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Lacking the discrete steps of the stepper motor drive, a voice coil drive cannot accurately predict the movement of the heads across the disk. To make sure voice coil drives land exactly in the correct area, the drive reserves one side of one platter for navigational purposes. This area essentially maps the exact location of the data on the drive. The voice coil moves the read/write head to its best guess about the correct position on the hard drive. The read/write head then uses this map to fine-tune its true position and make any necessary adjustments.

Now that you have a basic understanding of how a drive physically stores data, let’s turn to how the hard drive organizes that data so we can use that drive.

Geometry

Have you ever seen a cassette tape? If you look at the actual brown Mylar (a type of plastic) tape, nothing will tell you whether sound is recorded on that tape. Assuming the tape is not blank, however, you know something is on the tape. Cassettes store music in distinct magnetized lines. You could say that the physical placement of those lines of magnetism is the tape’s “geometry.”

Geometry also determines where a hard drive stores data. As with a cassette tape, if you opened up a hard drive, you would not see the geometry. But rest assured that the drive has geometry; in fact, every model of hard drive uses a different geometry. We describe the geometry for a particular hard drive with a set of numbers representing three values: heads, cylinders, and sectors per track.

Heads The number of heads for a specific hard drive describes, rather logically, the number of read/write heads used by the drive to store data. Every platter requires two heads. If a hard drive has four platters, for example, it needs eight heads (see Figure 11-3).

Based on this description of heads, you would think that hard drives would always have an even number of heads, right? Wrong! Most hard drives reserve a head or two for their own use. Therefore, a hard drive can have either an even or an odd number of heads. Four platters = eight heads

Figure 11-3 Two heads per platter

Cylinders To visualize cylinders, imagine taking an empty soup can and opening both ends. Look at the shape of the can; it is a geometric shape called a cylinder. Now imagine taking that cylinder and sharpening one end so that it easily cuts through the hardest metal. Visualize placing the ex-soup can over the hard drive and pushing it down through the drive. The can cuts into one side and out the other of each platter. Each circle transcribed by the can is where you store data on the drive, and is called a track.

Figure 11-4 Cylinder

Each side of each platter contains tens of thousands of tracks. Interestingly enough, the individual tracks themselves are not directly part of the drive geometry. Our interest lies only in the groups of tracks of the same diameter, going all of the way through the drive. Each group of tracks of the same diameter is a called a cylinder (see Figure 11-4). There’s more than one cylinder. Go get yourself about a thousand more cans, each one a different diameter, and push them through the hard drive.

Sectors per Track Now imagine cutting the hard drive like a birthday cake, slicing all of the tracks into tens of thousands of small slivers. Each sliver then has many thousands of small pieces of track. The term sector refers to a specific piece of track on a sliver, and each sector stores 512 bytes of data.

The sector is the universal atom of all hard drives. You can’t divide data into anything smaller than a sector. Although sectors are important, the number of sectors is not a geometry value that describes a hard drive. The geometry value is called sectors per track (sectors/track). The sectors/track value describes the number of sectors in each track (see Figure 11-5).

Figure 11-5 Sectors per track Six sectors per track (sectors/track)

The Big Three Cylinders, heads, and sectors/track combine to define the hard drive’s