Information Technology Reference
In-Depth Information
6
IDE and Mass Storage
6.1 Introduction
This chapter and the next chapter discuss IDE and SCSI interfaces which are used to inter-
face to disk drives and mass storage devices. Disks are used to store data reliably in the long
term. Typical disk drives either store binary information as magnetic fields on a fixed disk
(as in a hard disk drive), a plastic disk (as in a floppy disk or tape drive), or as optical repre-
sentation (on optical disks).
The main sources of permanent read/writeable storage are:
•
Magnetic tape - where the digital bits are stored with varying magnetic fields. Typical
devices are tape cartridges, DAT and 8 mm video tape.
•
Magnetic disk - as with the magnetic tape the bits are stored as varying magnetic fields
on a magnetic disk. This disk can either be permanent (such as a hard disk) or flexible
(such as a floppy disk). Large capacity hard disks allow storage of several gigabytes of
data. Normally fixed disks are designed to a much higher specification than floppy disks
and can thus store much more information.
•
Optical disk - where the digital bits are stored as pits on an optical disk. A laser then
reads these bits. This information can either be read only (CD-ROM), write once read
many (WORM) or can be reprogrammable. A standard CD-ROM stores up to 650 MB of
data. Their main disadvantage in the past has been their relative slowness as compared
with Winchester hard disks; this is now much less of a problem as speeds have steadily
increased over the years.
6.2
Tracks and sectors
A disk must be formatted before it is used, which allows data to be stored in a logical man-
ner. The format of the disk is defined by a series of tracks and sectors on either one or two
sides. A track is a concentric circle around the disk where the outermost track is track 40 and
the innermost track is 0. The next track is track 1 and so on, as shown in Figure 6.1. Each of
these tracks is divided into a number of sectors. The first sector is named sector 1, the second
is sector 2, and so on. Most disks also have two sides: the first side of the disk is called side 0
and the other is side 1.
