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
