Transcript PPT - muhanna.info

Chapter 6
Storage and Multimedia
COMPUTERS:
TOOLS FOR AN INFORMATION AGE
Secondary Storage
 Separate from the computer itself
 Software and data stored on a semi-permanent
media

Unlike memory, not lost when power is lost
Benefits of Secondary Storage
 Space
 Reliability
 Convenience
 Economy
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
 Track
 Sector
 Cluster
 Cylinder
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
 Character
 Field
 Record
 File
 Database
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 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
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