1. Chapter 6 Storage and Multimedia
Computers: Tools for An Information Age

2. Secondary Storage Separate from the computer itself
Software and data stored on a semi-permanent media
Unlike memory, not lost when power is lost

3. Benefits of Secondary Storage
Space
Reliability
Convenience
Economy

4. Space Store a roomful of data on disks
Diskette contains equivalent of 500 printed pages
Optical disk can hold equivalent of 500 books

5. Reliability Data in secondary storage is relatively safe
Secondary storage is highly reliable
More difficult for untrained people to damage data stored on disk

6. Convenience
Authorized users can easily and quickly locate data stored on the computer

7. 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

8. 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

9. Diskettes
3 ½” diskette holds 1.44 MB of data

10. 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

11. 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

12. Disk Packs Each platter has its own access arm with read/write head
Most disk packs combine platters, access arms, and read/write head

13. How Data is Organized
Track
Sector
Cluster
Cylinder

14. Track
The circular portion of the disk surface that passes under the read/write head

15. Sector
Each track is divided into sectors that hold a fixed number of bytes

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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)

28. 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

29. Applications Education Other Go on virtual tours Study musical scores
Study a foreign language
Other
Play games

30. Organizing and Accessing Stored Data
Character
Field
Record
File
Database

31. Character
A letter, digit, or special character

32. 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

33. Record A collection of related fields
For the university, all of the fields for one student compose one record

34. File A collection of related records
For university, all the student records compose a file

35. 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

36. File Organization
Three major methods of organizing data files in secondary storage
Sequential
Direct
Indexed

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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