Have you ever questioned how data is physically stored onto a disk? Floppy disks, no matter what density or size, store data in much the same way. All disks when first manufactured have nothing written on them; they are just blank sheets of magnetically coated plastic. Before data can be written on the disk, the disk must first be mapped out of concentric circles called tracks, and in pie-shaped wedges called sectors. Much like the many rings in the center of a tree, these tracks are what hold the memory. The process of preparing the disk to receive data is referred to as formatting the disk. For example, lets say we have a hard disk with eighty tracks on the topside of the disk and eighty more tracks on the bottom side of the disk. The tracks are numbered one through seventy-nine. Each side of the disk has nine sectors, numbered one through nine. Even though the tracks on the outside of the disk are larger than the tracks closer to the center, each stores the same amount of data.

Data is written to the tracks of the disk as bits, either a zero or a one. Each bit is a magnetized spot on the disk that is shaped roughly like a rectangle. Between the tracks there is a space that is not magnetized. This spacing prevents one spot from affecting the magnetism of a nearby spot. The difference between a zero spot and a one 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. Two heads are attached at the end of an actuator arm that freely moves back and forth over the surface of the disk. The arm has one read/write head above the disk and another read/write head 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 360rpm depending on the type of disk. Data is written first to the bottom and then the top of the disk beginning at the outermost track and going 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 too far from the track. All the tracks are the same width, the distance between them being uniform.

The disk itself is actually a piece of Mylar, similar to that used for overhead transparencies. Covering the surface of the Mylar is a layer of either cobalt oxide or iron oxide (rust) that can hold a magnetic charge. Some disks have another layer of Teflon on top to protect the oxide layer and to allow the read/write heads to move more smoothly over the surface. During formatting, the tracks are created by laying down a repeating character, the division symbol, which is F6 in ASCII code or 1111 0110 in binary. The tracks are divided into sectors, and the beginning sector is marked on each track with a designated code. For three and a half inch disks, the sector address mark written on the disk during the formatting marks the beginning of a sector; after formatting, actual data is written on the disk by overwriting the F6 patterns on the tracks.

The different disk types use varying degrees of magnetic strength when data is written to a disk or when a disk is formatted. For example, a three and a half inch high-density disk can hold more data than a double-density disk, because the data is written closer together. Data on the higher density disk is recorded at about twice the magnetic strength as data on the double-density disk. The high-density disk surface is not as sensitive to a magnetic field as the double-density disk. The high-density disk surface is not as sensitive to a magnetic field as the double-density is and can, therefore, handle double the strength of the fields.

Many users have discovered that the less expensive double-density disks can be formatted as high-density, and the format will work and data can be written to the disk. Beware! Don’t trust that disk with important data. The surface of a double-density disk is more sensitive to the magnetic field, and eventually the magnetic spots on the disk will affect each other, corrupting the data. The life span of an incorrectly formatted disk is very short. For this reason, always format a disk using the density for which it was manufactured.

When data is read from the disk surface, the read/write head changes roles. It passes over a track on the disk, waiting for the right position on the disk to appear. When the correct sector arrives, the controller board opens a gateway, and the magnetic charge on the disk passes voltage to the read/write head. The voltage is immediately amplified and passed on to the controller board, which in turn passes the data to the system bus.

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