Storage System

The storage system inside a PC, be it a diskette, a tape storage or a hard disk, consists of three logical parts. First part is the medium itself, that is the diskette, the cartridge tape or the hard disk. The second part is the mechanism that operates this medium—spins the disk or spools the tape. This is called the drive. The various kinds of drive are the floppy disk drive, the tape drive and the hard disk drive. The third part is the electronics, that controls the drive through commands given by the computer system. This is called the controller. In this section, you will learn about the floppy disk and hard disk systems.

Floppy Disk System

Floppy disks come in two sizes: 5¼ inch and 3½ inch. The 5¼ inch diskette is circular in shape with a hole in the center, called the clamping hole. It is covered with a square protective jacket. The jacket has an oval slot, called the head slot, which exposes a portion of the diskette surface. There is also a small hole near the center, called the index hole, which is the starting point of the diskette.

5¼ inch Floppy Disk

The diskette is inserted into the floppy disk drive, which is a sealed enclosure. It is clamped to the drive through the hub. The drive contains a spindle mechanism for rotating the diskette. It also contains another delicate piece of hardware called the read-write head that accesses the disk surface through the head slot and retrieves or stores data. The drive uses the index hole to sense the beginning of a track. The drive controller card supplies the drive with the necessary signals for its operations. For example, the controller tells the drive when to move the read-write head, when to start rotating the disk, and so on.

A 3½ inch diskette, also called as the microdisk, is also circular in shape and is covered with a square protective jacket. The media cover is pushed to expose the diskette surface. The operations are similar to those of 5¼ inch diskettes. A 3½ inch diskette is shown in Figure.

A 3½ Inch Diskette

A diskette is divided into a series of concentric circles called tracks. The tracks are identified by numbers, with the outermost track as track zero. The tracks do not spread across the whole width of the recording surface, but take up a small area. The track is divided into sectors. Each sector is identified by a number that is assigned to it, starting with zero. Each type of floppy has a sector of a fixed size. A disk has two sides. Data can be recorded on either one or both sides of the disk.

The storage capacity of a disk is determined by factors like the number of tracks, sectors and the number of sides on which data is recorded. Depending upon the capacity of storage, the 5¼ inch diskettes are divided into double-density (360 KB) and high-density (1.2 MB).

The microdisk may be double-density (720 KB) or high-density (1.44 MB). Accordingly, disk drives can be double-density or high-density drives. A high-density drive can read and write on to a double-density diskette but the reverse is not possible.

Hard Disk System

The hard disk is made up of a collection of disks known as platters. It is sealed in a vacuum container, and is located inside the system unit. When the tracks having the same track number on the different platters of a hard disk are joined together, they form a cylinder.

The read-write mechanism in a hard disk is slightly different from that of a floppy disk. Here, the read-write arm (containing twice as many read-write heads as there are platters, since each platter has two recording surfaces) is positioned over the recording surface and the disk is rotated. The read-write head then picks up the necessary data, which is transported to the RAM through the data bus. The hard disk controller performs functions similar to that of a floppy disk controller.