chapter 6 introduction to computer (dr.alya)

7/31/2019 Chapter 6 Introduction to Computer (Dr.Alya)

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Computers:

Tools for an Information Age

Chapter 6

Storage and Multimedia:The Facts and More


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Objectives

List the benefits of secondary storage Identify and describe storage media

that are available for personal

computers Differentiate among the principal types

of secondary storage

Describe how data is stored on a disk

Discuss the benefits of multimedia

Explain how data is organized,accessed, and processed

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Secondary Storage

Separate from the computer itself

Software and data stored on a

semipermanent basis

Unlike memory, not lost when power islost

Benefits

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Benefits of Secondary

Storage Space

Reliability

Convenience

Economy

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Space

Store a roomful of data on diskssmaller than the size of a breadbox

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 relativelysafe

Secondary storage is highly reliable

More difficult for untrained people totamper with data stored on disk

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Convenience

Authorized users can easily andquickly locate data stored on the

computer

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Economy

Several factors create significantsavings 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 magnetizedspots on surface of spinning disk

Spots on disk converted to electrical

impulses Primary types

Diskettes

Hard Disks

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Diskettes

Made of flexible Mylar and coated with

iron oxide

Has protection of rigid plastic jacket

3 diskette holds 1.44 MB of data High-capacity variations

Sonys HiFD holds 200 MB

Imations SuperDisk available in 120 and 240

MB versions

Iomegas Zip drive available in 100, 250, and

750 MB versions

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Hard Disks

Rigid platter coated with magneticoxide

Several can be combined into a disk pack

Disk drive - a device that allows datato 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

Data can be destroyed if headtouches miniscule foreign matter

on surface of disk

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Disk Packs

Each platter has its own

access arm with

read/write head

Most disk packscombine platters, access

arms, and read/write

head

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Hard Disks for Personal Computers

Sealed modules that mount in a 3

bay

Capacity in gigabytes

Accessing files much faster thanaccessing files on diskettes

Some contain removable

cartridges Iomegas Jaz drive is very popular

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Redundant Array of Independent Disks

(RAID)

A group of disks that work

together as one

Raid level 0 spreads data from a

single file over several drives Called data striping

Increases performance

Raid level 1 duplicates data on

several drives Called disk mirroring

Increases fault tolerance

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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

Floppy diskette has 80 trackson each surface

Hard disk may have 1,000 or

more tracks on each surface of

each platter

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Sector

Each track is divided intosectors that hold a fixednumber of bytes Typically 512 bytes per

sector

Zone recording assignsmore sectors to tracks inouter zones than those in

inner zones Uses storage space more

fully

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Cluster

A fixed number of adjacent sectorsthat 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 toaccess data on disk

Three factors

Seek time

Head switching

Rotational delay

Once data found, next step is datatransfer

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Seek Time

The time it takes the access arm toget into position over a particular track

All access arms move as a unit

All simultaneously in position over a set oftracks that make up a cylinder

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Head Switching

The activation of a particularread/write head over a particular track

All access arms move together, but only

one read/write head can operate at anyone time

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Rotational Delay

The time it takes for the desired dataon the track to rotate underneath the

read/write head

On average, half the time for a completerevolution of the disk

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Data Transfer

The process of transferring data betweenits location on the disk track and memory

Measures of performanceAverage access time About 10 milliseconds (10 thousands of a second)

Can be improved by disk caching

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

Similar to memory caching discussed

in Chapter 4

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Optical Disk Storage

Provides inexpensive and compact storagewith greater capacity

Laser scans disk and picks up lightreflections from disk surface

Categorized by read/write capability Read-only media - user can read from, but not

write to disk

Write-once, read-many (WORM) - user can write

to disk once Magneto-optical - combines magnetic and optical

capabilities

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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 Some compatibility problems trying to read

CD-RW disks on CD-ROM drives

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Digital Versatile Disk (DVD)

Short wavelength laser can readdensely packed spots

DVD drive can read CD-ROMs

Capacity up to 17GBAllows for full-length movies

Sound is better than on audio CDs

Several versions of writable andrewritable DVDs exist

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Multimedia

Presents information with text,illustrations, photos, narration, music,

animation, and film clips

Not practical until the advent of theoptical disk

Requirements

Applications

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Requirements

CD-ROM or DVD-ROM drive

Sound card or sound chip

Speakers

For high-quality sound, get good speakers

and powered subwoofer

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

Prepare taxes with video clips from IRS

experts Play games

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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 ontape

Used primarily for backup of data

stored on disk systems

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Backup Systems

Imperative to have copies of important datastored away from the computer

Disks occasionally fail

Software installation can cause computer to

crash

Users make mistakes entering data

Tape is ideal backup medium

Can copy entire hard disk to single tape inminutes

Backup can be scheduled when you are not

going to use the system

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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 namewould 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 constitute 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 withminimum 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 Plan Overview

Must devise a plan for placing data ona storage unit

Key factors

Whether users must access data directly(immediately)

How data must be organized on disk

Type of processing that will take place

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File Organization

Three major methods of organizingdata files in secondary storage

Sequential

Direct Indexed

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Sequential File Organization

Records are stored in order accordingto 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 toread 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 keyto determine disk address of given

record

Collision occurs when hashing algorithmproduces same disk address for two

different keys

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Indexed File Organization

Combines elements of sequential anddirect methods

Records stored sequentially, but file also

contains an index Index stored sequentially, contains record

key

Data accessed by record key

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Processing Stored Data

Transactions processed to update amaster file Transactions - a business event such as a sale

Master file - data that is updated when atransaction occurs, such as a sales file orinventory file

Two main methods of processing data Batch processing Transaction processing

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Batch Processing

Transactions collected intogroups 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 afterprocessing

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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 updatingof master file

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