disks were invented in the 1950s. They started as large disks
up to 20 inches in diameter holding just a few megabytes.
They were originally called "fixed disks" or "Winchesters"
(a code name used for a popular IBM product). They later became
known as "hard disks" to distinguish them from "floppy
disks." Hard disks have a hard platter that holds the
magnetic medium, as opposed to the flexible plastic film found
in tapes and floppies. At the simplest level, a hard disk
is not that different from a cassette tape. Both hard disks
and cassette tapes use the same magnetic recording techniques
described in How Tape Recorders Work. Hard disks and cassette
tapes also share the major benefits of magnetic storage the
magnetic medium can be easily erased and rewritten, and it
will "remember" the magnetic flux patterns stored
onto the medium for many years.
A typical desktop machine will have a hard disk with a capacity
of between 10 and 40 gigabytes. Data is stored onto the disk
in the form of files. A file is simply a named collection
of bytes. The bytes might be the ASCII codes for the characters
of a text file, or they could be the instructions of a software
application for the computer to execute, or they could be
the records of a data base, or they could be the pixel colors
for a GIF image. No matter what it contains, however, a file
is simply a string of bytes. When a program running on the
computer requests a file, the hard disk retrieves its bytes
and sends them to the CPU one at a time.
There are two ways to measure the performance of a hard disk:
rate - The data rate is the number of bytes per second that
the drive can deliver to the CPU. Rates between 5 and 40 megabytes
per second are common.
time - The seek time is the amount of time between when the
CPU requests a file and when the first byte of the file is
sent to the CPU. Times between 10 and 20 milliseconds are
The other important parameter is the capacity of the drive,
which is the number of bytes it can hold.
Data is stored on the surface of a platter in sectors and
tracks. Tracks are concentric circles, and sectors are pie-shaped
wedges on a track, like this:A
typical track is shown in yellow; a typical sector is shown
in blue. A sector contains a fixed number of bytes -- for
example, 256 or 512. Either at the drive or the operating
system level, sectors are often grouped together into clusters.
process of low-level formatting a drive establishes the tracks
and sectors on the platter. The starting and ending points
of each sector are written onto the platter. This process
prepares the drive to hold blocks of bytes. High-level formatting
then writes the file-storage structures, like the file-allocation
table, into the sectors. This process prepares the drive to