Appendix A. Hard Disk And Operating
System
Provided Appendices contain additional information about hard disk construc-
tion and data storage, partitions, file systems and interaction of operating
system with hard disk.
A.1
Hard Disk Organization
All hard disks, or hard disk drives, have basically the same structure, how-
ever diverse they are. Inside the case there are several disks with magnetic
coating set on a single axis (spindle). A special motor provides the necessary
rotation speed to the spindle, e.g. 5400 rpm, 7200 rpm, or 10000 rpm.
Information on disks resides on concentric
tracks
. Each track has its num-
ber. The outermost track is number 0, and the numbers grow inwards.
Each of the tracks is divided into
sectors
that contain minimal information
blocks that can be written to disk or read from it. Sectors also have numbers.
On every disk there is a marker that indicates the beginning of sector enu-
meration. The sector that is the closest to this marker is number 1.
At the beginning of a sector there is a header (prefix portion) that marks the
beginning of the sector and its number. At the end of a sector in the suffix
portion there is the checksum that is used to check data integrity. Data area
between the prefix and suffix portions is 512 bytes large.
Both upper and lower sides of each disk on the spindle are used to store
data. All tracks that have the same number on all the surfaces of all disks
comprise a
cylinder
. For each work surface of a disk in the drive there is a
head
that enables reading and writing data from/ to the disk. Heads are as-
sembled into a block and are enumerated, starting with 0.
To perform an elementary read or write operation the head block should be
positioned at the necessary cylinder. When the necessary sector (with the
necessary number in the service area) of the rotating disks approaches the
head, data is exchanged between the head and the electronic board of the
drive.
Sector structure of a hard disk is created via
low-level formatting
during
which each of the tracks of the disk is marked up.
Modern disk drives usually contain relatively few magnetic disks (1–2) to
make the head block lighter and speed up access to sectors (a drive like this
has 2–4 heads respectively).
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