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Storing and Retrieving

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					                Storing and Retrieving

Storing saves data and information for later use and retrieving obtains
the data and information that has been previously saved.
The process of storing data is called writing and the process of
retrieving data is called reading.
There are a variety of hardware devices used to store data. Software is
used to control these devices.
Non - Computer tools for storage of information:
 Paper-based medium ( folders, book cases)
 Microfiche: transparent sheets of film about 10* 15 cm which can store around
  200 pages of print. Need a special reader to access information.
 Library storage.


                            HARDWARE
The hardware used for storage and retrieval of data is known as
secondary storage. The data stored on a medium must be read into
memory before the CPU can access it.
A backup is a copy of data held on a separate storage medium for
security purpose. Information systems need to store data on secondary
storage because:
 Memory is small in size;
 Memory is volatile.

Volatile means that when the computer is switched off or the power
supply is interrupted, the contents of the memory are lost.
Secondary storage provides a more permanent storage area than the
memory.
The hardware involved in storage and retrieval includes:
     Magnetic disks
     Magnetic tapes
     Optical disks
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   Flash memory


                        Magnetic Disks
Magnetic disks are most widely used storage mediums for personal
computers.
A magnetic disk consists of a circular piece of metal or plastic, which is
coated with a thin layer of magnetic material. Magnetic disks use either
random access of direct access to retrieve data.


Direct access or Random access: Finds the data directly by finding its
exact location, without having to search through all the data that comes
before it. Allows data to be retrieved faster.


Sequential access: Commonly used on magnetic tapes where each data
item is checked before the required data item can be located. This is a
slow process.



                           Disk Drives
A disk drive is a device on which the magnetic disk is mounted. The disk
drive spins the disk and uses one or more heads to read data from disk or
write data to the disk. If the disk is permanently attached to the drive it
is called a fixed disk (hard disk).


Formatting a disk
A disk needs to be formatted to store data. Formatting organises the
disk into sectors and tracks, removes any data from disk, analyses the
disk for faults and creates a directory to record information about files.
This directory is called a file allocation table (FAT). The file allocation
table stores the file name, file size, time and date the file was last
modified and the address of the file.


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                                   Floppy Disk
                A floppy disk or a diskette, is a magnetic disk made of
                flexible plastic and covered with magnetic material. They
                are portable and cheap storage


               mediums. Floppy disks do not have a large storage
capacity and are slower than a hard drive for accessing data. The most
common size is 31/2 inch and has a capacity to store 1.44 MB of data.



                                         Hard Disk
                     Hard disk is a magnetic disk made of metal or glass
                     and covered with magnetic material. Because it is
                     rigid, it can spin 10 to 100 times faster than a
                     floppy disk, allowing a faster access to data. Hard
                     disks store more data because the data is stored
more densely. The storage capacity is measured in gigabytes.


Hard disks are formatted before they are used. A partition is an area
that functions as a separated disk. The size of each partition varies and
each partition is assigned a letter. On most computers, hard disk
partitions usually start with the letter “C”.


A Disk Controller is responsible for transferring data between the
computer and the disk drive. It consists of electrical circuits built into
the disk drive or contained onto an expansion card. There are many
standards for disk controllers, such as EIDE, Ultra DMA and SCSI.
EIDE (enhanced integrated drive electronics) supports a storage
capacity of up-to 8.4 Gb and data transfer rate of 66 MB per second.
Ultra DMA (direct memory access) increased data transfer rates of up-
to 66 megabytes per second and uses better error checks.


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SCSI (small computer system interface) controllers are usually contained
on an expansion card. They provide data transfer rate of up to 80
megabytes per second and support multiple disk drives.



                          Removable cartridges
               Each type of cartridge must be used with its own type of
               disk drive. The two formats that have become popular
               are:
 Zip disks- store up to 100MB
 Jaz disks- can store 1GB




                        Magnetic Tape
 Magnetic tape is a long thin strip of plastic coated with a thin layer of
  magnetic material. The tape is wound onto reels, sometimes inside a
  cartridge.
 The tape is written and read on a tape drive.
 Tape has large storage capacity and often used as a backup medium.
  The main disadvantage of this medium is that it uses sequential access
  making it slow and unsuitable for data that needs regular updating.
 Magnetic tape comes in a variety of sizes and formats: QIC tapes
  (quarter inch cartridge), DAT cartridges (Digital audio tape), 8 mm
  cartridge uses same technology as VCR tapes.




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                                 Optical Disks
                      An optical disk is a storage medium on which
                        data is read and written using laser technology.
                      These disks consist of a plastic disk covered
                        with a reflective layer of metal.
                      The data is written to the disk in a pattern of
                        tiny holes called pits. It uses a high powered
                        laser to burn the holes.
                      Optical disks are portable and they use random
  access to retrieve data.
 There are many different types of optical disks.

                                  Magneto optical Disk




     Highest capacity 5.25" removable drive available

     Read/write compatible to all ISO/ANSI 2.6, 1.3, and 650 cartridges

     Supports existing WORM and LIM DOW media

     Highest data transfer rate of any 5.25" MO drive

     Compatible with all major operating system platforms



                            Examples of Optical disks


   Name         Explanation        Features and uses

   CD-ROM       Compact disc       Capable of storing 650 Mb. Data can only be read, it
                read only          cannot be changed or added.
                memory             CD-ROM drive is needed to read data from the CD. The
                                   higher the drive speed, the faster the data access.
                                   They are convenient storage mediums for data that is

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                             relatively permanent e.g. encyclopedias, reference
                             material and games.
  CD-R      Compact Disk     Data is written once but read many times. They are also
            Recordable.      called WORM disks because data is written once and read
                             many times.

                             A CD-R drive or „CD burner‟ is needed to write the data.
                             The disadvantage is that the writing process is permanent.
  CD-RW     Compact Disk     Allows user to write, erase, and rewrite data. Erasing the
            Rewritable       disk is achieved by heating the surface and quickly cooling
                             it.
                             A CD-RW is slower than hard disk and after frequent use,
                             areas of the disk tend to become inaccessible.
  DVD       Digital          This is a disk format that can store large amounts of data.
            Versatile Disk   A DVD-ROM is the same size as a CD-ROM but provides
                             storage of between 4.7 Gb and 17 Gb. They are compatible
                             with CD-ROMS and CD‟s.
                             The major advantage of DVD is that data, video, and audio
                             have the same file structure.
                              This file structure is called UDF Universal Disc Format).
                             DVD‟s can store full-length movies.




Flash Memory
Flash memory is a non-volatile memory chip that retains its data when the
power is removed. Flash memory is erased and written in fixed blocks
ranging from 512 bytes to 256 Kb. Flash memory cards look similar to
credit card and come in a variety of formats such as PC cards. They are
widely used in digital cameras, modems, mobile phones and portable
computers.

                                  Here are a few examples of Flash memory:
                                        Your computer's BIOS chip
                                        CompactFlash (most often found in digital cameras)
                                        SmartMedia (most often found in digital cameras)
                                        Memory Stick (most often found in digital cameras)
                                        PCMCIA Type I and Type II memory cards (used as
                                         solid-state disks in laptops)
                                        Memory cards for video game consoles




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Software
Software directs or controls the hardware storage devices. Disk
formatting a for data storage is done by the software. Application
software/programs store data to a medium using the save as command.

The save as command allows the user to select a file name/change a file
name, location of storage and the format to save in. The open command is
used to retrieve the data by selecting the file name.

Formatting a disk:
 Prepares a disk to accept data by organising it into tracks and sectors.
 A track is a band formed by concentric circles and a sector is a
  section of a track that can store data.
 The number of tracks and sectors is determined by the operating
  system.
 A 31/2 floppy is capable of storing 1.44 megabytes. It consists of 80
  tracks on each side and 18 sectors per track.
 Each sector has an address, so the computer can go directly to a
  specific area (direct access).


               Software for storage and retrieval
i.    Hardware Interface software: controls hardware devices used for
      storage. Each device has its own specifications that are controlled by the
      hardware interface software.
ii.   File Management software: Usually part of the operating system. It
      organises file under a filename, on storage medium such as hard disk.
      Depending on the operating system, the file name can be from 8 characters to
      255 characters. The files are organised in folders or directories. The file
      management software allows the user to create, delete, open, close,
      rename, copy and move files or folders.
iii. File Formats:      are used for storing different data types. File name
      extensions are used to identify file formats. Some common extensions are:
      BMP- Bitmap
      JPG – JPEG- Joint Photographic Expert Group
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      GIF – Graphic Interchange Format
      PCT – Picture
      TXT – Text
      DOC – Document

iv. Database management system (DBMS):                         is the software used to
      access a database. Allows user to enter, store and retrieve data. Data in a
      database is organised into file, records, fields and characters. The user can
      choose which data is needed and how it can be displayed.
v.    Internet Browser: allows users to access documents stored on other
      computers connected to the World Wide Web. The URL is used to access a
      web page.
vi. Passwords: secret words or numbers used to gain access to other
      information systems.


                   Social and Ethical Issues

1. Security: Cost of replacing data, which has been accidentally, or deliberately
   damaged, is very expensive. Security involves safeguards to protect data. For
   example: passwords, biometrics devices, data encryption, securing waste,
   screening employees, and having back-up procedures.

2. Unauthorised, illegal access to information. Hackers can be involved in
   information theft or financial theft.




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