Storage media is used to store data, such that if the computer turns off, the data are not lost and can be retrieved at a later time. We here present several of the more common storage media types.
One of the first types of storage media to have been used was magnetic tape. These magnetic tapes were commonly 2400ft or 3200ft long and their recording density (how much information could be stored over a certain length of tape) were usually either 800 bits per inch (bpi), 1600 bpi or 6250 bpi. A 2400ft 6250 bpi magnetic tape held roughly 20MB of information. Some of the tape was lost because after each recording, a gap was left. These tapes were most efficient when the information had to be read in the order that it was recorded because to access the information recorded at the end, the computer had to go through all the information recorded before. In the image below, the large round magnetic tape's capacity is roughly 20MB and the small rectangular tape's capacity is roughly 4GB.
Magnetic disks are composed of a round piece of metal to which a magnetizable coating has been applied (on both sides in general). Disks are divided into concentric circles called tracks (usually a few hundred per surface) on which the information is recorded. Each of these tracks are divided into sectors (usually between 10 and 100). Magnetic disk drives read and write information using a movable head (one per surface) which is designed to read the information off of exactly one track at a time. Disk drives usually contain more than one magnetic disk. Disk drives have built in disk controllers which help transferring information. For a transfer, a certain amount of information must be provided. For example:
On the pictures above you can see an open floppy disk with the magnetic disk inside.
On the pictures below, you can see an open hard drive, with its several magnetic disks and the heads on each surface.
Magnetic drums are very similar to disks, concerning tracks and sectors, except that they are not disks but cylinders and therefore information is also written along the side of the cylinder. Magnetic drums have fixed heads all along their side and therefore no time is wasted seeking. However magnetic drums can't contain as much information as magnetic disks.
Optical Disks have only recently been introduced into the world of computers. They are known as CD-ROMs and DVD-ROMs.
The first CD-ROMs were developed by Dutch electronics conglomerate, Phillips, and later in collaboration with Sony. CD-ROMs are made of plastic and are coated with aluminum and a layer of transparent plastic for protection. They have a spiral groove starting in the center and moving outwards, similar to that of records. The information is then recorded in this groove as a series of pits (holes) and lands (unburned areas). The information is then read by a low-power laser. The pits and lands have different reflectivity, making it possible to distinguish one from the other. A pit-land or land-pit transition represents a 1 bit. The interval between two transitions indicate the number of zeroes between each 1 bit (see the binary page for more information about binary code). CD-ROMs have the advantage to be able to store a larger amount of data compared to floppy disks. However they cannot be written onto (in factories they are pressed using a mold).
To respond to the demand for a writable medium, the WORM (Write Once Read Many) optical disks were developed. These disks can be written onto only once. Once a pit has been burned, it cannot be erased. This then lead to the development CD-RWs which are disks which can be written onto, and then erased and rewritten only a certain amount of times (usually in the hundreds or thousands).
DVD-ROMs have only been developed very recently. They are based on the same principle as the CD-ROM with only a few differences. First of all, the spiral groove is much tighter than on a CD-ROM therefore allowing more information to be stored. Secondly, some DVD-ROMs contain information on both sides of the disk. Thus, DVD-ROMs have the advantage of being able to hold much more information than CD-ROMs. DVD-ROMs also have the disadvantage of not being writable. Therefore, a writable version of the DVD-ROM has been introduced, the DVD-RAM. The DVD-RAM works on the same principle as the CD-RW. [an error occurred while processing this directive]