Memory and Storage
Memory and storage overview
Computers need somewhere to hold programs and data. This splits into primary storage (memory the CPU uses directly) and secondary storage (long-term storage). You must know the difference and the main technologies.
Primary storage (memory)
RAM (Random Access Memory)
- Holds the programs, data and operating system currently in use.
- Volatile – its contents are lost when the power is turned off.
- Fast to read/write; the CPU works with data here.
- More RAM = more can run at once without slowing down.
ROM (Read Only Memory)
- Holds start-up instructions — the bootloader / BIOS that runs when the computer is switched on.
- Non-volatile – keeps its contents without power.
- Normally read-only (not changed during use).
Virtual memory
- When RAM is full, the OS moves some data to a reserved area of the hard drive called virtual memory, freeing RAM for active tasks.
- Lets more programs run than RAM alone allows, but is slower because secondary storage is slower than RAM.
Secondary storage
Used for long-term, non-volatile storage of files and programs. Three main types:
| Type | How it works | Pros | Cons |
|---|---|---|---|
| Magnetic (HDD) | Data stored magnetically on spinning platters | Cheap, large capacity | Slower, moving parts can fail |
| Solid state (SSD, USB, memory cards) | Data stored in flash memory (no moving parts) | Fast, durable, silent, low power | More expensive per GB |
| Optical (CD/DVD/Blu-ray) | Data read/written with a laser on a disc | Cheap, portable | Small capacity, easily scratched |
Choosing storage — factors to weigh
- Capacity (how much it holds), speed, cost per GB, portability, durability and reliability.
- Example: a laptop uses an SSD for speed and durability; a backup server uses HDDs for cheap, large capacity.
Worked example
Why does adding more RAM often make a slow computer feel faster?
- With more RAM, the OS relies less on virtual memory (the slow hard-drive swap area), so the CPU spends less time waiting for data. ✓
Common mistakes
- Saying RAM is non-volatile — RAM is volatile (lost on power off); ROM is non-volatile.
- Confusing memory (primary, e.g. RAM) with storage (secondary, e.g. SSD/HDD).
- Thinking virtual memory speeds things up — it lets more run, but is slower than real RAM.
Exam tips
- Learn the volatile / non-volatile labels: RAM = volatile, ROM = secondary storage = non-volatile.
- For "which storage" questions, justify using the factors (capacity, speed, cost, portability, durability).
- Be able to explain why an SSD is usually chosen over an HDD in modern devices (faster, no moving parts).
Key facts to remember
- RAM = volatile, holds what's in use now; ROM = non-volatile, holds boot instructions.
- Virtual memory = spare hard-drive space used when RAM is full (slower).
- Secondary storage: magnetic (HDD), solid state (SSD/flash), optical (CD/DVD) — chosen on capacity, speed, cost, portability and durability.