RAM

Summary: Random Access Memory — volatile primary memory that stores data and programs currently in use. Contents are lost when power is turned off. Much faster than secondary storage but more expensive per byte and of limited capacity. Works alongside ROM, cache, and virtual memory. Tags: igcse computer-science Created: 2026-05-08T14:03:00Z Last Updated: 2026-07-16

What is RAM?

Random Access Memory (RAM) is the main memory of a computer where the operating system, running programs, and current data are held. It is called “random access” because any memory location can be accessed directly (in any order) at roughly the same speed — unlike sequential access media like magnetic tape.

RAM is volatile: it requires power to maintain stored data. When the computer is turned off, the contents of RAM are lost. This is why unsaved work disappears if the power fails.

Key Properties of RAM

What is Stored in RAM?

• Operating system: Core OS components loaded at boot time
• Running programs: Each application is loaded into RAM when launched
• Open files: Documents, spreadsheets, images being edited
• Variables and data structures: Values used by running programs
• The stack and heap: Memory regions for function calls and dynamic allocation

RAM vs ROM

RAM and ROM are both primary memory (directly accessible by the CPU), but serve very different purposes:

What is in ROM?

• BIOS (Basic Input/Output System) or UEFI firmware: The first code that runs when the computer powers on
• Bootstrap loader: The program that loads the operating system from secondary storage into RAM
• In embedded systems (washing machines, microwaves), ROM stores the entire control program

Types of RAM

DRAM (Dynamic RAM)

• The most common type in computers for main memory
• Each bit stored as a charge in a tiny capacitor; charge leaks away, so it must be refreshed thousands of times per second
• Slower than SRAM but much cheaper per bit and higher density (more capacity)
• DDR (Double Data Rate) is the modern standard: DDR4, DDR5

SRAM (Static RAM)

• Uses flip-flop circuits to store each bit; does not need refreshing
• Faster than DRAM (used for CPU cache)
• Much more expensive and lower density
• Used for L1, L2, and L3 Cache Memory

Virtual Memory

When RAM is full, the operating system can use part of the hard disk or SSD as an extension — this is called virtual memory.

How It Works

1. The OS divides memory into fixed-size blocks called pages (typically 4 KB each)
2. When RAM is full, less-frequently-used pages are moved to a special area on the hard disk called the swap file or page file
3. When a program needs a page that was swapped out, it is moved back into RAM (swapping out another page if necessary)

Consequences of Virtual Memory

Thrashing occurs when the system spends more time swapping pages between RAM and disk than actually executing programs. The computer becomes extremely sluggish — the disk light stays on constantly, and the mouse/keyboard barely respond. Thrashing indicates a critical shortage of RAM.

Sources

• BBC Bitesize GCSE Computer Science — Memory and Storage, BBC (free educational resource)
• Cambridge IGCSE Computer Science 0478 — Hardware, Cambridge Assessment International Education
• CK-12 Computer Science — Memory Systems, CK-12 Foundation (free, CC BY-NC 3.0)

Related Notes

• ROM — Non-volatile read-only memory for firmware
• Virtual Memory — Using disk as an extension of RAM
• Cache Memory — CPU’s high-speed local memory (SRAM-based)
• Von Neumann Architecture — The architecture where RAM stores both instructions and data
• Operating System — Manages memory allocation and virtual memory
• SSD — Solid-state secondary storage
• HDD — Magnetic secondary storage
• CS-Index

Common Misconceptions