Figure 10-1.—A magnetic disk pack.
several gigabytes. An example of a disk pack is shown
in figure 10-1.
Disk cartridges are another form of disk pack with
the heads and head actuator assemblies contained
within a sealed cartridge. Since the disk pack is never
removed from the cartridge, disk cartridges suffer less
contamination problems from dust and dirt than
standard disk packs.
FIXED DISKS. —Fixed disks are small sealed
units that contain one or more disk platters. Fixed disks
are known by several terms, such as Winchester drive,
hard drive, or fixed disk. For clarity, we refer to them
as fixed disks throughout this chapter. Fixed disks are
used in minicomputers and personal computers. They
can also be adapted for use in mainframe computers
instead of having separate disk file units.
Floppy Disks
Floppy disks come in several sizes and densities.
They are called floppy disks because the magnetic
coating is placed on a thin flexible polyester film base.
THE 8-INCH FLOPPY DISK. —The 8-inch
floppy disk was the first disk widely used for
commercial purposes. It is available as both single- or
double-sided and single- or double density. The 8-inch
disk is quickly becoming obsolete.
THE 5.25-INCH FLOPPY DISK. —The 5.25-
inch floppy disks are used with both personal
computers
and
minicomputers.
The
standard
double-sided, double-density disk has a capacity of 360
kilobytes (K). Quad-density disks hold 720K, while the
newest high-density disks can hold 1.2 megabytes (M).
THE 3.5-INCH FLOPPY DISK. —The current
disk of choice is the 3.5-inch floppy disk. These disks
are
also
used
with
personal
computers
and
minicomputers. These smaller disks have data
capacities of 720K for double-density disks and 1.44M
for high-density disks.
ORGANIZING DATA ON DISKS
Before data can be stored on a magnetic disk, the
disk must first be divided into numbered areas so the
data can be easily retrieved. Dividing the disk so the
data can be easily written and retrieved is known as
formatting the disk. The format program divides each
data surface into tracks and sectors.
Tracks —Concentric rings, called tracks, are
written on the disk during the formatting process.
Floppy disks have 40 or 80 tracks per side. Fixed disks
and disk packs can have from 300 to over 1,000 tracks
per side. Figure 10-2 shows an example of how tracks
are written on a disk surface. Each track is assigned a
number. The outermost track on a disk is assigned
number 00. The innermost track is assigned the highest
consecutive number.
Sectors —Each track is divided into sectors. Sectors
are numbered divisions of the tracks designed to make
data storage more manageable. Without sectors, each
track would hold more than 4,500 bytes of information
and small files would use an entire track.
Figure 10-2.—Tracks on a segment of a magnetic disk.
10-3
