Secondary storage: magnetic, optical and solid-state

Last time

Before we start: last time, in Primary storage and virtual memory, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Where do your photos live?

Switch your phone off, and your photos are still there tomorrow. But RAM lost everything when the power went. So the photos live somewhere else: in secondary storage. There are three technologies to learn: magnetic, optical and solid-state.

Secondary storage

Secondary storage is non-volatile storage that the CPU cannot access directly. It stores files, programs and the operating system permanently, until they are deleted. Data has to be copied into RAM before the CPU can use it.

Why a computer needs it

Without secondary storage, data and programs would be lost every time the power was turned off. You would have to install every program again, every time you switched on. It also lets you move data to another computer, and it creates virtual memory.

Magnetic storage

A hard disk drive has one or more platters, coated in magnetic material, that spin at high speed. A read-write head on an arm moves across the surface of each platter. An electromagnet magnetises tiny areas. One direction is a one, and the other is a zero. The data is stored in circular tracks, which are divided into sectors.

The hard disk, up close

Here is a hard disk drive. The platters spin, and the head on its arm floats just above them. To read, the head detects the magnetic direction of each tiny area as it passes.

Magnetic: good and bad

A hard disk has a very large capacity, and it is cheap for every gigabyte. But it has moving parts, so a hard knock can damage it. It is slower than solid-state, because the head must move to the right track. It also uses more power, and it is noisy.