computers: tools for an information age chapter 6 storage and multimedia
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COMPUTERS:TOOLS FOR AN INFORMATION AGE
Chapter 6Storage and Multimedia
Secondary Storage
Separate from the computer itselfSoftware and data stored on a semi-
permanent media Unlike memory, not lost when power is lost
Benefits of Secondary Storage
SpaceReliabilityConvenienceEconomy
Space
Store a roomful of data on disks Diskette contains equivalent of 500 printed pages Optical disk can hold equivalent of 500 books
Reliability
Data in secondary storage is relatively safe Secondary storage is highly reliable More difficult for untrained people to damage data
stored on disk
Convenience
Authorized users can easily and quickly locate data stored on the computer
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
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
Diskettes
3 ½” diskette holds 1.44 MB of data
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
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
Disk Packs
Each platter has its own access arm with read/write head
Most disk packs combine platters, access arms, and read/write head
How Data is Organized
TrackSectorClusterCylinder
Track
The circular portion of the disk surface that passes under the read/write head
Sector
Each track is divided into sectors that hold a fixed number of bytes
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
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
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
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
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
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
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
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
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
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
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
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)
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
Applications
Education Go on virtual tours Study musical scores Study a foreign language
Other Play games
Organizing and Accessing Stored Data
CharacterFieldRecordFileDatabase
Character
A letter, digit, or special character
Field
A set of related characters
Describes one characteristic of a person, place, or thing For a university, a student’s first name would be
stored in a field
Key field - a unique identifier for a record
Record
A collection of related fields For the university, all of the fields for one student
compose one record
File
A collection of related records For university, all the student records compose a file
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
File Organization
Three major methods of organizing data files in secondary storage Sequential Direct Indexed
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
Direct File Organization
Also called random accessGo 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 keyRequires disk storage
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
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
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
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
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