Primary Memory

AQA 4.6.2 — Primary memory

Retrieval: Processor Performance mentioned cache as one of the four factors affecting speed, without explaining what it actually is. This lesson explains cache properly, alongside RAM and ROM — the memory a CPU works with directly, moment to moment.

Key vocabulary

RAM (Random Access Memory) — fast, general-purpose working memory holding currently-running programs and their data.
ROM (Read-Only Memory) — holds fixed startup instructions that must survive being switched off.
Cache — very small, very fast memory sitting between the CPU and RAM.
Volatile — loses its contents the instant power is removed.
Non-volatile — retains its contents with no power at all.

Understand — volatility is the key distinction

RAM is volatile: switch the power off, and everything held in it is gone immediately — which is exactly why unsaved work disappears in a power cut. ROM is non-volatile: it holds the fixed instructions a computer needs the instant it's switched on (before an operating system has even loaded from disk), so it must survive having no power at all between uses.

Visualise — the memory hierarchy

FASTEST, SMALLEST, MOST EXPENSIVE PER BYTE
        ▲
   Registers   (inside the CPU itself)
   Cache       (very close to the CPU)
   RAM         (main working memory)
   Secondary storage (SSD/HDD - covered next lesson)
        ▼
SLOWEST, LARGEST, CHEAPEST PER BYTE

Each level trades size and cost for speed: registers are tiny but instant; secondary storage is huge but comparatively slow. Cache exploits locality of reference — the well-established observation that a program tends to reuse the same instructions and data repeatedly in a short span of time (a loop, for example, re-reads the same few instructions over and over). Keeping that recently-used material in a small, very fast cache means the CPU usually doesn't have to wait for slower RAM at all.

Compare — RAM vs ROM vs cache

	RAM	ROM	Cache
Volatile?	Yes	No	Yes
Typical size	Gigabytes	Megabytes or less	Kilobytes–megabytes
Speed	Fast	Moderate	Extremely fast
Typical use	Running programs and their current data	Fixed startup instructions (e.g. BIOS/firmware)	Recently/soon-to-be-reused CPU data

Apply it — classify the memory type

For each scenario, identify RAM, ROM or cache:

The exact instructions a computer runs the instant it's switched on, before anything else has loaded.
The document a student is currently typing in a word processor, not yet saved.
A value the CPU calculated moments ago inside a loop, and is very likely to need again on the very next pass.

(1: ROM — fixed, must survive being off. 2: RAM — actively being worked on, volatile. 3: Cache — recently used, kept close to the CPU for near-instant reuse.)

Explain — why not just make everything as fast as cache?

Cache-speed memory is dramatically more expensive per byte to manufacture than RAM, and RAM is in turn more expensive per byte than secondary storage. Building a computer entirely out of cache-speed memory at today's RAM-sized capacities would be enormously expensive — the memory hierarchy exists specifically because no single memory type can be simultaneously fast, large and cheap; every real computer has to trade between the three.

Common mistake

Describing RAM as "storage" — RAM is volatile working memory, not storage. A file only genuinely survives being switched off once it's been saved to non-volatile secondary storage, covered properly in the next lesson; RAM alone is never enough to keep it.

Challenge

Explain, using the difference between volatile and non-volatile memory, why word processors and other applications include an "autosave" feature that periodically writes a copy of your work to disk — what specific risk is autosave designed to reduce?

Looking ahead: the next lesson (Secondary Storage) covers the non-volatile layer at the bottom of the memory hierarchy above — where data actually lives long-term, once it's no longer being directly worked on by the CPU.

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