11. Read Text B:
Text B
Storage Devices
Storage devices are used to store data and programs that are
not being used by the processor. They usually consist of:
a) storage media in the form of a circular disk or a tape
where the data is stored
b) a disk or tape drive that moves the media past a read/write
head that reads the data from and writes data to the storage media.
Types of storage devices include:
– Magnetic devices
– Optical devices
– Flash memory
Magnetic Storage Devices
Magnetic storage devices store data by magnetizing particles
on a disk or tape.
A floppy disk is so called because it consists of a flexible
sheet of plastic, coated with iron oxide – a magnetisable material.
A floppy disk drive spins at 360 revolutions per minute (rpm), so
it’s relatively slow.
However, a hard drive consists of a head actuator, head arm,
chassis, and a disc platter; it spins at over 7,200 rpm and stores da-
ta on a stack of metal rotating disks called platters. This means
you can store much more data and retrieve information much fast-
er.
New disks need to be formatted before you can use them, un-
less they come preformatted from the manufacturer. When the disk
is formatted, the operating system (OS) organizes the disk surface
into circular tracks and divides each track into sectors. The OS
creates a directory which will record the specific location of files.
When you save a file, the OS moves the read/write head of the
drive towards empty sectors, records the data and writes an entry
for the directory. Later on, when you open that file, the OS looks
for its entry in the directory, moves the read/write heads to the cor-
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rect sector, and reads the file in the RAM area. However, format-
ting erases any existing files on a disk, so do not format disks on
which data that you don't want to lose is stored.
The OS allows you to create one or more partitions on your
hard drive, in effect dividing it into several logical parts. Partitions
let you install more than one operating system (e.g. Windows and
Linux) on your computer. You may also decide to split your hard
drive because you want to store the OS and programs on one parti-
tion and your data files on another; this allows you to reinstall the
OS when a problem occurs, without affecting the data partition.
The average time required for the read/write heads to move
and find data is called seek time (or access time) and it is meas-
ured in milliseconds (ms); most hard drives have a seek time of 7
to 14 ms. Don’t confuse this with transfer rate – the average
speed required to transmit data from the disk to the CPU, meas-
ured in megabytes per second.
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