PC Hardware

Hard Drive Geometry

Hard disk drives are composed of one or more disks or platters on which data is stored. The geometry of a hard drive is the organization of data on these platters. Geometry determines how and where data is stored on the surface of each platter, and thus the maximum storage capacity of the drive. There are five numerical values that describe geometry:

  • Heads

  • Cylinders

  • Sectors per track

  • Write precompensation

  • Landing zone

Write precompensation and landing zone are obsolete, but often seen on older drives. Let's take a look at each of these components.

All hard disk drives have geometry. Knowledge of the geometry is required to install or reinstall a hard drive.


The number of heads is relative to the total number of sides of all the platters used to store data (see Figure 8.5). If a hard disk drive has four platters, it can have up to eight heads. The maximum number of heads is limited by BIOS to 16.

Figure 8.5 Drive heads

Hard disk drives that control the actuator arms using voice coil motors reserve a head or two for accuracy of the arm position. Therefore, it is not uncommon for a hard disk drive to have an odd number of heads.

Some hard disk drive manufacturers use a technology called sector translation. This allows some hard drives to have more than two heads per platter. It is possible for a drive to have up to 12 heads but only one platter. Regardless of the methods used to manufacture a hard drive, the maximum number of heads a hard drive can contain is 16.


Data is stored in circular paths on the surface of each head. Each path is called a track. There are hundreds of tracks on the surface of each head. A set of tracks (all of the same diameter) through each head is called a cylinder (see Figure 8.6). The number of cylinders is a measurement of drive geometry; the number of tracks is not a measurement of drive geometry. BIOS limitations set the maximum number of cylinders at 1024.

Figure 8.6 Cylinders

Sectors per Track

A hard disk drive is cut (figuratively) into tens of thousands of small arcs, like a pie. Each arc is called a sector and holds 512 bytes of data. A sector is shown in Figure 8.7. The number of sectors is not important and is not part of the geometry; the important value is the number of sectors per track. BIOS limitations set the number of sectors per track at 63.

Figure 8.7 Sector

Write Precompensation

All sectors store the same number of bytes: 512; however, the sectors toward the outside of the platter are physically longer than those closer to the center. Early drives experienced difficulty with the varying physical sizes of the sectors. Therefore, a method of compensation was needed-the write precompensation value defines the cylinder where write precompensation begins.

The write precompensation value is now obsolete, but is often seen on older drives.

Landing Zone

A landing zone defines an unused cylinder as a "parking place" for the R/W heads. This is found in older hard disk drives that use stepper motors. It is important to park the heads on these drives to avoid accidental damage when moving hard disk drives.

CHS Values

Cylinders, heads, and sectors per track (see Figure 8.8) are known collectively as the CHS values. The capacity of any hard disk drive can be determined from these three values.

Figure 8.8 Cylinders, heads, sectors per track

The maximum CHS values are:

  • 1024 cylinders.

  • 16 heads.

  • 63 sectors per track.

  • 512 bytes per sector.

Therefore, the largest hard disk drive size recognized directly by the BIOS is 504 MB. Larger drive sizes can be attained by using either hardware or software translation that manages access to the expanded capacity without direct control by the system BIOS.

1024 x 16 x 63 x 512 bytes/sector = 528,482,304 bytes (528 million bytes or 504 MB)

There are many hard disk drives that are larger than 504 MB. These drives manage to exceed this limitation in one of two ways: either they bypass the system BIOS (by using one of their own) or they change the way the system BIOS routines are read. (For a fuller discussion of this, refer to Tutorial 9, "High-Capacity Disk Drives.")