OCR J277 · Component 1 · Topic 1.2.1

Primary
Storage

RAM · ROM · Virtual Memory · Cache

CSZone OCR GCSE Computer Science J277

Learning Objectives

By the end of this video you will be able to...

Explain why computers need primary storage

Describe the key characteristics of RAM and ROM and explain the difference between them

Explain how virtual memory works and when it is needed

Describe the purpose of cache memory

⚡ All four areas are examined — especially virtual memory, where students regularly write incorrect statements that cost marks

Why is Primary Storage needed?

The CPU cannot use data directly from secondary storage

THE CORE REASON

Primary storage is needed to store the active data, instructions, software, and operating system that the processor needs to access quickly. Secondary storage is too slow for the CPU to use directly.

RAM

Stores the currently running data, instructions, programs and OS that the CPU needs right now

ROM

Stores the start-up instructions permanently so they are not lost when the power is off

Primary storage also includes cache — stores frequently used data/instructions close to the CPU for very fast access

RAM — Random Access Memory

Key characteristics of RAM

VOLATILE Loses data when power is off

RAM loses its contents when the computer is turned off. It only holds data while there is power.

Read/write — data in RAM can be read from and written to constantly

Stores the currently running programs, data, instructions and operating system — everything the CPU is actively using

Contents change constantly while the computer is running — when you open a new program, it is loaded into RAM

📝 More RAM = more programs can be open at once without slowdown

ROM — Read Only Memory

Key characteristics of ROM

NON-VOLATILE Retains data when power is off

ROM retains its contents when the computer is turned off. The data is permanently stored and does not need power.

Read only — data in ROM cannot (easily) be written to or changed by the user

Stores the start-up instructions / BIOS / bootstrap — the instructions needed to start the computer before the OS loads

Contents do not change — these instructions are set during manufacture and are not altered during normal use

📝 Without ROM the computer would not know how to start — it holds the first instructions the CPU runs on power-on

RAM vs ROM

The key differences at a glance

	RAM	ROM
Volatile?	Yes — loses data when power off	No — retains data when power off
Read/Write?	Read and write	Read only
What it stores	Currently running programs, data, instructions, OS	Start-up instructions / BIOS / bootstrap
Contents change?	Yes — constantly while running	No — fixed during manufacture

Cache Memory

A small, very fast type of primary storage

WHAT IS CACHE?

Cache is a small amount of very fast memory built into or located very close to the CPU. It stores frequently used data and instructions so the CPU can access them quickly without fetching from the slower RAM every time.

WHY IT HELPS

Faster than RAM → CPU spends less time waiting for data → performance improves

THE TRADE-OFF

Much smaller in size than RAM — only the most frequently used data fits in cache; everything else stays in RAM

📝 Cache was already covered in 1.1.1 (as a CPU component) and 1.1.2 (cache size affects performance) — 1.2.1 classifies it as a type of primary storage

Virtual Memory

Why is virtual memory needed?

THE TRIGGER CONDITION

Virtual memory is used when RAM is full or nearly full. The computer needs more space to store active data, but there is no more room in RAM.

When RAM fills up, the computer cannot simply stop working — it needs to keep running programs

Virtual memory allows the computer to use part of secondary storage as an overflow area for data that would otherwise go in RAM

⚠ Common mistake: Virtual memory is not part of RAM — it is a section of secondary storage. This distinction is directly tested in mark schemes.

Virtual Memory — How it works

The process step by step

Virtual memory is a section of secondary storage that is partitioned to act as an extension of RAM

1

RAM becomes full or nearly full — there is not enough space for all active programs and data

2

Data / instructions that are less recently used are transferred from RAM to virtual memory (on secondary storage)

3

This frees up space in RAM for the data that is currently needed by the CPU

4

When that data is needed again, it is transferred from virtual memory back to RAM

⚠ Virtual memory is slower than RAM because secondary storage access is slower — excessive VM use causes the computer to slow down ("thrashing")

Exam Practice

Pause and have a go before the answer appears

Question 1

State two differences between RAM and ROM.

2 marks

✓ MARK SCHEME

1 mark

RAM is volatile (loses data when power is off); ROM is non-volatile (retains data when power is off)

1 mark

RAM is read/write; ROM is read only

Alternative second mark: RAM stores currently running programs/data/instructions; ROM stores start-up instructions/BIOS

Exam Practice

Question 2

Describe the purpose of ROM in a computer system.

2 marks

✓ MARK SCHEME

1 mark

Stores the start-up instructions / BIOS / bootstrap — the instructions needed to start/boot the computer

1 mark

ROM is non-volatile — so these instructions are not lost when the computer is turned off

Both the what (start-up instructions) and the why (non-volatile) are needed for full marks on a describe question

Exam Practice

Question 3 — Virtual Memory (2023 style)

The following statements about virtual memory are either true or false. Correct any false statements.

(a) A section of primary storage is partitioned to act as virtual memory.
(b) VM is needed when RAM is full or nearly full.
(c) Data from ROM is transferred into virtual memory.
(d) Data from VM is transferred back to secondary storage when needed.

4 marks

✓ MARK SCHEME

1 mark

(a) False — a section of secondary storage is partitioned to act as virtual memory

1 mark

(b) True ✓

1 mark

(c) False — data from RAM is transferred into virtual memory

1 mark

(d) False — data from VM is transferred back to RAM when needed

Exam Practice

Question 4

Explain why a computer needs primary storage. Your answer should refer to both RAM and ROM.

3 marks

✓ MARK SCHEME

1 mark

Primary storage is needed to store active data / instructions / software / OS that the processor needs to access quickly

1 mark

RAM stores the currently running programs/data/instructions so the CPU can access them quickly

1 mark

ROM stores start-up instructions / BIOS permanently so they are not lost when the power is off

Exam Practice

Question 5

Explain how virtual memory works when RAM is full.

3 marks

✓ MARK SCHEME

1 mark

A section of secondary storage is partitioned to act as virtual memory

1 mark

Data / instructions are transferred from RAM to virtual memory (secondary storage) when RAM is full

1 mark

Data is transferred from virtual memory back to RAM when it is needed again

All three points must be precise. Use "secondary storage" and "RAM" explicitly.

Common Mistakes

Don't lose easy marks

1

Saying virtual memory is part of RAM or primary storage. It is a section of secondary storage. This is one of the most commonly corrected statements in OCR mark schemes.

2

Saying data from ROM is transferred to VM, or data from VM goes back to secondary storage. The correct flow is: RAM → VM when full, and VM → RAM when needed again.

3

Saying RAM is non-volatile or ROM is volatile. These are opposites. RAM = volatile (loses data on power off). ROM = non-volatile (retains data on power off).

4

Saying ROM stores "the operating system" or "programs". ROM stores start-up instructions / BIOS / bootstrap. The OS is stored on secondary storage and loaded into RAM at start-up.

Topic Summary — 1.2.1

What You Need to Know

RAM

Volatile · Read/write
Stores currently running programs/data/instructions/OS
Contents change constantly

ROM

Non-volatile · Read only
Stores start-up instructions / BIOS / bootstrap
Contents fixed during manufacture

CACHE

Small, very fast primary storage
Stores frequently used data/instructions
Located close to the CPU · faster than RAM

VIRTUAL MEMORY

Section of secondary storage partitioned as overflow
Used when RAM is full/nearly full
RAM → VM (when full) · VM → RAM (when needed)

CSZone

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1.2.2

Secondary Storage

Optical · Magnetic · Solid State

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PrimaryStorage

By the end of this video you will be able to...

The CPU cannot use data directly from secondary storage

Key characteristics of RAM

Key characteristics of ROM

The key differences at a glance

A small, very fast type of primary storage

Why is virtual memory needed?

The process step by step

Pause and have a go before the answer appears

Don't lose easy marks

What You Need to Know

Next Video

Primary
Storage