Computers store information on magneticdevices and optical devices.
Magnetic storage devices are coated with tiny magnetically sensitive particles which record binary data. The read/write head can change the magnetic charge of the particles to represent 1s or 0s. When magnetic energy passes from the head to the storage device, it writes. When magnetic energy passes from the device to the head, it reads. Common magnetic devices are floppy disks, hard disks, and magnetic tape.
Optical storage devices use laser beams to read information from a spinning disk. The laser beam is focused onto a small point on the disk while a sensor measures the amount of light reflecting off the surface. The surface of the disk has spots that reflect light, called lands, and spots that do not reflect light, called pits. A land is interpreted as as a 1, and a pit is interpreted as a 0. The disk which contains this optical information is permanently stamped, like a phonograph, and can only be read. Optical storage devices include CD-ROMs and magneto-optical disks.
Most microcomputers have at least one floppy-disk drive which uses either 5.25 or 3.5 inch disks. The actual floppy disk is round and floppy, however it is encased in a thin, square, plastic casing. The casing on a 5.25 inch disk is flexible, but the casing on a 3.5 inch disk is hard. The disk inside is coated with iron oxide, and the closer together the iron oxide particles, the greater the capacity of the disk. The 5.25 inch disk can be double density or high density, and the 3.5 inch disk can also be very high density. There is a cut-out section on the disk's case to allow the read/write head access.
The floppy-disk drive contains a read/write head which looks similar to the arm of a record player. The drive also has a spindle that spins the disk to allow the read/write head access to any protion of the disk; it does not have to access the data in sequential order.
Floppy disks are commonly used for moving files between separate computers, loading new programs onto the computer, or storing backup copies of data.
The hard disk is not a single flexible disk, but a stack of metal disks sealed in a box. The hard disk and hard-disk drive exist together as a unit and are usually a permanant part of the computer.
Like the floppy disk, the hard-disk platters are coated with iron oxide, however the dust-free sealed environment allows the hard disk to spin much faster than the floppy disk as the risk of crashing due to dirt is greatly reduced. Because of the rigidity of the hard disk it is also much more precise than the floppy disk, and it holds much more data.
Because of its great capicity and speed, the hard disk is used to store both programs and data.
Magnetic tape reads and writes data in the same way an audio casssette tape records and plays sound, with the exception that magnetic tape stores digital data (1s and 0s). Because a tape physically allows only sequential access, data retrieval can be slow and tedious, however tapes have more capacity than floppy disks and are good for storing full-motion video images and backup data.
The compact disc, read-only memory is like a music CD. It contains digital information which can be read, but it cannot be used to store information, just like a music CD cannot be used to record. However the CD-ROM can hold vast amounts of information such as full-motion videos, animation, music, text, and data. This makes it ideal for storing encyclopedias, reference libraries, games, and any other information that the user is not required to alter or change.
Interactive CDs or CD-Is, allow you to decide what information you wish to access. For example, when touring the starship "Enterprise" on the Star Trek Interactive CD, you may choose to see the bridge and Data's quarters only, without touring the entire ship.
Magneto-optical storage is unique in that it uses both magnetic technology -- like a floppy disk -- for easy data storage, and optical technology -- like a CD-ROM -- for precision. The result is a storage medium which can easily be rewritten, with a large capicity, all in a portable format.
The magneto-optical disk is coated with magnetically sensitive particles. It is then coated with thin, clear plastic which prevents the particles from responding to the write head. In order to write, an intense laser beam must be focused on a specific point, melting the plasitc for a moment to allow the head to re-aline the particles. The plastic then quickly cools and hardens. The particles are arranged in such a way as to reflect laser light in the appropriate spots, much like lands and pits on a CD-ROM. A low intensity laser beam is focused on the spinning disk while a sensor measures whether or not light is reflecting off the surface.
Introduction to Computers
IHC - 2E03
Humanities Computing Centre