Chapter 9: Hard Disks and Floppy Disks^187
Data Organization
Both hard disks and floppy disks set organization schemes that allow them to store data,
and more importantly, find it again later. The disk is organized into cylinders, tracks,
sectors, and clusters. Remember that this organization is over and above the servo and
gray code systems that were placed on the disk when it was manufactured.
Each disk is organized into the following organization building blocks: Tracks As illustrated in Figure 9-8,tracksare concentric areas on the disk that
complete one circumference of the disk. On a hard disk, there can be 1,000
tracks or more. The first track, which is where data is usually written first, is
along the outermost edge of the disk. Tracks of the same number on all platters
of the drive form acylinder.
Sectors Sectorsdivide the disk into a number of cross-sections that intersect all of
the tracks on the disk. Sectors break tracks into addressable pieces, as illustrated in
Figure 9-8. A sector is typically 512 bytes in length, and disk drives have from 100
to 300 sectors per track. Without a sector division, a track could only be addressed
at its beginning, wherever that would be. Sectors provide segment beginning
points on the tracks as well as the disk as a whole.
Cylinders Cylindersreflect how the read/write heads move in and out of the
disk platters in unison. This grouping technique is unique to hard disks. A
cylinder is the logical grouping of the same track on each disk surface. For
example, if a hard disk drive has three platters, as illustrated in Figure 9-9,
it has six disk surfaces and six track 52s. All six track 52s combine logically to
create cylinder 52. Data is written vertically between disks following the track
and cylinder path, which eliminates the need to move the read/write heads.
Clusters Aclusteris formed from groups of sectors. This logical grouping is
used by operating systems to track data on the disk. There are normally around
64 sectors to a cluster, but the size of the disk drive and the operating system in
use determine the actual number of sectors in a cluster. A cluster transfer is
also called ablock modetransfer.
Multiple Zone Recording This is a technique used on some disk drives to
eliminate the effect of the shape of the disk in recording sectors. More sectors are
placed on tracks closer to the outer edge of the disk and fewer sectors are placed
on the tracks closer to the inside edge. On drives without zoned recording
capabilities, the size of the sector and the number of sectors per track are fixed
numbers, which means that at the point on the physical media where the fixed
track size can no longer be accommodated, there can be no additional tracks.
Zoned recording allows more of the disk, the part closer to the inside edge of the
disk, to be put to use. The disk is divided into zones, each with its own sizing and
spacing criteria. Virtually all IDE and SCSI drives use zone recording methods.