Your photos, games and homework must survive when the computer is switched off, so they live in secondary storage. Three technologies do the job in very different ways: spinning magnetic platters, lasers on discs and flash memory with no moving parts.
Secondary storage is not directly accessed by the CPU and is non-volatile, so it is needed for more permanent storage of data and programs. Magnetic storage (HDD) uses spinning platters divided into tracks and sectors; data is read and written by electromagnets on a moving head. Optical storage (CD, DVD, Blu-ray) uses a laser to create and read pits and lands. Solid-state (flash) storage (SSD, SD card, USB drive) uses NAND or NOR technology, with transistors acting as control gates and floating gates, and has no moving parts.
Fig. 1: a hard disk platter divided into tracks and sectors, and the three types of secondary storage
Section 01
Magnetic storage.
A hard disk drive (HDD) contains one or more circular platters coated in a magnetic material, spinning at thousands of revolutions per minute.
Secondary storage: non-volatile and not directly accessed by the CPU; data is copied into RAM when it is needed.
Platters, tracks and sectors: each platter surface is divided into concentric rings (tracks), and each track into sectors that each hold a fixed amount of data.
Read/write head: an arm moves a head across the spinning platter to the correct track. Its electromagnet magnetises tiny areas in one of two directions, representing 1 and 0; to read, it detects the direction of magnetisation.
Features: large capacity at a low cost per gigabyte, but moving parts make it slower than solid-state, noisier, and easily damaged if dropped.
Example: hard disk drive (HDD) in desktop PCs and servers.
Examiner trap
Data on a hard disk is stored magnetically, not with a laser. Lasers belong to optical storage.
Section 02
Optical storage.
Optical discs store data on a single spiral track that runs from the centre to the edge of the disc.
Writing: a laser burns or changes the surface to create pits (dips); the flat areas between them are lands.
Reading: a lower-power laser is shone on the spinning disc. Lands reflect the light back strongly; pits scatter it. A sensor detects the reflected light, and the changes are read as binary.
CD, DVD, Blu-ray: CDs use a red laser of longer wavelength; DVDs a shorter red laser; Blu-ray a blue-violet laser with an even shorter wavelength. Shorter wavelength means smaller pits packed closer together, so more data fits on the disc.
Features: cheap and portable, good for distributing films, games and software, but slow to access, easily scratched and small capacity compared with an HDD.
Your turn
Why can a Blu-ray disc hold much more data than a CD of the same size?
Blu-ray uses a blue-violet laser with a shorter wavelength, so the pits are smaller and closer together and more data fits on the disc.
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What is secondary storage?
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How is data read from an optical disc?
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A hard disk's platter is divided into which two parts?
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Which is an example of solid-state storage?
Section 03
Solid-state storage.
Flash memory stores data electronically in billions of transistors, with no moving parts at all.
Transistors: each memory cell uses a transistor with a control gate and a floating gate. Electrons are trapped in the floating gate (or removed from it) to represent 1 or 0. They stay there with no power, so flash is non-volatile.
NAND and NOR: the two ways the cells are connected. NAND flash, the most common, is used in SSDs, SD cards and USB drives.
Features: very fast access, silent, low power, light and robust (no moving parts to break), but a higher cost per gigabyte than an HDD and a limited number of write cycles.