Java Reference
In-Depth Information
Boom
(arm)
Platters
Read/Write
Track
Spindle
Heads
(a)
(b)
Figure8.2
(a) A typical disk drive arranged as a stack of platters. (b) One track
on a disk drive platter.
to each head. The data on a single platter that are accessible to any one position of
the head for that platter are collectively called a track, that is, all data on a platter
that are a fixed distance from the spindle, as shown in Figure 8.2(b). The collection
of all tracks that are a fixed distance from the spindle is called a cylinder. Thus, a
cylinder is all of the data that can be read when the arms are in a particular position.
Each track is subdivided into sectors. Between each sector there are inter-
sector gaps in which no data are stored. These gaps allow the read head to recog-
nize the end of a sector. Note that each sector contains the same amount of data.
Because the outer tracks have greater length, they contain fewer bits per inch than
do the inner tracks. Thus, about half of the potential storage space is wasted, be-
cause only the innermost tracks are stored at the highest possible data density. This
arrangement is illustrated by Figure 8.3a. Disk drives today actually group tracks
into “zones” such that the tracks in the innermost zone adjust their data density
going out to maintain the same radial data density, then the tracks of the next zone
reset the data density to make better use of their storage ability, and so on. This
arrangement is shown in Figure 8.3b.
In contrast to the physical layout of a hard disk, a CD-ROM consists of a single
spiral track. Bits of information along the track are equally spaced, so the informa-
tion density is the same at both the outer and inner portions of the track. To keep
the information flow at a constant rate along the spiral, the drive must speed up the
rate of disk spin as the I/O head moves toward the center of the disk. This makes
for a more complicated and slower mechanism.
Three separate steps take place when reading a particular byte or series of bytes
of data from a hard disk. First, the I/O head moves so that it is positioned over the
track containing the data. This movement is called a seek. Second, the sector
containing the data rotates to come under the head. When in use the disk is always
compromises change over time. In addition, most of the description given here for disk drives is a
simplified version of the reality. But this is a useful working model to understand what is going on.
