Peripherals: Floppy disks and Hard disks

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Floppy disks

There are two types of magnetic disk: hard disks (also called Winchester disks), and the smaller floppy disks (also called diskettes). Figure 3.2 shows the standard 5.25-inch diameter floppy disk, and as you can see it is enclosed in a protective flexible casing with an opening cut out to give the read/write head access to the magnetic surface. This type of disk is extremely light, and the disk-drive is able to bring it up to the required speed of rotation almost instantly. The disk­ drive's motor only needs to be switched on by the computer when files are actually being accessed; the rest of the time the disk remains stationary in the drive.

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A disk must be formatted, i.e. magnetically configured to run on the particular computer system, before it can be used. This process marks out sectors on the surface within which the data is stored. In the case of PCs of the IBM­ compatible variety, each track on the disk is divided into nine sectors, and each sector is able to store 512 bytes of data. There are 40 tracks on each side of a standard (low­ density) floppy disk, giving 40 x 512 = 184,320 bytes for one side of the disk, i.e. 180K.

In the past, cheaper disk-drives had only one read/write head, and so were able to access only one side of the disk. Modern drives are able to access both sides of the disk, so 360K can be stored on a standard 5.25 inch floppy formatted for PCs. High-density drives on PCs allow you to store 1.2 Mbytes on two sides of a disk, though the more expensive high-density disk must be used in this case.

The smaller 3.5 inch disks are now becoming standard on personal computers (they have been used for many years on pace-setting computers such as the Apple Macintosh and the Atari ST). These disks are enclosed in a rigid plastic casing, and for added protection the read/write opening in the case is enclosed by a metal slider when the disk is removed from the drive. Despite their smaller size, these disks can store much more data than their larger 5.25 inch brethren, namely 720K in the case of double-sided standard density (i.e. low density) disks, and 1.44 Mbytes for high­ density disks. 3.5 inch disks with capacities of 5 Mbytes or more are becoming available, and no doubt these will become the standard in the future.

Hard disks

Hard disks are so-called because, unlike floppies, they are of a stm;dy, rigid construction. They are also much heavier than a floppy, and a hard disk-drive can take several seconds to get up to speed. Hard disks are therefore kept constantly spinning (while the computer is turned on), even when data is not being accessed.

The speed of rotation is very high, 3600 rev/min compared to 360 rev/min for floppy disks. At this speed, the movement of the air molecules adjacent to the disk's surface lifts the read/write head sufficiently to prevent it touching the sur­ face. It can still access the data, but it does not cause any wear on the disk. The disk and the read/write head are enclosed in an airtight casing to prevent dust particles adhering to the disk's surface and interfering with the read/ write process.

Hard disks have two advantages over floppies:

• They can store much more data - typical hard disks for microcomputers have capacities of 40, 80, or 120 Mbytes.

• They can access data 10 times faster than is possible with floppy disks.

Most personal computers nowadays have hard disks inside their casing. This saves fiddling with floppy disks whenever you want to load a program or load or save a file of data or text, and it greatly speeds up the loading of programs and data. Besides this, very large files, which may be too big to be held on a single floppy, will fit without difficulty onto the hard disk.

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