Tapes are an example of sequential-access memory technology; an 1
example of random-access or direct-access secondary memory devices is the magnetic disk. It provides a large amount of storage and rapid retrieval of any stored information. All disks are made of a substance coated with metal oxide, and can therefore be magnetized. 5
 Magnetic disks are of two kinds, namely floppyand hard.The hard disks, in turn, are subdivided into fixed-headand moving-headdisks which are either cartridgeor pack.Floppy disks, or diskettesas they are called, are made from plastic, which makes them very light, flexible, and quite inexpensive, whereas hard disks are made from a rigid 10
which are similar to the grooves in a record. Information is stored on a track in magnetized spots called bits. These bits are similar to the bits in internal memory and are situated on the track such that usually every 20
eight of them make up one byte.
 To access information from a cartridge, it is mounted on a disk drive which is equipped with two recording heads,one for each side of the disk. The heads move radially along a line from the center to the outside from track to track. To access information from a disk pack, the 25
recording heads are moved back and forth in the space between the platters by the access arms to which they are attached.
A disk cartridge is made of a circular disk called a platter,about the same size as a long-playing record, which can be magnetized on both sides. When a number of these circular platters are stacked one on top of the other, they are called a disk pack. How many platters there are in 15 a disk pack varies depending on the manufacturer and the model.
 The recording surface of a disk has concentric circles called tracks,
Figure 15.2 Disk pack
A stack of tracks is called a cylinderand it is accessed by all the recording heads acting at once. The recording capacity of a disk pack is measured in terms of a number of cylinders, the number of tracks, and 30 the amount of data in each track.
 Information on a disk is organized in terms of blocks, each having its own address, which consists of a cylinder number, a track number, and arecord number. To access directly the necessary information, the recording heads first seek the required cylinder, then search to find the 35 beginning of the required record, and then transfer the information to the memory of the computer or to another form of storage, all of which is done in a few milliseconds.
 Dust and dirt cause the recording condition of disks to deteriorate. As a
result, data packs,which are disks with the recording heads sealed 40
inside, were developed. They are more expensive than the normal disk packs but the drives on which they are mounted are cheaper than the normal disk drives.
 Disk drives are of two kinds: drives with a single non-removable platter, and drives in which disks can be changed. The latter kind is further 45
subdivided into top-loading single platter, front-loading single platter, and top-loading multiple platter. Some disk drives open from the top, where single platter disks are placed. Other drives open in the front and single platter disks, either hard disks or diskettes, are inserted. For very long storage, the top-loading multiple platter drives are used. After 50
being mounted on a disk drive, disks are kept spinning at a very high and constant speed, thus allowing the recording heads to have direct access to the required information. For example, the pack on the IBM 3330 spins at 60 revolutions per second.