Page 58 - UNI 101 Computer Science Handout.

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3.4 MAGNETIC DISKS

A magnetic disk consists of one or more flat, circular platters made of glass, metal, or plastic, and coated

with a magnetic substance. Particles within a small area of the magnetic substance can be polarized

magnetically in one of two directions with an electromagnet; an electromagnet can also detect the

direction of polarization previously recorded. Thus, magnetic polarization can be used to distinguish 1s

and 0s. Electromagnetic read/write heads are used for this purpose. A drive motor rotates the disk
platter(s) about its central axis. On most drives, the motor rotates the disk at a fixed speed.

An arm has the read/write head mounted at the end. The arm makes it possible for the head to move

radially in and out across the surface of the disk. A head motor controls precisely the position of the arm

on the disk. Most hard disk drives contain several platters, all mounted on the same axis, with heads on
each surface of each platter. The heads move in tandem, so they are positioned over the same point on

each surface. Except for the top and bottom, each arm contains two read/write heads, which service the

surfaces of two adjoining platters. With the head in a particular position, it traces out a circle on the disk
surface as the disk rotates; this circle is known as a track. Since the heads on each surface all line up, the

set of tracks for all the surfaces form a cylinder.

Each track contains one or more blocks of data. On most disks the surface of the disk platter is divided

into equally sized pie shape segments, known as sectors, although the disks on some large computers
divide up the track differently. Each sector on a single track contains one block of data, typically 512

bytes, which represents the smallest unit that can be independently read or written. Figure below shows

the layout of a hard disk. If you assume that the number of bytes in a sector is the same anywhere on

the disk, then you can see from the layout that the bits on the disk are more closely packed on the inner

tracks than they are on the outer tracks.

Regardless of the track, the same angle is swept out when a sector is accessed; thus, the transfer time

is kept constant with the motor rotating at a fixed speed. This technique is called CAV, for constant

angular velocity. CAV has the advantage of simplicity and fast access. It is possible to increase the

58 Academic Year 2025/2026