4Storing+&+Retrieving

Two step process: storing saves information retriveing gets the information
 * Definition of Storing and Retrieving**

=**Hardware used for Storing and Retrieving**=

__Magnetic Disks__ Most widely used storage medium which consists of a circular piece of metal or plastic whose surface has been coated with a thin layer of magnetic material Disk Drive - where a magnetic disk is mounted. It spins the disk in a circular motion using one or more heads to read and write the data. It needs to be formatted to store data which organises the disk into sectors and tracks and removes any data on the disk, analysing it for faults and creates a directory to record information. Virtual Memory - technique used by operating systems to increase the amount of memory.
 * Use Random Access to retrieve data. Allows a particular piece of data to be found directly rather then going through ever other piece that comes before it.
 * Hardware Device || Description ||
 * Floppy Disk || Magnetic disk made of flexible plastic and covered in magnetic material.
 * Small storage capacity
 * Slow
 * Portable
 * Cheap ||
 * Hard Disk || Magnetic disk made of metal or glass and covered with magnetic material. It is rigid allowing it to be 10-100X faster than a floppy disk.
 * Data is stored densely therefore it can store more
 * Size is measured in Gigabits
 * Partition: an area that functions as a separate disk, they are partitioned before formatted ||
 * Disk Controller || Responsible for the transfer of data between the computer and the disk drive. It consists of electrical circuits built into the disk drive. Different controllers include:
 * EIDE: supports storage capacity up to 8.4 Gb and transfer rates of up to 66 megabytes per second
 * Ultra DMA: increased data transfer rates of 66 megabytes per second and improved data integrity
 * SCSI: controllers are usually contained on an expansion card and provide transfer rates of up to 80 megabytes per second and support multiple disk drives ||
 * Removable Cartridges || Disks encased in metal or plastic cartridges that are removes like a floppy disk. They are fast and combine the good aspects of floppy and hard disks. Different types include:
 * Zip Disks: slightly larger than a floppy disk and can store 100-250 Mb with a transfer rate of 1.4 Megabytes per second. They are popular of backing up hard disks and transporting files as they are cheap.
 * Jaz Disks: can store 2Gb of data with a transfer rate of 5.5 megabytes per second. They are suited for large files of multimedia, graphics and sound. ||
 * Sets aside part of a hard disk and treats it as though it were memory.
 * Paging: involves allocating a fixed number of bytes every time data is transferred from memory to a disk.

__Magnetic Tape__ Long thin strip of plastic coated with a thin layer of magnetic material. It is wound into reels and is read and written on a tape drive
 * Main disadvantage: uses a sequential access to retrieve data, have to start at the beginning and fast forward to the section you want. It is also slow and unsuitable for data which has to be updated often

__Optical Disks__ Medium in which data is read and written using laser technology. They are polycarbonate plastic disks whose surface is coated with a reflective layer of metal. Data is written by a laser than burns holes (pits) into the surface. Different types include: Flash Memory - non-volatile memory chip that retains its data when the power is removed.
 * Type of Disk || Description ||
 * CD-ROM (Compact Disk Read Only Memory) || ** 650 Mb
 * Once data is stored it cannot be changed and no more can be added
 * Slower than a disk drive (however getting faster)
 * Good for storing data that remains constant eg. Encyclopaedia, games etc ||
 * CD-R (Compact disk recordable) || ** Allow data to be written once but read many times
 * Allows users to create CD ROMS and audio CD's as well as archive large amounts of data
 * Disadvantage is that the writing process is permanent ||
 * CD-RW (Compact Disk Rewritable) || ** Allows the user to write, erase and re-write data
 * Erasing is achieved by heating the disk and quickly cooling it
 * Slower than a hard disk and after frequent use areas of the disk become inaccessible ||
 * DVD (Digital Versatile Disk) || ** Stores large amounts of data
 * Same physical size as a CD but stores between 4.7 to 17 Gb
 * Compatible with CD-ROMS and audio CD's
 * Major advantage: data, video and audio have the same file structure (UDF) which overcomes problems of incompatibility
 * Can store full length movies ||
 * Widely used within cameras, mobile phones and portable computers

=**Software used for Storing and Retrieving**= Directs and controls the hardware storage devices. Data needs to be formatted before it can be stored:
 * Formatting: prepared the disk to accept data by organising it into tracks and sectors
 * Track: a band formed by centric circles
 * sector: a section of a track that can store data. Each sector has an address so hat the computer can go directly to a specific area



The storage medium and operating system determines the number of tracks and sectors


 * Hardware Interface Software**: controls the hardware devices used for storage
 * File Management Software:** organises files on a storage medium such as a hard disk and is part of the operating system.
 * Files are recognised by their names which can be between 8 adn 255 characters long
 * For quick access of these files they can be stored in folders or directories
 * File Formats:** used for different data types, this can be found in the filename extension to identify the format such as jpeg, doc, mp3, rtf etc.
 * Database Management System:** software used to access a database. the DBMS has no data in it itself however it allows the user to enter, store and retrieve data
 * Internet Browsers**: program which allows users to access documents stored on other computers connected to the World Wide Web.

Non-comp tools for Storing and Retrieving
- **Paper-based storage system:** convenient method of data storage but requires a large amount of space and specific information is hard to retrieve. //i.e. folders, boxes, bookcases and filing systems// - **Microfiche:** a transparent sheet of film that stores around 200 pages of print. Being a popular method for several decades, particularly libraries, allowing the data to be sourced directly instead of searching through the whole document. //NOTE: this has largely been replaced by optical disks// - **Library:** store a variety of information using the Dewey classificaltion/decimal system, classifying the information into 10 main subjects areas using decimal systems with data being retrieved via. book attributes. Many contries create a national library for orderly collection and storage of data. //i.e. of data types: books, cds, newspappers, magazines//

Social and Ethical issues for Storing and Retrieving
- **Security of data:** this is major with their enormous cost in damaged data. DAta security involves a series of safeguards to protect data //examples are: passwords, personal objects, biometric devices, firewalls, data encryption, securing waste, carefully screening employees and having appropriate backup procedures// //-// **Unauthorised retrieval of data:** refers to people //(hackers)// gaining illegal access to information systems, theese hackers are often involved in information theft or financial theft. //NOTE: these crimes are often not reported due to adverse publicity and are not regarded in the same light as conventional robbery//