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 Other 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