Information Technology Reference
In-Depth Information
6.4 Fixed disks
Fixed disks store large amounts of data and vary in their capacity, from several MB to sev-
eral GB. A fixed disk (or hard disk) consists of one or more platters which spin at around
3000 rpm (10 times faster than a floppy disk). A hard disk with four platters is shown in Fig-
ure 6.2. Data is read from the disk by a flying head which sits just above the surface of the
platter. This head does not actually touch the surface as the disk is spinning so fast. The dis-
tance between the platter and the head is only about 10
in (which is no larger than the thick-
ness of a human hair or a smoke particle). It must thus be protected from any outer particles
by sealing it in an airtight container. A floppy disk is prone to wear as the head touches the
disk as it reads but a fixed disk has no wear as its heads never touch the disk.
One problem with a fixed disk is head crashes, typically caused when the power is
abruptly interrupted or if the disk drive is jolted. This can cause the head to crash into the
disk surface. In most modern disk drives the head is automatically parked when the power is
taken away. Older disk drives that do not have automatic head parking require a program to
park the heads before the drive is powered down.
There are two sides to each platter and, like floppy disks, each side divides into a number
a tracks which are subdivided into sectors. A number of tracks on fixed disks are usually
named cylinders. For example a 40 MB hard disk has two platters with 306 cylinders, four
tracks per cylinder, 17 sectors per track and 512 bytes per sector, thus each side of a platter
stores
µ
306
×
4
×
17
×
512 B
= 10 653 696 B
= 10 653 696/ 1 048 576 MB
=
.2 MB
Read/write heads
Platters
Head movement
Figure 6.2
Hard disk with four platters
