Hardware Reference
In-Depth Information
Intersector gap
Read/write
head
Width of
1 bit is
0.1 to 0.2 microns
Disk
arm
Track
width is
1-2 microns
Figure 2-19.
A portion of a disk track. Two sectors are illustrated.
All disks have movable arms that are capable of moving in and out to different
radial distances from the spindle about which the platter rotates. At each radial
distance, a different track can be written. The tracks are thus a series of concentric
circles about the spindle. The width of a track depends on how large the head is
and how accurately the head can be positioned radially. With current technology,
disks have around 50,000 tracks per centimeter, giving track widths in the
200-nanometer range (1 nanometer = 1/1,000,000 mm). It should be noted that a
track is not a physical groove in the surface, but simply an annulus (ring) of mag-
netized material, with small guard areas separating it from the tracks inside and
outside it.
The linear bit density around the circumference of the track is different from
the radial one. In other words, the number of bits per millimeter measured going
around a track is different from the number of bits per millimeter starting from the
center and moving outward. The density along a track is determined largely by the
purity of the surface and air quality. Current disks achieve densities of 25 giga-
bits/cm. The radial density is determined by how accurately the arm can be made
to seek to a track. Thus a bit is many times larger in the radial direction as com-
pared to the circumference, as suggested by Fig. 2-19.
Ultrahigh density disks utilize a recording technology in which the ''long''
dimension of the bits is not along the circumference of the disk, but vertically,
down into the iron oxide. This technique is called
perpendicular recording
, and it
has been demonstrated to provide data densities of up to 100 gigabits/cm.
It is
likely to become the dominant technology in the coming years.
In order to achieve high surface and air quality, most disks are sealed at the
factory to prevent dust from getting in. Such drives were originally called
Winch-
ester disks
because the first such drives (created by IBM) had 30 MB of sealed,
fixed storage and 30 MB of removable storage. Supposedly, these 30-30 disks
reminded people of the Winchester 30-30 rifles that played a great role in opening
the American frontier, and the name ''Winchester'' stuck. Now they are just called
