Commonly Available Storage Devices Include
Storage devices
Commonly available storage devices include:
Ø Magnetic disk
Ø Optical disk
Ø Network storage
Ø Flash memory
Ø Magnetic tape
Magnetic disk
Magnetic disks are probably the most commonly used form of non-volatile storage media. Both hard and floppy disk drives use read/write head(s) which can both write magnetic spots into the thin oxide coating and read magnetic spots within the thin magnetic coating. Each spot can only be ON or OFF. This directly corresponds to binary codes of 0 and 1 used with digital computers and hence digital data can be stored on magnetic media. For example, the CPU under software control sends instructions to the disk drive controller which in turn controls the read/write head(s) which in turn can either write or read data to/from the disk.
Because data is stored in concentric circular tracks, the head(s) are moved in or away from the centre of the disk to the appropriate track. To read data from the appropriate track, the disk must be rotated. A floppy disk is only rotated when necessary (reading or writing) whereas hard disk platters typically spin continually. New energy saving computer systems have features that, for example, after certain period of non-usage the hard disk is powered down. Other elements, such as the monitor, are also powered down and the system uses minimal power in a ‘sleep mode’. When the user wishes to use the computer it will take a few seconds to ‘spin up’ the hard drive as well as power up other elements of the system.
While standard mass produces CD-ROMs cannot be altered, recordable CDR’s (record once only) CDRW’s (CD record and rewrite) are now available. The standard CDR only provides one session of recording while the CDRW allows multiple sessions of recording. CDR and CDRW can be used as another alternative to tape backup.
CD-ROM drives in computers are also capable of playing music CD’s and so people can use a CD-ROM drives to play music as they work with their computer system. A blank CDR disc costs a few dollars and the CDRW recorder costs about $400.
DVD (Digital video disc)
In general terms DVD is similar to CD in physical size and appearance but with a much larger capacity. Different manufactures have produced competing products that are not fully compatible with each other. This has caused some confusion in the market. DVD-ROM can be thought of a larger capacity CD-ROM with data more densely packed. Current single sided DVD-ROM provides 4.7 GB while double-layer-DVD-ROM provides 8.5 GB of storage, while proposed multi-layer DVD-ROM offers 15GB. This is sufficient space to store a complete movie in high quality digital form or store very high quality audio or other computer-based digital data. DVD-ROM drives for computers are now freely available and are quickly becoming a standard CD-ROMs. In addition DVD players for movie playback on TV are also commonly available. Such players provide higher quality image and audio than current videotape technology.
At the time of writing, recordable DVD drives were becoming available. There are currently 2 competing standards; however DVD-RAM is available and provides 5.6 GB of re-writeable storage space. DVD technology is still a developing area with new standards evolving, which promise 15GB to 20GB of storage in the near future!
Magneto-optical
Another form of optical disk technology is available that allows for altering existing contents. These new drives are more expensive and have a slower access speed than magnetic hard disks and are called magneto optical disk (MO disks). The actual disk is typically glass or hard plastic coated with a material that is sensitive to both intense light and magnetic fields. To read/write data to/from the disk MO drives use a combined magnetic and laser based read/write head.
To write data to the disk a laser (typically LED laser) heats a spot on the disk surface and the magnetic part of the read/write head applies a magnetic field which re-aligns the magnetic particles are ‘set’ in the material. Data is read from the disk by just reading disk drive. From the above description you can see that writing data to a MO disk is slower than reading data from an MO disk.
