistors.
• Strengths: much faster, lighter, lower power, silent, more shock-resistant.
• Weaknesses: more expensive per GB, finite write cycles per cell (mitigated by wear levelling).

Optical storage — CDs, DVDs, Blu-ray.

• Laser reads/writes pits and lands on a reflective disc.
• Strengths: cheap to mass-produce; long shelf life if stored well; portable.
• Weaknesses: low capacity (CD ~700MB, DVD ~4.7GB, Blu-ray 25GB single layer), slower, easily scratched.

Magnetic tape.

• Sequential-access magnetic strip on a reel.
• Strengths: very high capacity (tens of TB on modern LTO tapes), very cheap per GB, good for archives.
• Weaknesses: SLOW for individual files because tape is linear; designed for sequential read/write.

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 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.

1. The OS divides RAM and the swap area into fixed-size pages.
2. When RAM is full and a new program needs space, the OS chooses an inactive page and writes it out to the swap area.
3. The freed-up RAM is given to the new program.
4. If the swapped-out page is needed again later, it's read back from disk into RAM (and another page may be evicted in turn).

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).

Cloud storage keeps files on remote servers managed by a provider (Google Drive, OneDrive, iCloud, Dropbox, AWS S3) and accessed over the internet.

Pros.

• Access from any device with an internet connection.
• Built-in backups and redundancy across the provider's data centres.
• No local hardware to maintain or replace.
• Easy file sharing and collaboration.

Cons.

• Requires a reliable internet connection.
• Ongoing subscription cost.
• Privacy and security depend on the provider.
• Risk of vendor lock-in or service shutdown.

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.