Hardware Reference
In-Depth Information
termine and adjust the head position. Because of these problems, the wedge servo never
was a popular design; it no longer is used in drives.
Embedded Servo
An embedded servo is an enhancement of the wedge servo. Instead of placing the servo
code before the beginning of each cylinder, an embedded servo design writes the servo
information before the start of each sector. This arrangement enables the positioner cir-
cuits to receive feedback many times in a single revolution, making the head positioning
much faster and more precise. Another advantage is that every track on the drive has its
own positioning information, so each head can quickly and efficiently adjust position to
compensate for any changes in the platter or head dimensions, especially for changes due
to thermal expansion or physical stress.
Most drives today use an embedded servo to control the positioning system. As in the
wedge servo design, the embedded servo information is protected by the drive circuits,
andanywriteoperationsareblockedwhenevertheheadsareabovetheservoinformation.
Thus, it is impossible to overwrite the servo information with a low-level format, as some
people incorrectly believe.
Dedicated Servo
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 sec-
tor. 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 indic-
atesthepropertrackpositions.Becausetheheadthatrestsabovethissurfacecan'tbeused
for normal reading and writing, the gray code can never be erased and the servo inform-
ation is protected—as in the other servo designs. No low-level format or other procedure
can possibly overwrite the servo information.
Figure 9.13
shows a dedicated servo mech-
anism. Typically, the head on top or one in the center is dedicated for servo use.
Figure 9.13
A dedicated servo, showing one entire head/side used for servo reading.
