Hard Disk Storage - Upgrading and Repairing PCs - page 486

A dedicated servo is a design in which the servo information is written continuously throughout the
entire track, rather than just once per track or at the beginning of each sector. Unfortunately, if this
procedure were used on the entire drive, no room would be left for data. For this reason, a dedicated
servo uses one side of one of the platters exclusively for the servo-positioning information. The term
dedicated comes from the fact that this platter side is completely dedicated to the servo information
and can't contain data.
When building a dedicated servo drive, the manufacturer deducts one side of one platter from normal
read/write usage and records a special set of gray-code data there that indicates the proper track
positions. Because the head that rests above this surface can't be used for normal reading and writing,
the gray code can never be erased and the servo information is protected—as in the other servo
designs. No low-level format or other procedure can possibly overwrite the servo information.
Figure 9.14 shows a dedicated servo mechanism. Typically, the head on top or one in the center is
dedicated for servo use.
Figure 9.14. A dedicated servo, showing one entire head/side used for servo reading.
When the drive moves the heads to a specific cylinder, the internal drive electronics use the signals
received by the servo head to determine the position of the read/write heads. As the heads move, the
track counters are read from the dedicated servo surface. When the servo head detects the requested
track, the actuator stops. The servo electronics then fine-tune the position so the heads are precisely
above the desired cylinder before any writing is permitted. Although only one head is used for servo
tracking, the other heads are attached to the same rack so that if one head is above the desired
cylinder, all the others are as well.
One way of telling whether a drive uses a dedicated servo platter is if it has an odd number of heads.