How Computers Store Data (Files and Directories)

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How Computers Store Data(Files and Directories)

• Files
• Data Storage
• Secondary Storage
• Types of Data Files
• File Names
• Directories
• File Organization
• Storage TechnologiesDisk and Tape

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Files

• Did you ever stop to think what happens behind
  the scenes when you
• Draw money from your checking account at an ATM?
• Check out at a supermarket?
• Request a telephone number through directory
  assistance?

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Files

• All of this information is stored in files.
• A file is logical and a physical entity.
• A file is the foundation of permanent storage on
  a computer system.
• To a computer, a file is simply a string of
  digitized data that are stored and retrieved as a
  single unit.

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Data Storage

• bit
• byte (character)- 8bits
• field - set of characters
• record - related fields
• file - area of storage medium set aside to data

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Secondary Storage

• Within a computer system, information is stored
  in both primary and secondary storage.
• Information (file) is retrieved from secondary
  storage and stored temporarily in primary storage
  for processing.
• Variety of devices and media for secondary
  storage.

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Types of files

• A file is to secondary storage as a vehicle is to
  a parking lot.
• Variety of vehicle types
• Numbered parking slots

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Types of Files

• Program Files
• Support Files

• Data Files
• Configuration files
• Text files
• Graphics files
• Spreadsheet files
• Database files
• Sound files
• Backup files

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File Names

• We assign names to files to store and recall
  them.
• Windows 95/98/NT allows descriptive file names of
  up to 255 characters.
• MS-DOS only allows eight characters for the
  filename

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File Extensions

• Since there are so many file types, we use a file
  extension appended by a . to the end of the
  filename
• filename.filetype
• filetype is also called file extension.
• extension identifies type - jpg, gif, doc, xls,
  wav, bmp,
• usually application program appends the
  appropriate extension

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Organizing Files into Folders

• Root Folder
• Subfolders

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Folders or Directories?

• A directory is a type of file that contains a
  list of files
• Computer automatically updates the listing of all
  files in the directory whenever the contents of
  the disk change.
• When graphical interfaces were introduced, they
  used a folder to display directories

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Relationships

• Parent directory and child directory - when you
  are viewing a subdirectory (child), the directory
  that is one level up is the parent directory.
• Path - sequence of directories that the computer
  must follow to locate a file
• absolute - start at the root
• A\letters\intro.doc
• relative - from where you are
• \documents\letters\letter.doc

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File Organization

• Sequential
• Direct (Random)
• Indexed Sequential

Customer Database
12/24/94 Backup
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File Organization

• sequential files - data processed in the order in
  which it was recorded
• direct (random) - data can be retrieved from
  position within the file -
• indexed sequential file - data can be accessed
  either directly or sequentially

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Files

• A file is stored permanently for periodic
  retrieval in secondary storage.
• Accessing the information depends on the device
  and the media.

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Secondary Storage - Magnetic

• Magnetic Disks (Random)
• Floppy Disks (interchangeable)

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5 1/4 and 3 1/2 Floppy Disks
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Floppy Disk

• Thin, flexible magnetic-coated mylar diskette
• Disk density - number of bits (binary digits)
  that can be stored per unit of area.
• 800K - new ones 2.88 MB (MB -1,048,576 million
  bytes)

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Secondary Storage - Magnetic

• Magnetic Disks (Random)
• Floppy Disks (interchangeable)
• Zip Disk

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Zip Disk

• Requires a zip disk drive
• use optical technology together with magnetic
  technology to read and write to 3 1/2 inch 120 MB
  diskettes as well as the standard 3 1/2 inch
  diskettes.

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Secondary Storage - Magnetic

• Magnetic Disks
• Floppy disks
• Zip disks
• Hard disks

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Hard Disks and Drives

• Greater storage capacities
• Permanently installed
• Aluminum coated with a easily magnetized material
• measured now in gigabytes (1billion bytes

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Magnetic Disk Organization

• Concentric tracks
• recorded using serial representation
• Number of tracks varies
• recording densities - high density disk has
  double the recording density of a double-density
  disk

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Magnetic Disk Organization

• recording surface is also divided into pie shaped
  sectors
• the number of sectors depends on density of the
  disk
• typically each sector is 512 bytes
• disk address is sector and track number

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Magnetic Disk Organization

• Tracks on tracks
• hard drive contains several platters
• cylinder refers to every track with the same
  number on all recording surfaces

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Magnetic Disk Organization

• Formatting
• Creates sectors and tracks into which data are
  stored
• Sets up and area for the file allocation table
  (FAT). Tells the system where to find the files
  and file folders you eventually store on the
  disk, (sector and track)

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Secondary Storage - Magnetic

• Magnetic Disks
• Floppy disks
• Zip disks
• Hard disks
• Magnetic Tape

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Magnetic Tape

• Media comes in several widths and many different
  lengths
• sequential processing
• 1/4 inch - used for PC backup
• needs a tape drive
• a tape drive is is rated by its storage capacity
  and its data transfer rate

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Secondary Storage - Other Forms

• Optical Disk
• CD-ROM
• WORM
• DVD-ROM
• MO
• Bubble Memory

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CD-ROM UNIT
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The FIRST commandment in computing

• Back up your files!
• When you create a document, spreadsheet or graph,
  you will store the file on disk. If the disk is
  in some way destroyed (scratched, demagnetized,
  burned) or lost, you have lost your files unless
  you have a backup.

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How Permanent is Your Data?

• Once its digitized, its available forever!
• Wrong
• media type can deteriorate
• findings indicate that even CD-ROMs can
  deteriorate in as little as five years
• obsolescence - no way to read to the old format

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