Primary storage: RAM and ROM
Primary storage is memory directly accessed by the CPU, of two chip types. RAM is volatile (loses contents without power) and read/write; it holds the data, programs and OS in use. ROM is non-volatile and read-only; it holds the start-up / boot instructions run at switch-on. A computer needs both: ROM boots the machine and loads the OS into RAM. Note: primary storage is both volatile (RAM) and non-volatile (ROM) — never just "volatile".
Secondary storage: 3 technologies
Secondary storage is not directly accessed by the CPU; non-volatile, more permanent. Magnetic (HDD, tape) — rotating platters with tracks/sectors; a head uses electromagnets to magnetise areas (polarity = 1 or 0). Optical (CD/DVD) — a laser reads pits and lands by reflection. Solid-state/flash (SSD, USB flash drive) — no moving parts; NAND/NOR transistors with control and floating gates (trapped charge = 1/0).
Virtual memory and cloud storage
Virtual memory is an area of secondary storage used as extra RAM when RAM is full. Pages not in use are swapped out of RAM to virtual memory and back when needed (paging). It is necessary when programs need more memory than the RAM provides; heavy use slows the computer (secondary storage is slower than RAM). Cloud storage keeps data on physical servers owned by a provider, accessed remotely over the internet; the provider keeps backups.
Drawn from real examiner reports.
Primary storage is not just volatile
Saying "primary storage is volatile" is incomplete — primary storage is both volatile (RAM) and non-volatile (ROM). If a question asks for a characteristic of primary storage, state both, or name the specific chip you mean. RAM loses its contents when power is removed; ROM keeps them. Always give the property together with the chip it belongs to.
November 2023 Paper 1 primary-storage characteristic question: "Some candidates stated that primary storage is volatile. This answer was incomplete as primary storage is both volatile and non-volatile."
RAM and ROM are not secondary storage
RAM and ROM are primary storage (directly accessed by the CPU), not secondary-storage devices. In a "circle the storage devices" question, do not tick RAM or ROM as secondary storage — the secondary devices are HDD, SSD, CD/DVD, USB flash drive, memory card. RAM and ROM are memory chips inside primary storage, not backing store.
June 2023 Paper 1 storage-devices question: "The most common incorrect answers were RAM and ROM" — candidates wrongly classified RAM/ROM as secondary-storage devices.
Do not swap RAM and ROM properties
Do not swap the properties of the two chips. RAM is volatile and read/write; ROM is non-volatile and read-only. A frequent slip is saying ROM is volatile, or that RAM keeps its contents when switched off. And "RAM is memory" earns nothing — give the two-element form: RAM = volatile + read/write; ROM = non-volatile + read-only (boot instructions).
Magnetic: say electromagnets, not magnets
"Magnetic storage uses magnets" is too brief — say it uses electromagnets to magnetise areas of the platters (tracks/sectors). For solid-state, "it stores data" earns nothing: say transistors / NAND flash, control and floating gates, no moving parts. For optical, name the laser reading pits and lands. Cambridge rewards the mechanism, not one word.
November 2023 Paper 1 Q8(a): "improved understanding of how electromagnets are used in magnetic storage. Some candidates gave brief states such as it uses magnets." Q8(b): "Few candidates could give features of solid-state storage. The most common correct answer was that it uses transistors."
Name specific example devices, not USB
Match the example to the technology and name it specifically. "USB" is not a storage device — write USB flash drive (or memory stick). Magnetic → hard disk drive (HDD); optical → CD / DVD / Blu-ray; solid-state → SSD / USB flash drive / memory card. Naming a device from the wrong technology (e.g. calling a DVD magnetic) also loses the mark.
November 2023 Paper 1 Q8(c): "Some candidates were too vague in their example such as USB. Candidates must make sure their response is specific such as a USB drive."
Cloud can be hacked too, like local
For an advantage/disadvantage of cloud compared with local, the point must be true of the cloud but not equally true of local. Invalid: "it could be hacked" — local data can be hacked too, so examiners reject it. Valid disadvantages: no access if the internet is lost/slow; dependence on the provider; subscription cost. Ask: is this only true of the cloud?
June 2023 Paper 1 cloud-storage question: "Many candidates stated the photos could be hacked... the photos could also be hacked when stored locally, so this is not a disadvantage... The most common correct answer given was not being able to access them if internet connection is lost."
Cloud vs local: test the point
For a cloud vs local advantage/disadvantage, state the point, then test it against local storage: keep it only if true of the cloud and not of a local disk. "Access from any device with internet" passes; "could be hacked" fails (local data can be hacked too).
RAM/ROM: give two elements each
Build the four-cell grid — RAM: volatile, read/write; ROM: non-volatile, read-only — and give two elements per term. Primary = directly accessed by the CPU; secondary = not directly accessed, more permanent. A one-element answer ("RAM is memory") usually scores nothing.
Describe operation: mechanism + example
For "describe how a storage type works", name three things: the technology, its physical mechanism and a specific example — e.g. magnetic → platters/sectors → electromagnets magnetise the surface → HDD. The mechanism keyword is where the marks are.
Capacity sums: convert units first
Files that fit (round down); total size . Convert units first — , , — then divide or multiply; show working.
Computers store data in two broad layers: primary storage (fast memory the CPU uses while running) and secondary storage (slower, permanent storage for files). This topic also covers virtual memory and cloud storage.
Primary storage — memory directly accessed by the CPU (central processing unit). It is made up of RAM and ROM.
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