COMPUTER APPLICATIONS Waxahachie High School

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COMPUTER APPLICATIONSWaxahachie High School

• Secondary Storage Devices
• TA TEKS (1)(B)

Source Computers Tools for an Information Age
(Capron and Johnson)
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SECONDARY STORAGE DEVICES

• LESSON OBJECTIVE
• List the benefits of secondary storage.
• Identify and describe storage media that are
  available for personal computers.
• Differentiate among the principal types of
  secondary storage.
• Describe how data is stored on a disk.

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SECONDARY STORAGE DEVICES
Secondary storage, sometimes called auxiliary
storage, is storage that is separate from the
computer itself and is where software and data
can be stored on a semi permanent basis.
Secondary storage is necessary because memory, or
primary storage, can be used only temporarily.
The benefits of secondary storage are space,
reliability, convenience, and economy.
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SECONDARY STORAGE DEVICES
Floppy disks and hard disks are magnetic media,
based on a technology of representing data as
magnetized spots on the disk. Floppy disks are
made of flexible Mylar. Advantages of floppy
disks, compared with hard disks, are portability
and backup. A hard disk is a rigid platter coated
with magnetic oxide that can be magnetized to
represent data. Several platters can be assembled
into a disk pack.
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SECONDARY STORAGE DEVICES
Floppy Disk
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Hard Disk
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SECONDARY STORAGE DEVICES
Disk Pack
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SECONDARY STORAGE DEVICES
A disk drive is a device that allows data to be
read from a disk or written to a disk. A disk
pack is mounted on a disk drive that is a
separate unit connected to the computer. The disk
access arm moves a read/write head into position
over a particular track, where the read/write
head hovers above the track. A head crash occurs
when a read/write head touches the disk surface
and causes data to be destroyed.
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SECONDARY STORAGE DEVICES
Read/Write Heads and Access Arm
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SECONDARY STORAGE DEVICES
During operation, the read/write head comes very
close to the surface of the disk. On a disk,
particles as small as smoke, dust, fingerprint,
and a hair appear large. If the read/write head
encounters these particles it will crash.
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SECONDARY STORAGE DEVICES
A redundant array of independent disks, or simply
RAID, uses a group of small hard disks that work
together as a unit. RAID level 0, data striping,
spreads the data across several disks in the
array, increasing performance. RAID level 1
duplicates data on separate disk drives, a
concept called disk mirroring, which provides
fault tolerance.
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SECONDARY STORAGE DEVICES
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SECONDARY STORAGE DEVICES
A track is the circular portion of the disk
surface that passes under the read/write head as
the disk rotates. Each track is divided into
sectors that hold a fixed number of bytes. A
cluster is a fixed number of adjacent sectors
that are treated as a unit of storage by the
operating system. A cylinder consists of the
track on each surface that is beneath the
read/write head at a given position of the
read/write arms.
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SECONDARY STORAGE DEVICES
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SECONDARY STORAGE DEVICES
Three factors determine access time, the time
needed to access data directly on disk seek
time, the time it takes to get the access arm
into position over a particular track head
switching, the activation of a particular
read/write head over a particular track on a
particular surface and rotational delay, the
brief wait until the desired data on the track
rotates under the read/write head. Once data has
been found, data transfer occurs.
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SECONDARY STORAGE DEVICES
Access time is usually measured in milliseconds
(ms). The data transfer rate, which tells how
fast data can be transferred once it has been
found, is usually stated in terms of megabytes of
data per second. Disk caching uses an area of
memory called disk cache to temporarily store
data from disk that the program might need soon.
If desired data is found in the disk cache, time
is saved because no actual read is necessary.
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SECONDARY STORAGE DEVICES
Optical disk technology uses a laser beam to
enter data as spots on the disk surface. To read
the data, the laser scans the disk, and a lens
picks up different light reflections from the
various spots. Read-only media are recorded on
by the manufacturer and can be read from but not
written to by the user. Write-once, read-many
media, also called WORM media, may be written to
once.
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SECONDARY STORAGE DEVICES
CD-ROM, for compact disk read-only memory drive,
which has a disk format identical to that of
audio compact disks, can hold up to 700 megabytes
per disk. CD-R (compact disc-recordable)
technology permits writing on optical
disks. CD-RW (compact disk-rewritable) technology
is more flexible, allowing you to erase and
record over data multiple times.
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Optical Disk
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CD-ROM Drive
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SECONDARY STORAGE DEVICES
DVD-ROM, for digital versatile disk, has a large
storage capacity, up to 17GB if both layers and
both sides are used. Three standards exist for
writable DVDs DVD-RAM, DVD-R, and DVDR. there
are two standards for rewritable DVDs DVD-RW and
DVDRW.
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SECONDARY STORAGE DEVICES
Multimedia software typically presents
information with text, illustrations, photos,
narration, music, animation, and film clips. This
is possible because of the large capacity of
optical disks. MPEG (Motion Picture Experts
Group) is a set of widely accepted video
compression standards.
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SECONDARY STORAGE DEVICES
Magnetic tape stores data as extremely small
magnetic spots (bits) on tape similar to that
used in music cassettes. Tape capacity is
expressed in terms of density, which is the
number of characters per inch (cpi) or bytes per
inch (bpi) that can be stored on the tape.
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SECONDARY STORAGE DEVICES
A magnetic tape drive reads and writes data using
a read/write head when the computer is writing
on the tape, the erase head first erases any data
that was previously recorded. Magnetic tape is
used primarily as a backup medium. A backup
system is a way of storing data in more than one
place to protect it from damage and loss. Most
backup systems use tape, but CD-R or CD-RW media
can also be used.
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Magnetic Tape Units
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A character is a letter, digit, or special
character (such as , ?, or ). A field contains
a set of related characters. A record is a
collection of related fields. A file is a
collection of related records. A database is a
collection of interrelated files stored together
with minimum redundancy specific data items can
be retrieved for various applications. A key
field uniquely identifies each record.
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How data is organized.
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SECONDARY STORAGE DEVICES
Sequential file organization means that records
are stored in order according to the key field.
If a particular record in a sequential file is
wanted, then all the prior records in the file
must be read before the desired record is
reached. Tape storage is limited to sequential
file organization.
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SECONDARY STORAGE DEVICES
Direct file organization (also called random file
organization), allows direct (random) access, the
ability to go directly to the desired record by
using a record key. Indexed file organization
stores records in the file in sequential order,
but the file also contains an index of keys the
address associated with the key can be used to
locate the record on the disk.