Storage Devices

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Storage Devices
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Storage Devices

• Also called secondary storage
• store data programs permanently for future use
• can be used as both input output devices

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Magnetic Disk Storage

• Most widely used storage medium
• consists of a round piece of plastic or metal,
  the surface on which is covered by a magnetic
  materials.
• e.g. floppy disks, hard disks and removable disk
  cartridge

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Floppy Disk

• Random-access storage medium
• provides storage of 1.44MB data
• A circular piece of thin plastic that is coated
  with magnetic material
• the circular piece of plastic is enclosed by a
  rigid plastic shell
• Formatting prepares a floppy disk for storage by
  defining the tracks, cylinders, and sectors on
  the disk surface
• time required to locate data and transfer it to
  memory is called the access time.

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Data is recorded on disk in concentric circular
bands called tracks. The tracks on a disk are
similar to the grooves on a phonograph record.
Each track is divided into pie-shaped wedges
called sectors. Two or more sectors combine to
form a cluster.
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Most computers maintain on the disk a table with
the sector and track locations of data. This
table, the file allocation table (FAT), enables
the computer to locate data easily.
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Structure of a Floppy Disk
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Hard Disks

• Fixed in the System Unit
• Consists of one or more rigid platters coated
  with a magnetic material
• the platters, read/write heads, and the access
  arms that move the heads across the disk surface
  are all enclosed in an airtight, sealed case

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• Access time for a hard disk is significantly less
  than that for a floppy disk because a hard disk
  spins faster and unlike a floppy disk, a hard
  disk is spinning constantly (7200 revolutions per
  minute).
• hard disks are permanently encased within the
  disk drive in a sealed environment free from dust
  and dirt. The disk can spin very rapidly, with
  the read/write head "floating" above the disk's
  surface.

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Hard Disk Interfaces

• To connect a hard disk to a microcomputer
  motherboard, you must have a hard disk interface.
  
• This component includes circuitry that conforms
  to a standard recognized by both the hard disk
  and the motherboard manufacturer.
• Common standards are Integrated Drive Electronics
  (IDE) and Small Computer System Interface (SCSI).

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Removable Disks

• Provide both the storage capacity and fact access
  time of hard disk and the probability of floppy
  disks.
• Examples
• Iomegas ZIP (100MB/250MB)
• JAZ (1-2 GB)
• Imations Superdisk (120MB)

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Maintaining Stored Data

• To prevent loss of data, 2 procedures should be
  performed regularly
• Backup
• process of creating a copy of important programs
  and data
• Defragmentation
• process of reorganizing data stored on a disk so
  that files are located in contiguous (adjacent)
  clusters.

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DVD/CD-ROM and Optical Disks

• By using laser technology, optical disk can store
  large quantities of data
• a high power laser writes data on an optical disk
  by burning microscopic holes on the disk surface
• a lower power laser reads the data by reflecting
  light off the disk surface. The reflected light
  is converted into a series of bits

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• DVD/CD-ROM
• DVD/CD-Read Only Memory
• can store 650 MB data
• suitable for reference material such as
  encyclopedias, catalogs and pictures
• DVD/CD-R
• CD-Recordable
• write once, read many
• DVD/CD-RW
• DVD/CD-Rewritable
• erasable write many, read many
• MO
• Magneto-Optical
• Floptical (Optical Magnetic)

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Magnetic Tape

• Consists of a thin ribbon of plastic, one side of
  which is coated with a material that can be
  magnetized to record binary data.
• A sequential storage media (must write and read
  records on after another)
• usually cartridge tape
• serves as a primary means of
• backup,
• a method of transferring data between system, and
• a cost-effective way to store data that does not
  have to be accessed immediately

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Other types of Storage Devices

• PC Cards (PCMCIA)
• Flash Memory Card
• Memory Stick
• Smart Card