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