1.2.1 Primary Storage — OCR J277

Primary Storage

Primary storage refers to memory that is directly accessible by the CPU. It is used to hold the programs currently running and the data being processed. There are two main types of primary storage for OCR J277: RAM and ROM.

RAM — Random Access Memory

RAM is volatile primary storage — data is lost when the power is switched off. It is used to temporarily hold:

The operating system (loaded from secondary storage at boot)
Currently running programs and applications
Data being actively processed by the CPU

The CPU can read from and write to RAM. It is much faster than secondary storage but much slower than cache. The contents of RAM are cleared every time the computer is switched off.

Effect of RAM size on performance

More RAM means the computer can hold more programs and data in primary storage at once, reducing the need to swap data to slower secondary storage. If RAM is insufficient, the computer uses virtual memory (a portion of the hard drive), which is much slower and causes the computer to run sluggishly.

ROM — Read-Only Memory

ROM is non-volatile primary storage — data is retained when the power is switched off. The CPU can read from ROM but cannot write to it (in normal operation).

In a computer, ROM typically stores the BIOS/bootstrap program — the small program that runs when the computer is first switched on, checks hardware (POST — Power-On Self Test), and loads the operating system from secondary storage into RAM.

ROM in embedded systems

In embedded systems, the entire program (firmware) is stored in ROM because it must be permanent and non-changeable. This is why washing machines and microwaves do not lose their programs when unplugged.

RAM vs ROM — Comparison

Feature	RAM	ROM
Volatile/Non-volatile	Volatile (data lost on power off)	Non-volatile (data retained)
Read/Write	Read and write	Read only (normally)
Contents	Programs and data currently in use	BIOS / firmware / bootstrap loader
Changed by user?	Yes — constantly changed	No — fixed at manufacture
Speed	Fast	Fast but typically slower than RAM
Size	Larger (GBs)	Much smaller (MBs)

Virtual Memory

When RAM is full, the OS uses part of the secondary storage (hard drive or SSD) as an extension of RAM. This is called virtual memory. While it prevents the computer crashing, it is significantly slower than real RAM because secondary storage access times are much higher. This is why computers slow down when many large programs are open.

Exam tip: Always link volatile to RAM (data lost on power off) and non-volatile to ROM (data retained). A very common 2-mark question: "State the difference between RAM and ROM." Answer: RAM is volatile (data lost when power off); ROM is non-volatile (data retained). RAM can be read and written; ROM can only be read.

⚠️ Common Mistakes

Saying ROM stores the operating system — in a PC, the OS is stored on secondary storage (HDD/SSD) and loaded into RAM at boot. ROM only holds the BIOS/bootstrap.
Confusing "volatile" with "fast" — volatility is about data persistence, not speed
Saying RAM stores data permanently — RAM is volatile; data is lost on power off
Forgetting that more RAM reduces the need for virtual memory, improving performance
Saying the CPU reads data from secondary storage directly — the CPU reads from RAM (primary storage). Data must be loaded into RAM first.

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Key points covered in this video

What primary storage is and why the CPU needs it
RAM: volatile, read/write, holds OS and running programs
ROM: non-volatile, read-only, holds BIOS/firmware
How RAM size affects performance and virtual memory
The RAM vs ROM comparison table

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Worksheet — 1.2.1 Primary Storage (RAM and ROM)

8 questions · 22 marks · Submit to reveal mark schemes

Q1State what is meant by the term 'volatile' in relation to memory.[1]

✅ Mark scheme

Data/contents are lost when power is switched off. [1]

Q2State two differences between RAM and ROM.[2]

✅ Mark scheme

Any two: RAM is volatile / ROM is non-volatile [1]; RAM can be read and written / ROM is read-only [1]; RAM holds current programs and data / ROM holds BIOS/firmware [1].

Q3Explain what is stored in ROM in a desktop computer and why it is stored there.[3]

✅ Mark scheme

• BIOS / bootstrap program / startup program [1]
• It is stored in ROM because it must be available as soon as the computer is switched on [1]
• ROM is non-volatile so the program is retained when the computer is off [1]
• It must not be changeable/overwritten in normal operation [1] (any 3)

Q4Describe what happens during the computer's startup (boot) process, referring to both RAM and ROM.[4]

✅ Mark scheme

• CPU reads the BIOS program from ROM [1]
• BIOS checks hardware (POST — Power-On Self Test) [1]
• BIOS locates the operating system on secondary storage [1]
• OS is loaded from secondary storage into RAM [1]

Q5Explain why having more RAM can improve the performance of a computer.[3]

✅ Mark scheme

• More programs/data can be held in RAM simultaneously [1]
• The CPU can access data from RAM faster than from secondary storage [1]
• Reduces the need for virtual memory (using secondary storage as RAM) [1]
• Virtual memory is much slower, so avoiding it speeds up the computer [1] (any 3)

Q6Define virtual memory and explain one disadvantage of using it.[2]

✅ Mark scheme

• Virtual memory is a section of secondary storage used as an extension of RAM when RAM is full [1]
• Disadvantage: it is much slower than RAM, so the computer runs noticeably slower [1]

Q7A user says "ROM is faster than RAM so the operating system should be stored in ROM." Explain why this is incorrect.[3]

✅ Mark scheme

• The OS is stored on secondary storage, not ROM [1]
• The OS needs to be loaded into RAM so the CPU can access it during use [1]
• ROM cannot be written to (in normal operation), so the OS cannot be updated if stored there [1]
• ROM is much smaller in capacity than what the OS requires [1] (any 3)

Q8State whether each of the following is stored in RAM or ROM: (a) BIOS program, (b) currently open web browser, (c) word processor currently being used, (d) firmware of a microwave.[4]

✅ Mark scheme

(a) ROM [1]; (b) RAM [1]; (c) RAM [1]; (d) ROM [1].

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Term	Definition

Primary Storage

RAM — Random Access Memory

Effect of RAM size on performance

ROM — Read-Only Memory

ROM in embedded systems

RAM vs ROM — Comparison

Virtual Memory

Key points covered in this video

Worksheet — 1.2.1 Primary Storage (RAM and ROM)

Mini Test — 1.2.1 Primary Storage