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Unit 8: Disk and disk drive 63 - Information on a disk is organized in terms of blocks.Thông tin trên điã đ c t ch c thành các kh i. - Recording heads are capable of transferring information from a disk to the memory of the computer. Đ u ghi có kh năng chuy n thông tin t đĩa đ n b nh c a máy tính. Bài khoá: Tapes are an example of sequential-access memory technology; an 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 substance coated with metal oxide, and can therefore be magnetized. Magnetic disks are of two kinds, namely floppy and hard. The hard disks, in turn, are subdivided into fixed-head and moving-head disks which are either cartridge or pack. Floppy disks, or diskettes as they are called, are made from plastic, which makes them very light, flexible, are quite inexpensive, whereas hard disks are made from a rigid material. 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 a disk pack varies depending on the manufacturer and the model. The recording surface of a disk has concentric circles called tracks, 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 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 recording heads are moved back and forth in the space between the platters by the access arms to which they are attached. A stack of track is called a cylinder and 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 the amount of data in each track.