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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Geometry and Low-Level Data Structures | Hard Disk Data Geometry Specifications and Translation ]

Physical Geometry

The physical geometry of a hard disk is the actual physical number of heads, cylinders and sectors used by the disk. On older disks this is the only type of geometry that is ever used--the physical geometry and the geometry used by the PC are one and the same. The original setup parameters in the system BIOS are designed to support the geometries of these older drives. Classically, there are three figures that describe the geometry of a drive: the number of cylinders on the drive ("C"), the number of heads on the drive ("H") and the number of sectors per track ("S"). Together they comprise the "CHS" method of addressing the hard disk. This method of description is described in more detail in this description of CHS mode addressing.

At the time the PC BIOS interfaces to the hard disk were designed, hard disks were simple. They had only a few hundred cylinders, a few heads and all had the same number of sectors in each track. Today's drives do not have simple geometries; they use zoned bit recording and therefore do not have the same number of sectors for each track, and they use defect mapping to remove bad sectors from use. As a result, their geometry can no longer be described using simple "CHS" terms. These drives must be accessed using logical geometry figures, with the physical geometry hidden behind routines inside the drive controller. For a comparison of physical and logical geometry, see this page on logical geometry.

Often, you have to request detailed specifications for a modern drive to find out the true physical geometry. Even then you might have problems--I called one major drive manufacturer when first writing the site, and the technician had no idea what I was talking about. He kept giving me the logical parameters and insisting they were the physical ones. Finally, I asked him how his drive could have 16 heads when it had only 3 platters, and he got very confused. :^)

Tip: It's easy to tell if you are looking at physical or logical hard disk geometry numbers. Since no current hard drive has the same number of sectors on each track, if you are given a single number for "sectors per track", that must be a logical parameter. Also, I am aware of no current hard disk product that uses 8 platters and either 15 or 16 heads. However, all modern, larger IDE/ATA hard disks have a nominal logical geometry specification of 15 or 16 heads, so either of those numbers is a dead giveaway.

Next: Logical Geometry

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