Magnetic Disks and Optical Disks

There are two major types of magnetic disks: floppy disks and hard disks.
Both types of disks rely on a rotating platter coated with a magnetic surface and use a moveable read/write head to access the disk. Disk storage is nonvolatile, meaning that the data remains even when power is removed. Because the platters in a hard disk are metal (or, recently, glass), they have several significant advantages over floppy disks.

Every user has used hard disks and liked them very much since they have gigantic storage capacity and work fast, especially since operating systems grow larger and larger. One example is Win98, with its full installation needing 300 MB memory, long application programs and multi-media development need more and more storage space, etc. All of these spur the development of hard disks. The hard disk storage capacity almost is doubled every year and the hard disk works faster and faster. The rotative velocity of a main shaft of a motor in a hard disk is working speed of the hard disk. The velocities is now commonly from 5400rpm to 7200rpm. The high turning velocity can reduce average seek times and waiting times. Most of average seek times are less 10ms.

The hard disk capacity develops very fast, it is almost doubled every year. The larger the capacity, the lower the cost of storage per bit. You should select a suitable one according to your economic ability and experience.

To access data, the operating system must direct the disk through a three-stage process. The first step is to position the arm over the proper track. This operation is called a seek, and the time to move the arm to the desired track is called seek time. Once the head has reached the correct track, we must wait for the desired sector to rotate under the read/write head. This time is called the rotation latency or rotational delay. The average latency to the desired information is halfway around the disk. Smaller diameter disks are attractive because they can spin at higher rates without excessive power consumption, thereby reducing rotational latency. The last component of a disk access, transfer time, is the time to transfer a block of bits, typically a sector. This is a function of the transfer size, the rotation speed, and the recording density of a track. Transfer rates in 1992 are typically 2 to 4 MB per second.

An optical disk is a disk on which data are encoded for retrieval by a laser. Optical disks offer information densities far beyond the range of current magnetic mass-storage devices. Similar devices have been on the market for several years in the form of laser videodisks and audio compact disks(CDs) for consumer use. These laser videodisks are analog, that is, the disk contains one spiral track, like the track on a phonograph record. Optical disks for computer applications are digital and store their information on concentric tracks, like their magnetic cousins. Currently, three versions of optical disk technology are competing for the mass-storage market, they are read-only optical disks, write-once optical disks, and erasable optical disks.

Unlike conventional magnetic disks, read-only optical disks cannot be written on and so have the functional equivalence of read-only memory (ROM). The most popular version of read-only optical disks employs the same technology as the CD that has become popular for audio recording. The technology is digital and based on a 4 -~- inch optical disk that can store 540 MB on a single side. The devices are called compact disk read-only memories (CD-ROMs).

Write-once optical disks (also called write-once, read-mostly, or WORM) are blank disks that are recorded on by the user. To write data, a powerful beam of laser light burns tiny spots or pits into the coating that covers the surface of these disks. Once burnt in, the sport are not erasable. TO retrieve the data, a less powerful lair is used to read the pattern of Sports and convert the patterns into audiovisual signals that can be played back on a television set. Write-once optical disks are being used to replace microfilm storage. Because optical disks have the ability to store images as well as, sound, their use is quite versatile. Anything that can be digitized, such as documents, pictures, photographs, line drawings, and music, can be recorded and stored on an optical disk.

Erasable optical disks use lasers to read and write information to and from the disk but also use a magnetic material on the surface of the disk and a magnetic write head to achieve eras ability. To write on such as disk, a laser beam heats a tiny spot on it; then a magnetic field is applied to reverse the magnetic polarity of the spot. Erasable optical disk systems offer the same storage capabilities of the non-erasable optical disks, along with the same reusability capabilities of conventional magnetic disks, such as Winchester systems.
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