Primary Memory (RAM and ROM)

This lesson covers the two types of primary memory — RAM and ROM — their characteristics, differences, and roles within a computer system. This is a core topic in AQA and OCR GCSE Computer Science.

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What Is Primary Memory?

Primary memory (also called main memory or internal memory) is memory that the CPU can access directly. It is connected to the CPU via the system bus and is used to store data and instructions that are currently being used or about to be used.

There are two types of primary memory:

1. RAM (Random Access Memory)
2. ROM (Read-Only Memory)

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RAM (Random Access Memory)

What Is RAM?

RAM is the main working memory of a computer. When you open a program, the operating system loads it from secondary storage (such as a hard drive or SSD) into RAM so that the CPU can access it quickly.

Key Characteristics of RAM

Feature	Detail
Volatile	Data is lost when the power is turned off
Read and write	Data can be both read from and written to RAM
Speed	Much faster than secondary storage but slower than cache
Capacity	Typically 4 GB to 64 GB in modern computers
Purpose	Stores currently running programs, the operating system, and data being processed

Why Is RAM Important?

• The more RAM a computer has, the more programs and data it can hold in fast-access memory at the same time.
• If RAM runs out, the operating system uses a section of secondary storage as virtual memory, which is much slower and causes the computer to slow down.

Types of RAM

Type	Full Name	Use
DRAM	Dynamic RAM	Used for main memory; cheaper but needs constant refreshing
SRAM	Static RAM	Used for cache; faster and does not need refreshing, but more expensive

Exam Tip: When asked "What happens when a computer is turned off?", you must state that the contents of RAM are lost because RAM is volatile. ROM retains its contents because it is non-volatile.

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ROM (Read-Only Memory)

What Is ROM?

ROM is a type of primary memory that stores data permanently. Unlike RAM, the data in ROM is not lost when the power is switched off.

Key Characteristics of ROM

Feature	Detail
Non-volatile	Data is retained when the power is turned off
Read-only	Data can be read but generally not written to during normal operation
Speed	Fast (though typically not as fast as RAM for general use)
Capacity	Very small (usually just a few kilobytes or megabytes)
Purpose	Stores the BIOS/UEFI (bootstrap program) — the first instructions the computer runs when it is powered on

What Does ROM Store?

ROM stores the BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface). This is a small program that:

1. Runs automatically when the computer is powered on.
2. Performs a POST (Power-On Self-Test) to check hardware is working.
3. Identifies and initialises hardware components (keyboard, screen, drives).
4. Locates the operating system on secondary storage and loads it into RAM.
5. This entire process is called booting (or bootstrapping).

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The Memory Hierarchy

Primary memory sits between the ultra-fast CPU registers and the much slower secondary storage. The diagram below shows the typical hierarchy from fastest (top) to slowest (bottom).

graph TD
    A["Registers (inside CPU)"] --> B["Cache (L1 / L2 / L3)"]
    B --> C["RAM (Primary Memory)"]
    C --> D["Secondary Storage (SSD / HDD)"]
    C --> E["ROM (BIOS / UEFI)"]

RAM vs ROM: Key Differences

Feature	RAM	ROM
Volatile?	Yes — data lost when power is off	No — data retained when power is off
Read/Write?	Read and write	Read-only (during normal use)
Contents	Currently running programs and data	BIOS/UEFI (boot-up instructions)
Size	Large (4–64 GB typically)	Very small (a few MB)
Speed	Fast	Fast
Can be changed?	Yes — contents constantly change	No — contents are fixed at manufacture (or rarely updated)

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Virtual Memory

When RAM is full, the operating system uses a technique called virtual memory:

• A section of secondary storage (e.g., hard drive or SSD) is allocated to act as an extension of RAM.
• Data that is not currently being used is moved from RAM to the virtual memory area on disk (this process is called swapping or paging).
• When the CPU needs that data again, it is swapped back into RAM.

The Problem with Virtual Memory

• Secondary storage is much slower than RAM.
• Constant swapping between RAM and disk is called thrashing and causes the computer to slow down significantly.
• Virtual memory is a temporary solution — the real fix is to add more RAM.

Exam Tip: A common exam scenario: "A user notices their computer is running slowly when many programs are open. Explain why." Answer: RAM is full, so the OS is using virtual memory on slower secondary storage. The constant swapping (thrashing) causes poor performance. Adding more RAM would solve the problem.

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How RAM and ROM Work Together During Boot-Up

1. Computer is powered on.
2. The CPU fetches its first instruction from ROM (the BIOS/UEFI).
3. ROM runs the POST to check hardware.
4. ROM identifies the operating system on secondary storage.
5. The operating system is loaded from secondary storage into RAM.
6. From this point on, the CPU primarily uses RAM to access the OS and programs.

Power On --> ROM (BIOS/UEFI) --> POST --> Load OS from disk --> RAM
                                                                  |
                                                             CPU uses RAM
                                                           for all programs

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Summary