How a Hard Disk Drive Works
(By Dux Computer Digest)

Servo-Formatting

SERVO-FORMATTING.  Try to visualize a thin, hollow cylinder passing through all of the platters in a hard disk drive.  It would produce a circular track on each side of each platter.  Now divide each tack into equal arcs or sectors.  Well, that is exactly how a hard disk is organized.  That is, Cylinders, Heads (which are equal the number of tracks/cylinder or platter sides), and Sectors are the coordinates of the data on a hard disk drive.

 

There are two kinds of sectors on a hard disk.  The first and at the very lowest level is the servo sector.  When a hard disk is manufactured a special pattern is written in a code called a Gray code on the surface of the platters, while the drive is open in a clean room, with an expensive machine called a servowriter.

000
001
011
010
110
111
101
100

A Gray Code is a binary code in which successive numbers differ by only a single bit.  Although many Gray Codes are possible, one specific Gray Code is considered the Gray Code because of its efficiency in computation.  This efficiency is why it is used for the servo pattern instead of other binary codes.  A three-bit Gray Code is shown to the right.

Although there are other schemes,  the Gray Code is written in a wedge at the start of each sector (an embedded servo pattern) on most drives.  There are a fixed number of servo sectors per track and the sectors are adjacent to one another.  This pattern is permanent and cannot be changed by writing normal data to the drive. It also cannot be changed by low-level formatting (see below) the drive, as some may think.  If it is changed, the drive has had it--kaput!

The electronics use feedback from the heads, which read the Gray code pattern, to very accurately position, and constantly correct the radial position of the appropriate head over the desired track, at the beginning of each sector, to compensate for variations in platter geometry, caused by mechanical stress and thermo expansion and contraction.   Altogether, the head positioning components form what is know as closed-loop servo system--a marvelous (and, perhaps, dangerous) thing to watch operate in a drive which has been opened.

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