SECONDARY STORAGE DEVICES

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SECONDARY STORAGE DEVICES
Y.Colette LeMard
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• The ability to store data and programs so that
  they can be used/reused in the future is one of
  the most important features of a computer.

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?

• However, where to store this data is the
  question.
• We can not change whats in ROM so that is out
  of the question
• RAM is volatile so it will not preserve anything
  we place there when the computer is off
• In addition both ROM and RAM are much too small
  anyway.

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Thus the need for secondary storage a.k.a.
auxiliary storage a.k.a. storage
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Secondary Storage

• Sometimes secondary storage is also called
  external storage.
• However, it is important to bear in mind that one
  of our most important secondary storage media is
  the hard disk which is not external.

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Medium vs Device

• There is also the tendency when we discuss
  secondary storage to mix up medium and device.
• Medium the actual physical material on which
  the bits are stored
• Device the mechanism used to access the data on
  the medium

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Medium

Device
Floppy disk
Floppy drive
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Distinguishing Characteristics of Secondary
Storage

• AS we examine some secondary storage media, look
  out for the following
• their capacity
• their access speed
• their access method and
• their portability.

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Definitions

• Storage Capacity
• Maximum amount of data which a data storage
  medium can hold

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Definitions

• Access Speed
• How quickly the required data is retrieved by the
  storage device

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Definitions

• Portability
• Readily and easily transported from one computer
  system to another.

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Definitions

• Access Method
• What particular mechanics/algorithm is used to
  locate specific data at a particular place on a
  storage medium.
• SASD Sequential Access Storage Device
• DASD Direct (Random) Access Storage
  Device

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MAGNETIC MEDIA
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The Floppy Disk
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The Floppy Disk
Floppies have come a long way

• There was the 8 floppy,and the 5¼ floppy,
  and then the 3½ floppy.

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The Floppy Disk

• The present floppy is a 3.5 inch magnetic disk
  of flexible material which typically stores 1.44
  MB of data maximum

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The Floppy Disk

• The thin plastic circle is enclosed in rigid
  plastic to protect it.
• A metal sliding access shutter opens when the
  disk is in the drive so that the read/write head
  can access the disk itself.

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The Floppy Disk

• Data can be written to and read from a floppy
  disk.
• A write protect tab can be used to prevent
  accidental overwriting of data

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The Floppy Disk

• Before data can be written to a disk, it must be
  formatted. This prepares the surface for storing
  bits by creating a map of sectors and tracks.

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The Floppy Disk

• The address of a particular file is determined
  by the track and sector it occupies.
• Most floppies now come preformatted, so many
  users are not aware of the requirement for
  formatting

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The Floppy Disk
preformatted
Micro-floppy disk
2 -sided
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The Floppy Disk

• Formatting also creates a File Allocation Table
  (FAT) - a directory which stores the names of the
  files on the disk and their address.
• A FAT is necessary for management of the storage
  space on the disk.

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• Floppies are highly portable and are great for
  transporting small files between machines
• They are direct access but rather slow
• Most desktops still have a floppy drive but
  newer laptops no longer contain a floppy disk
  drive

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The Zip Disk
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The Zip Disk

• The zip disk comes in different capacities
  ranging from 100MB to 750MB.
• This means that each can store the contents of
  many floppies

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The Zip Disk

• The zip disk is portable and the disk itself
  operates similarly to a hard disk.

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The Zip Disk

• Zip disks require a zip drive. These are not
  automatically attached to new systems however,
  they have to be specially ordered.
• A zip drive can be external or internal.

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The Zip Disk

• Zip disks are relatively inexpensive. The 750MB
  version can store the same amount of data as 520
  floppy disks.
• Physically the zip disk is about 1½ times the
  size of a floppy.

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The Zip Disk

• Zip disks are popular for backing up hard disks
  and for transferring large files from one
  computer to another.

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The Hard Disk
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The Hard Disk

• The hard disk is made from rigid aluminum coated
  with a magnetic material.
• It is normally supplied as a sealed unit and
  will often consists of a number of disks
  (platters) on a common spindle

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The Hard Disk

• The hard disk and the hard drive are packaged
  together

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The Hard Disk

• Each platter has a pair of read/write heads
  which are used for writing to and reading from
  that particular surface of the disk.

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The Hard Disk

• Each disk is divided into tracks and sectors.
  Each sector can store between 256 and 512 bytes.

The tracks are immediately above and below each
other. A particular set of such tracks is called
a cylinder.
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The Hard Disk

• As the spindle turns the platters, the
  read/write heads move together in and out between
  the platters. At any point in time they are all
  positioned over one particular cylinder

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The Hard Disk

• It is now fairly standard for a pc to have a
  hard disk of at least 60GB. When PCs were first
  used in the 1980s, the hard disks capacity was
  measured in MB. But modern data and programs take
  up much more space especially as they are now GUI
  based.

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The Hard Disk

• Hard disks are pretty reliable
• However as a standard they are fixed inside the
  systems unit/computer case.
• Portable (removable) units are available though,
  and tend to be used mostly by laptop owners.

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The Hard Disk

• Hard disks are direct access devices. The data
  transfer rate is also much higher than for floppy
  disks
• They also have a FAT which store the addresses
  of the files stored thereon.

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Magnetic Tapes
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Magnetic Tape

• Tape is flexible reels of plastic with a
  magnetic coating.
• It may range from ¼ cartridges to large reels

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Magnetic Tape

• The data is stored in tracks that run along the
  entire length of the tape. Usually there are 9
  tracks.

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Magnetic Tape

• Magnetic tape requires a special drive of course
  and access is slow as it is a sequential access
  devices.
• It is an ideal medium for backup however,
  because it is fast, inexpensive and has a high
  storage capacity.

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OPTICAL MEDIA
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Optical Disks

• These have a hard plastic disk with a mirrored
  surface. The data is stored by removing the
  mirroring from a particular point through the use
  of a laser. The laser creates a pit on the
  surface. These pits are usually on the underside
  of the disk and cannot be seen with the naked eye.

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Optical Disks
pits
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Optical Disks

• Optical disks have a larger storage capacity
  than floppies but less than hard disks or tapes.
  They also fall in the middle in terms of access
  speed.
• They are portable but must be transported with
  care because scratching the recording surface
  will damage the data.

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Optical Disks

• The earliest optical disks could only be written
  to once so they became known as write once read
  many (WORM)disks.

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The CD-ROM

• The most common WORM disk is the Compact Disc
  Read Only Memory
• CD-ROM.
• A typical CD-ROM holds 650MB of data

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The CD-ROM

• CD-ROMs are great for distributing programs and
  other material that is not to be changed.
• CD-ROM drives became a standard for desktops in
  the 1990s

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The CD-R

• CD-R (CD-recordable) allows end users to record
  their own data on CDs. Data can be burned onto
  these once and thereafter cannot be changed.
• This makes CD-Rs great for permanent backup

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The CD-R

• The CD-R is write once, read many.
• The dye layer allows writing of data to this disk

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The CD-RW

• CD-RW (CD rewriteable)
• These became available in 1997

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The CD-RW

• While a prerecorded CD has its information
  permanently stamped into its polycarbonate
  plastic substrate layer, a CD-RW disc contains a
  phase-change alloy recording layer.

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The CD-RW

• An infra-red laser beam is used to heat (at
  400oC) and melt, the crystallized recording layer
  into a fluid state or to freeze it at a lower
  temperature back to its crystalline state.
• In this way previous data is erased and new
  data can be written (burned)

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The DVD

• Digital versatile disk
• Digital video disk
• These look similar to CDs but are capable of
  storing up to 6 times more data principally
  because the tracks are closer together and
  because some store data on both surfaces

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The DVD

• DVDs are used mainly for movies as the quality
  of the sound and the video is superior to that of
  videotapes.
• They are also used for software, and data
  archiving.

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The DVD

• The DVD is a direct access device and is fairly
  fast.
• Types
• DVD-ROM
• DVD-R
• DVD-RAM (DVD-RW)

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Optical Disks

• The CD-ROM drive is a standard on PCs, but now
  CD-R/W drives, DVD-ROM drives, and combo drives
  are also becoming very popular.
• DVD writers can also now be seen on sale in
  computer stores

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Any Questions ?
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My Questions

1. Magnetic tape has sequential-access capabilities
   (T/F)?
2. What is the most appropriate application of
   magnetic tape storage?
3. Why are floppy drives no longer included on many
   newer machines?
4. What happens during formatting? Why must hard
   disks and diskettes be formatted?

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My Questions

• Describe the potential impact of optical disk
  technology on public and university libraries
• Describe at least two applications where CD-RW
  would be preferred over hard disk for storage.
• How many diskettes would you need to backup your
  Computer Science notes which is 2.8MB large.
• The DVD is compatible with the CD-ROM. Currently
  the DVD drive is more expensive than the CD-ROM,
  but prices are converging. Will the DVD replace
  the CD?

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ASSIGNMENT
The THUMB DRIVE is now very popular for
personal computing. What is a thumb drive? How
does a thumb drive work/what mechanism does it
use to store data? What are its advantages? What
are the other names for this device?
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FLASH MEMORY

• You may recall that EEPROM can be erased by using
  high voltages.
• There is a type of memory which can be erased
  with low voltages and then rewritten even while
  still inside the computer. This type of memory is
  called flash memory.
• The thumb drive is a type of flash memory

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Reading Assignment

• Computers Information Technology in Perspective
  Long and Long 12th edition Pages 192 to 206
• Understanding Computer Science for Advanced Level
  Ray Bradley
• Chapter 12
• ALevel Computing Heathcote and LangfieldPages
  144 to 148

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The E N D