User Guide

80 Appendix A : General Information. Hard Disks
Appendix A. General Information. Hard Disks
The Appendices below provide you with extra information on the hard disk
organization, how information is stored on disks, how disks should be installed
in the computer and plugged into motherboard, configuring disks with BIOS,
partitions, file systems, and how operating systems interact with disks.
A.1 Hard Disk Organization
All hard disks, or hard disk drives, have basically the same structure,
however diverse they are in size. 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
number. 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
enumeration. The sector that is the closest to this marker is number 1.
Usually sector size is 571 bytes. 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 to/from the disk. Heads are
assembled into a block and are enumerated, starting with 0.
To perform an elementary read or write operation the head block must 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).