ORGANIZING DATA ON DISKS

Figure 10-1.—A magnetic disk pack. several gigabytes. An example of a disk pack is shown in figure 10-1. Disk cartridges are another form of disk pack with the heads and head actuator assemblies contained within a sealed cartridge. Since the disk pack is never removed from the cartridge, disk cartridges suffer less contamination problems from dust and dirt than standard disk packs. FIXED DISKS.— Fixed disks are small sealed units that contain one or more disk platters. Fixed disks are known by several terms, such as Winchester drive, hard drive, or fixed disk. For clarity, we refer to them as fixed disks throughout this chapter. Fixed disks are used in minicomputers and personal computers. They can also be adapted for use in mainframe computers instead of having separate disk file units. Floppy Disks Floppy disks come in several sizes and densities. They are called floppy disks because the magnetic coating is placed on a thin flexible polyester film base. THE 8-INCH FLOPPY DISK.— The 8-inch floppy disk was the first disk widely used for commercial purposes. It is available as both single- or double-sided and single- or double density. The 8-inch disk is quickly becoming obsolete. THE 5.25-INCH FLOPPY DISK.— The 5.25-inch floppy disks are used with both personal computers and minicomputers. The standard double- sided, double-density disk has a capacity of 360 kilobytes (K). Quad-density disks hold 720K, while the newest high-density disks can hold 1.2 megabytes (M). THE 3.5-INCH FLOPPY DISK.— The current disk of choice is the 3.5-inch floppy disk. These disks are also used with personal computers and minicomputers. These smaller disks have data capacities of 720K for double-density disks and 1.44M for high- density disks. ORGANIZING DATA ON DISKS Before data can be stored on a magnetic disk, the disk must first be divided into numbered areas so the data can be easily retrieved. Dividing the disk so the data can be easily written and retrieved is known as formatting the disk. The format program divides each data surface into tracks and sectors. Tracks — Concentric rings, called tracks, are written on the disk during the formatting process. Floppy disks have 40 or 80 tracks per side. Fixed disks and disk packs can have from 300 to over 1,000 tracks per side. Figure 10-2 shows an example of how tracks are written on a disk surface. Each track is assigned a number. The outermost track on a disk is assigned number 00. The innermost track is assigned the highest consecutive number. Sectors — Each track is divided into sectors. Sectors are numbered divisions of the tracks designed to make data storage more manageable. Without sectors, each track would hold more than 4,500 bytes of information and small files would use an entire track. Figure 10-2.—Tracks on a segment of a magnetic disk. 10-3