Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
RAM, ROM & virtual memory
The CPU can only work on what is held in memory, so every running program is first loaded into RAM, which is thousands of times faster than any disk. Memory and storage form a hierarchy: registers, then cache, then RAM, then storage, with each step holding more data, at lower cost, but running more slowly.
RAMRAM is the computer's working memory. It is fast but volatile, meaning its contents are wiped as soon as the power goes off. The more RAM a computer has, the more programs it can run before it starts to slow down.
ROMROM is small, non-volatile and read-only. It holds the bootstrap instructions that start the machine and load the operating system.
Virtual memoryWhen RAM is full, the operating system moves inactive pages onto the hard drive and treats that space as if it were extra RAM. This works, but because disk access is so much slower, it makes the whole machine feel sluggish — which is exactly why adding more RAM speeds up a slow computer.
Secondary storage
RAM forgets everything once it loses power, so data is stored permanently in secondary storage instead. It helps to know how each technology physically works.
Magnetic (HDD)A hard disk drive stores data as magnetised spots on spinning platters, read by a moving head. It offers huge capacity and is cheap per GB, but its moving parts make it slower, noisier and more fragile if it is dropped.
Solid state (SSD/flash)A solid state drive stores data as charge trapped in flash cells and has no moving parts, so it is fast, silent, tough, light and energy-efficient. It costs more per GB than a hard disk drive.
Optical (CD/DVD/Blu-ray)An optical disc stores data as pits on its surface, read by a laser. Its capacity is tiny, but each copy costs only pennies to produce, which makes it well suited to distributing media.
Choosing the right storage means weighing up six factors: capacity, speed, portability, durability, reliability and cost. In an exam scenario, name the two factors that matter most and explain the trade-off: video editing calls for an SSD, server archives call for an HDD, and posting out software calls for optical media.
To check how many files fit, convert both amounts to the same unit and then divide: a 2 GB stick holding 4 MB photos gives 2000 ÷ 4 = 500 photos, rounded down.
Turn these notes into memory. 11 flashcards cover this topic. Short daily sessions, spaced repetition up to your exam date, quizzes with friends. Free during the beta.Practise Memory & Storage in the app ↗