Computers: Tools for An Information Age

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Chapter 6Storage and Multimedia

• ComputersTools for An Information Age

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Secondary Storage

• Separate from the computer itself
• Software and data stored on a semi-permanent
  media
• Unlike memory, not lost when power is lost

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Benefits of Secondary Storage

• Space
• Reliability
• Convenience
• Economy

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Space

• Store a roomful of data on disks
• Diskette contains equivalent of 500 printed pages
• Optical disk can hold equivalent of 500 books

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Reliability

• Data in secondary storage is relatively safe
• Secondary storage is highly reliable
• More difficult for untrained people to damage
  data stored on disk

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Convenience

• Authorized users can easily and quickly locate
  data stored on the computer

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Economy

• Several factors create significant savings in
  storage costs
• Less expensive to store data on disks than to buy
  and house filing cabinets
• Reliable and safe data is less expensive to
  maintain
• Greater speed and convenience in filing and
  retrieving data

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Magnetic Disk Storage

• Data represented as magnetized spots on surface
  of spinning disk
• Spots on disk converted to electrical impulses
• Primary types
• Diskettes
• Hard Disks

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Diskettes

• 3 ½ diskette holds 1.44 MB of data

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Hard Disks

• A platter coated with magnetic oxide
• Several can be combined into a disk pack
• Disk drive - a device that allows data to be read
  from or written to a disk
• Disk drive for personal computers contained
  within computer housing
• Large computer systems may have several external
  disk drives

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Reading/Writing Data

• Access arm moves read/write head over particular
  location
• Read/write head hovers a few millionths of an
  inch above platter
• If head touches platter, a head crash occurs and
  data is destroyed

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Disk Packs

• Each platter has its own access arm with
  read/write head
• Most disk packs combine platters, access arms,
  and read/write head

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How Data is Organized

• Track
• Sector
• Cluster
• Cylinder

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Track

• The circular portion of the disk surface that
  passes under the read/write head

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Sector

• Each track is divided into sectors that hold a
  fixed number of bytes

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Cluster

• A fixed number of adjacent sectors that are
  treated as a unit of storage
• Typically two to eight sectors, depending on the
  operating system

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Cylinder

• The track on each surface that is beneath the
  read/write head at a given position of the
  read/write heads
• When file is larger than the capacity of a single
  track, operating system will store it in tracks
  within the same cylinder

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Disk Access Speed

• Access time - the time needed to access data on
  disk
• Three factors
• Seek time
• Head switching
• Rotational delay
• Once data found, next step is data transfer

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Seek Time

• The time it takes the access arm to get into
  position over a particular track
• All access arms move as a unit
• All simultaneously in position over a set of
  tracks that make up a cylinder

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Head Switching

• The activation of a particular read/write head
  over a particular track
• All access arms move together, but only one
  read/write head can operate at any one time

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Rotational Delay

• The time it takes for the desired data on the
  track to rotate underneath the read/write head
• On average, half the time for a complete
  revolution of the disk

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Data Transfer

• The process of transferring data between its
  location on the disk track and memory
• Measures of performance
• Average access time
• About 10 milliseconds (10 thousands of a second)
• Can be improved by disk cashing
• Data transfer rate - how fast data can be
  transferred once it has been found
• Stated in terms of megabytes per second

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Disk Caching

• Disk cache - a special area of memory
• When disk drive reads data from disk, it reads
  adjacent data and stores it in memory
• When next read instruction is issued, drive
  checks first to see if desired data is in disk
  cache

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Magnetic Tape Storage

• Tape similar to tape used in music cassettes
• Categorized in terms of density
• Number of bits per inch stored on tape
• Used mainly for backup of data

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Compact Disks

• CD-ROM - drive can only read data from CDs
• CD-ROM stores up to 700 MB per disk
• Primary medium for software distribution
• CD-R - drive can write to disk once
• Disk can be read by CD-ROM or CD-R drive
• CD-RW - drive can erase and record over data
  multiple times

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Digital Versatile Disk (DVD)

• Uses wavelength laser
• DVD drive can read CD-ROMs
• Capacity up to 17GB
• Allows for full-length movies
• Sound is better than on audio CDs
• Several versions of writable and rewritable DVDs
  exist

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Blue-ray Disc

• An optical disc storage medium designed to
  replace the DVD
• Can have a capacity of 25 GB per layer (total of
  50 GB for dual layer discs)

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Multimedia

• Presents information with text, illustrations,
  photos, narration, music, animation, and film
  clips
• Requirements
• CD-ROM or DVD-ROM drive
• Sound card or sound chip
• Speakers
• Equipped to handle MPEG
• Standards for compressing video

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Applications

• Education
• Go on virtual tours
• Study musical scores
• Study a foreign language
• Other
• Play games

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Organizing and Accessing Stored Data

• Character
• Field
• Record
• File
• Database

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Character

• A letter, digit, or special character

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Field

• A set of related characters
• Describes one characteristic of a person, place,
  or thing
• For a university, a students first name would
  be stored in a field
• Key field - a unique identifier for a record

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Record

• A collection of related fields
• For the university, all of the fields for one
  student compose one record

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File

• A collection of related records
• For university, all the student records compose a
  file

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Database

• A collection of related files stored with minimum
  redundancy (duplication)
• For university, student file, alumni file,
  faculty/staff file, courses file, financial file,
  etc. would make up a database
• Organized to make retrieving data easier

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File Organization

• Three major methods of organizing data files in
  secondary storage
• Sequential
• Direct
• Indexed

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Sequential File Organization

• Records are stored in order according to a key
  field
• If a particular record is desired, all prior
  records must be read first
• To update a record, a new sequential file must be
  created, with changed and unchanged records
• Tape storage uses sequential organization

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Direct File Organization

• Also called random access
• Go directly to desired record by using a key
• Computer does not have to read all prior records
• Hashing algorithm used to determine address of
  given key
• Requires disk storage

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Hashing Algorithm

• Applies mathematical formula to key to determine
  disk address of given record
• Collision occurs when hashing algorithm produces
  same disk address for two different keys

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Processing Stored Data

• Transactions processed to update a master file
• Transactions - a business event such as a sale
• Master file - data that is updated when a
  transaction occurs, such as a sales file or
  inventory file
• Two main methods of processing data
• Batch processing
• Transaction processing

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Batch Processing

• Transactions collected into groups or batches
• Batch processed and master file updated when the
  computer has few users online
• Very efficient use of computer resources
• Master file current only immediately after
  processing

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Transaction Processing

• Processing transactions as they occur
• Also called real-time processing and online
  processing
• Terminals must be connected directly to the
  computer
• Offers immediate updating of master file