GCSEPaper 1Memory and storagePrimary storage (memory)
1.2.1

Primary storage (memory)

Why primary storage is needed, the differences between RAM and ROM, virtual memory, and cache.

Learning Objectives

  • 1Describe the need for primary storage
  • 2Describe the purpose and characteristics of RAM
  • 3Describe the purpose and characteristics of ROM
  • 4Explain the difference between RAM and ROM
  • 5Explain why virtual memory is needed and how it works

What primary storage is

Primary storage (also called main memory) is the memory the CPU can reach directly and very quickly. When you open a program, it is loaded into primary storage so the CPU can work with it. It is much faster than secondary storage (like a hard drive), but it usually holds less. Primary storage is made up of two main types: RAM and ROM.
Primary storage (main memory) is the memory that the CPU can access directly. The programs and data currently in use are held here so the CPU can reach them quickly, far faster than reading from secondary storage. Primary storage usually consists of RAM and ROM.
Primary storage is memory that is directly accessible by the CPU, holding the data and instructions currently in use. It is much faster to access than secondary storage but has a smaller capacity. It usually consists of RAM, which holds the programs and data currently running, and ROM, which holds the start-up instructions.

Think of it like...

Primary storage is like your desk: it holds what you are working on right now, within easy reach. Secondary storage is like a filing cabinet: much bigger, but slower to fetch things from.

RAM (Random Access Memory)

RAM holds the programs, data and parts of the operating system that are being used right now. The CPU both reads from and writes to RAM as it works. RAM is volatile, which means it loses everything when the power is turned off. That is why you lose unsaved work if the computer suddenly shuts down.
RAM stores the data, programs and parts of the operating system that are currently in use. It can be both read from and written to. RAM is volatile: its contents are lost when the power is switched off. The more RAM a computer has, the more it can hold at once, so it relies less on slower virtual memory.
RAM is volatile primary storage that holds the programs, data and operating system currently in use. It can be read from and written to. Because it is volatile, its contents are lost when power is removed. The amount of RAM affects performance: with more RAM, more can be kept in fast memory at once, so the system needs to use slower virtual memory less often.

ROM (Read Only Memory)

ROM holds the instructions the computer needs to start up (the boot process). Unlike RAM, ROM is non-volatile, so it keeps its contents even when the power is off. And as the name says, it is read-only: the computer reads the instructions but does not normally change them.
 RAMROM
Volatile?VolatileNon-volatile
Read/write?Read and writeRead-only
HoldsPrograms/data in useStart-up instructions
ROM stores the start-up instructions a computer needs when it is switched on (the boot sequence). ROM is non-volatile, so it keeps its contents when the power is off, and it is read-only, meaning its contents are not changed during normal use.
 RAMROM
Volatile?VolatileNon-volatile
Read/write?Read and writeRead-only
HoldsPrograms and data in useStart-up (boot) instructions
ROM is non-volatile primary storage that holds the instructions needed to boot the computer when it is switched on. Its contents are retained without power and are read-only under normal operation, ensuring the start-up instructions are always available. Note the key distinction: RAM is volatile and read/write, while ROM is non-volatile and read-only.
RAMVolatile (lost when power off)Read and writeHolds programs & data in useROMNon-volatile (kept when off)Read-onlyHolds start-up instructions

Virtual memory

Sometimes a computer runs low on space in RAM because too much is open at once. To cope, it uses virtual memory: it moves data that is not being used right now out of RAM and onto secondary storage (like the hard drive) to free up space. When that data is needed again, it is moved back into RAM. This keeps the computer going, but it is slower because secondary storage is slower than RAM.
Virtual memory is used when RAM becomes full. The operating system moves data that is not currently in use from RAM to secondary storage, freeing up RAM for the programs that need it. When that data is needed again, it is transferred back into RAM. This lets more programs run than would fit in RAM alone, but it is slower because secondary storage is much slower than RAM.
Virtual memory is an area of secondary storage used as an extension of RAM when RAM is full. The operating system transfers data not currently in use out of RAM to secondary storage, and moves it back when required. This allows a system to run more, or larger, programs than physical RAM alone would allow, at the cost of performance, since accessing secondary storage is much slower than RAM.
RAMFullUnused dataMoved outMoved backWhen neededSecondary storageVirtual memory

Think of it like...

Virtual memory is like clearing space on your desk by putting some papers back in the filing cabinet, then fetching them again when you need them. It works, but walking to the cabinet takes longer.

Cache

Cache is a small amount of very fast memory that sits between the CPU and RAM. It stores data and instructions the CPU uses a lot, so they can be fetched quickly instead of going all the way to slower RAM. (You also met cache in Systems Architecture.)
Cache is a small, very fast memory located between the CPU and RAM. It holds frequently used data and instructions so the CPU can access them more quickly than fetching from RAM, reducing the time the CPU spends waiting for data.
Cache is fast memory positioned between the CPU and main memory. It stores frequently accessed data and instructions so the CPU can retrieve them faster than from RAM, reducing waiting time and improving performance.
CPUVery fastCacheSmall, fast memorySlowerRAMLarger, slower memory

Exam Phrasing: Use These Exact Phrases

  • ✓Primary storage is the memory that can be accessed directly by the CPU
  • ✓RAM is volatile, so its contents are lost when the power is turned off
  • ✓RAM can be read from and written to
  • ✓RAM holds the data, programs and operating system currently in use
  • ✓ROM is non-volatile, so it keeps its contents when the power is off
  • ✓ROM is read-only and holds the start-up (boot) instructions
  • ✓Virtual memory is used when the RAM is full (or nearly full)
  • ✓Virtual memory moves data not currently in use from RAM to secondary storage
  • 💡Learn two key words for each: RAM is volatile and read/write; ROM is non-volatile and read-only.
  • 💡"Volatile" means "loses its contents when the power is off". Use that exact idea in your answer.
  • 💡When explaining virtual memory, always say it is used when RAM is full (or nearly full) and that data is moved to secondary storage. OCR credits "nearly full" too.
  • 💡In virtual memory questions say RAM, not "primary storage", examiners are told not to accept primary storage in place of RAM there.
  • 💡Don't say ROM stores the operating system. It stores the start-up instructions that begin loading the OS; the OS itself is loaded into RAM.

Wrong:

ROM stores the operating system

Right:

ROM stores the start-up (boot) instructions; the operating system is loaded into RAM

Why it matters: Mixing up the boot instructions with the full operating system is a very common error.

Wrong:

RAM keeps its contents when the power is off

Right:

RAM is volatile, so it loses its contents when the power is turned off

Why it matters: This confuses RAM with ROM. Volatile means the contents are lost without power.

Wrong:

Virtual memory is a type of RAM chip

Right:

Virtual memory is space on secondary storage used as extra memory when RAM is full

Why it matters: Virtual memory is not physical RAM; it needs secondary storage to work.

Question:

Explain what virtual memory is and why a computer might need to use it. [3]

💡 Attempt this before revealing the answer, practice makes perfect!

Model Answer:

Virtual memory is an area of secondary storage that is used as an extension of RAM. It is needed when the RAM becomes full, for example when many programs are open at once. The operating system moves data that is not currently being used from RAM onto secondary storage to free up space, and moves it back into RAM when it is needed again.

Mark Scheme:

  • 1Virtual memory is (an area of) secondary storage used as extra memory / an extension of RAM
  • 2It is used when the RAM is full
  • 3Data not currently in use is moved from RAM to secondary storage (and back when needed)

Tap a card to flip it. Use the arrow keys to move and space to flip.

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Exam-style questions. Have a go, then reveal the mark scheme to check your answer.

1.State what is meant by volatile memory.

[1]

2.State one difference between RAM and ROM.

[1]

3.Describe the purpose of ROM in a computer.

[2]

4.Explain why a computer uses virtual memory.

[3]

5.Compare RAM and ROM.

[4]

6.Discuss why a computer needs both RAM and ROM, and explain what happens when the RAM becomes full.

[8]

Extended response · marked in levels, not by counting points

2025Question paperMark schemeExaminers' report
2024Question paperMark schemeExaminers' report
2023Question paperMark schemeExaminers' report
2022Question paperMark schemeExaminers' report

The examiners' reports are the most useful and least read of the three: they say what candidates actually got wrong each year. All files are hosted by OCR.

Past-paper questions grouped by topic

The same OCR questions, sorted by topic rather than by year (via Physics & Maths Tutor).

2024
Paper 1

Stronger answers on virtual memory recognised that it is used when a system is short of RAM, and crucially that it relies on secondary storage. Examiners credited the point that a device with no secondary storage cannot use virtual memory.

Paper 1

When ROM came up, examiners credited answers identifying that it stores the start-up instructions (or firmware) that run when the computer is switched on, rather than describing it as storing the whole operating system.

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Secondary storage