What you must learn
This is taken directly from the OCR specification. Tick each line when you can explain it without looking. Your ticks are saved in this browser.
Required by the specification
Your activity progress
1 · Why do computers need primary storage?
The CPU can only work on instructions and data that it can reach very quickly. Secondary storage (hard disk, SSD) keeps everything safe when the power is off, but it is far too slow for the CPU to fetch each instruction from it. So the computer has primary storage — memory the CPU can access directly — and the CPU's fetch–decode–execute cycle reads from there.
The memory hierarchy — click each layer
The closer to the CPU, the faster and smaller (and more expensive per byte) the storage is.
Activity 1 — order the storage Activity
Use the arrows to put these in order from fastest (top) to slowest (bottom).
What happens when you switch a computer on? — Activity 2 Activity
This is where ROM and RAM work together. Put the boot sequence in the correct order.
2 · RAM and ROM
Primary storage usually consists of two kinds of memory chip. They look similar but do opposite jobs.
RAM — Random Access Memory
- Purpose: holds the operating system, programs and data currently in use so the CPU can access them quickly.
- Volatile — contents are lost when the power is off.
- Read and write — the CPU constantly changes what is in it.
- Large — typically 4–32 GB in a modern PC.
- Can be upgraded — more RAM lets you run more programs at once before virtual memory is needed.
ROM — Read Only Memory
- Purpose: stores the bootstrap program (BIOS) — the instructions the CPU needs to start up the computer and load the operating system from secondary storage into RAM.
- Non-volatile — contents are kept when the power is off.
- Read only — cannot normally be changed by the user.
- Small — a few megabytes.
- Also found in embedded systems (washing machines, routers) to hold their firmware.
Flash cards — click to flip
Activity 3 — RAM or ROM? Activity
Drag each statement into the correct box (or click a statement, then click a box). Then press Check.
RAM
ROM
3 · Virtual memory
Why it is needed: every program you open needs space in RAM. If you open more programs (or bigger ones) than the RAM can hold, the computer would have to refuse to open them — or crash. Virtual memory solves this by using part of secondary storage (the hard disk or SSD) as if it were extra RAM.
How it works:
- RAM becomes full.
- The operating system picks data or programs in RAM that are not currently being used.
- It moves (swaps) them out of RAM to an area of secondary storage called the swap file / page file.
- The freed space in RAM is used by the program that needs it.
- When the swapped-out program is needed again, it is moved back into RAM (and something else may be moved out to make room).
Activity 4 — Virtual memory simulator Activity
This computer has 4 pages of RAM. Click programs to open them, then click an open program to use it. Watch what happens once RAM is full.
RAM (fast)0 / 4
Secondary storage — swap file (slow)0
Check your understanding
Based on what you saw: open five programs, then use the one that was swapped out. In the box, explain what the operating system did and why the computer slowed down. Use the words RAM, full, secondary storage, swap, slower.
4 · Cache
Even RAM is slow compared with the CPU — a modern processor can execute billions of instructions a second and would spend most of its time waiting for RAM. Cache is a small amount of very fast memory inside or next to the CPU that keeps a copy of the most frequently or recently used instructions and data.
- When the CPU needs something, it checks the cache first. If it is there — a cache hit — it gets it almost instantly.
- If not — a cache miss — it fetches it from RAM (slower) and usually stores a copy in the cache for next time.
- Cache is smaller than RAM (megabytes, not gigabytes) because it is expensive, and it is volatile like RAM.
- More cache (or a higher hit rate) means the CPU waits less, so performance improves.
Activity 5 — Cache hit or miss? Activity
The CPU is going to request a series of data items. The cache holds 4 items. Before each request, predict: will it be a hit or a miss? (When the cache is full, the oldest item is replaced.)
Next request: — Requests remaining: 0
5 · Quick quiz
Ten multiple-choice questions covering the whole topic. Answer them all, then press Mark.
6 · Exam-style questions
Write your answer in the box, then reveal the mark scheme and mark yourself honestly — tick each marking point you made. Remember: in "explain" questions you need the point and the reason.
7 · Questions from real OCR papers
These are the primary-storage questions that have actually appeared in OCR GCSE papers, reworded in brief. Each shows the paper, question number and marks. Answer first, then reveal the mark scheme — tick-box questions are marked automatically; written answers you mark yourself.