How Data Is Physically Stored On A Disk

How Data Is Physically Stored On A Disk

Years ago, floppy drives came in two sizes: 5¼ inches and 3½ inches. Today, new computers are equipped with only 3½-inch drives. However, because some computers still have 5¼-inch floppy disk drives, this section refers to 5¼ -inch disks as well.

Although they are larger, 5¼ -inch disks do not hold as much data as 3½ -inch disks because they do not store data as densely.

The table below summarizes the capacity of the four common types of floppy disks. The 5¼ -inch double-density disks and the 3½ -inch extra-high-density disks are hardly ever seen; most disks today are 3½ -inch high-density and hold 1.44 MB of data.

3½-inch extra-high-density 2.88 MB = 80 X 36 X 2 sectors

3½-inch high-density 1.44 MB = 80 X 18 X 1 sector

3½-inch double-density 720K = 80 X 9 X 2 sectors

5¼-inch high-density 1.2 MB = 80 X 15 X 1 sector

5¼-inch double-density 360K = 40 X 9 X 2 sectors

Floppy disks, no matter what density or size, store data in much the same way. When first manufactured, disks have nothing written on them; they are blank sheets of magnetically coated plastic.

Before data can be written on the disk, it must first be mapped in concentric circles called tracks, and in pie-shaped wedges called sectors (see the following figure). This process of preparing the disk to receive data is called formatting the disk.

The figure below shows a formatted 3½ -inch double-density floppy disk. According to the table above, there are 80 tracks or circles on the top side of the disk and 80 more tracks on the bottom. The tracks are numbered 0 through 79. Each side of the disk has 9 sectors, numbered 1 through 9. Although the circles or tracks on the outside of the disk are larger than the circles closer to the center, all tracks store the same amount of data.

Data is written to the tracks as bits, either a 0 or 1. Each bit is a magnetized, rectangular-shaped spot on the disk. Between the tracks and spots are spaces that are not magnetized. This spacing prevents one spot from affecting the magnetism of a nearby spot. The difference between a 0 spot and a 1 spot is the orientation of the magnetization of the spot on the disk surface.

Data is written to and read from the disk via a magnetic read/write head mechanism in the floppy drive (see the following figure). Two heads are attached at the end of an actuator arm that freely moves over the surface of the disk. The arm has one read/write head above the disk and another below the disk. Moving in unison back and forth across the disk, the two heads lightly touch the surface of the disk, which is spinning at either 300 rpm (revolutions per minute) or 360 rpm, depending on the type of disk. (Note that the read/write heads of a hard drive never touch the surface.)

Data is written first to the bottom and then to the top of the disk, beginning at the outermost circle and moving in. Eraser heads on either side of the read/write head ensure that the widths of the data tracks do not vary. As the data is written, the eraser heads immediately behind and to the sides of the write head clean up both sides of the magnetized spot, making a clean track of data with no “bleeding” from the track. The magnetized area does not spread far from the track. All tracks are then the same width, and the distance between tracks is uniform.