Data storage

Computer Science · Hardware. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Primary storage is memory the CPU can access directly: RAM and ROM. RAM holds the programs and data in use right now. ROM holds the start-up instructions.

Secondary storage is not accessed directly by the CPU. It keeps programs and files permanently, even with the power off. It can be magnetic, optical or solid-state.

When RAM is full, the computer uses part of secondary storage as virtual memory. Pages of data not needed at that moment are moved out of RAM to make room, and moved back when needed. Cloud storage keeps files on remote servers that you reach through the internet.

Rules to remember

RAM: volatile (loses contents without power), can be read and written, holds data and programs in use
ROM: non-volatile, read only, holds the start-up (boot) instructions
Magnetic (HDD): spinning platters; tiny areas are magnetised to stand for 1 or 0; a read/write head moves over them
Optical (CD, DVD, Blu-ray): a laser reads pits and lands on the disc surface
Solid-state (SSD, USB flash drive, SD card): transistors hold charge; no moving parts; fast, quiet, robust
Virtual memory: part of secondary storage used as extra RAM; slower than real RAM
Cloud: access from anywhere and no hardware to maintain, but needs an internet connection and you depend on the provider for security

Common mistake

Students say ROM is secondary storage because it is permanent. ROM is primary storage; it is non-volatile memory that the CPU reads directly.

Worked example

A camera fixed to a bicycle helmet must record video. Which type of secondary storage is most suitable, and why?

Magnetic drives have spinning platters and a moving head, which shaking can damage. They are also heavy.
Optical discs are too large for a small camera and also need moving parts to spin the disc.
Solid-state storage has no moving parts, so bumps do not harm it. It is small, light and uses little power.

Answer: Solid-state (flash) storage, such as an SD card.

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