• Primary storage (RAM & ROM) sits directly connected to the CPU — small, fast, expensive.
• RAM is volatile, read/write, holds data & programs currently in use.
• ROM is non-volatile, read-only, holds the bootstrap/BIOS start-up instructions.
• Secondary storage (magnetic, solid-state, optical) is non-volatile, large capacity, slower — used for long-term storage.
• Virtual memory is a "pretend RAM" area on secondary storage, used when RAM is full — it saves the system from crashing but slows it down.
• Cloud storage is secondary storage that lives on someone else's remote servers, accessed over the internet.

Why does a computer even need it?

Think of the CPU as a chef cooking a meal. The chef can't run to the supermarket (secondary storage) every time they need an ingredient mid-recipe — that's way too slow. Instead, they keep the ingredients they're actively using on the counter right next to them. That counter is primary storage.

Primary storage is directly connected to the CPU, so access times are dramatically faster than secondary storage. This matters because every single instruction the CPU runs goes through the Fetch-Execute Cycle — and if the CPU had to wait for a slow hard disk every single time, your computer would grind to a halt.

The catch: fast memory is expensive to make, so we can only afford a little of it. That's why RAM is sold in gigabytes (16GB, 32GB) while hard drives are sold in terabytes (1000s of GB).

RAM — Random Access Memory

RAM is the computer's short-term working memory. It holds whatever the CPU is currently using — the open apps, the current file, the parts of the operating system that are running right now.

Volatile means: turn the power off, and everything in RAM disappears instantly. That's why you lose unsaved work in a power cut — it was only ever sitting in RAM, never written to a permanent disk.

Before the CPU can use any data or instruction, it has to first be copied from secondary storage into RAM. That's literally what "loading" a program means.

ROM — Read Only Memory

ROM is a tiny memory chip on the motherboard that holds the very first instructions the computer needs the moment it's switched on — this is called the bootstrap process, and the instructions it holds are the BIOS (Basic Input Output System).

Think of ROM as a recipe card glued permanently to the kitchen wall — you can read it every time you cook, but you can never scribble over it and change it. That permanence is exactly why it's used for start-up instructions: they must survive being switched off, and they should never be accidentally altered.

Why does a computer need it?

RAM is volatile and ROM is read-only — neither can hold your files, photos, or installed programs permanently. Secondary storage exists to give the computer long-term, non-volatile storage for everything not currently in active use.

There are three families of secondary storage, and each stores data physically in a completely different way:

• Magnetic — uses magnetised particles (hard disk drives)
• Solid-state — uses electronic circuits with no moving parts (SSDs, USB flash drives)
• Optical — uses a laser to burn/read pits and lands on a disc (CD, DVD, Blu-ray)

Magnetic Storage

A magnetic hard disk contains several metal discs called platters, coated in magnetic material. Each platter is divided into circular tracks, which are further split into wedge-shaped sectors — the point where a track meets a sector is where data actually gets addressed and stored.

The platters spin at high speed (typically 5400–7200 RPM) while a read/write arm, controlled by an actuator, hovers over the surface. Tiny electromagnets in the arm magnetise iron particles on the platter to represent 0s and 1s.

Solid-State (Flash Memory)

Solid-state storage has no moving parts at all — instead it stores data electronically in millions of tiny cells, each containing one bit (a 1 or a 0). Each cell contains a transistor made of two key parts:

• Control gate — connects to the circuit and controls whether current flows through
• Floating gate — sits sandwiched between insulating layers and can hold an electrical charge, like a microscopic rechargeable battery

To write a 1: a high voltage is applied to the control gate, pushing electrons through the insulating oxide layer onto the floating gate. To erase/remove data: a high voltage is applied in the opposite direction, pulling the electrons back off.

Optical Storage

Optical storage (CDs, DVDs, Blu-rays) uses a laser to burn microscopic pits and lands (flat areas) into the surface of a spinning disc. The laser then shines onto these pits and lands to read the data back — when it hits the point where a pit changes to a land (or vice versa), the light scatters differently, and a sensor interprets that scatter as a change in binary value.

Virtual memory is one of the most misunderstood topics — so let's build it from scratch with an analogy.

Formally: virtual memory is an extension of RAM that lives on secondary storage. When RAM is close to full, programs and data not currently being executed get moved out to virtual memory in chunks called pages. When they're needed again, they're paged back into RAM.

Cloud storage is just long-term secondary storage that isn't physically in your device — it lives on remote servers owned by a company, and you access it over the internet (a wide area network). Under the hood, cloud providers mostly use magnetic hard drives (cheap, huge capacity), with solid-state increasingly used too.