Hard Disk Storage - Upgrading and Repairing PCs - page 566

These “margins” (headers and trailers) are created during the low-level formatting pro-
cess, which for modern drives can only be done at the factory.
Each physical sector contains 512 or 4,096 bytes of data. The low-level (and sometimes
the high-level) formatting process fills the sector data fields with some specific value or
values, such as F6h (hex), 00h, or some repeating test pattern used by the drive manu-
facturer. Certain bit patterns are more difficult for the electronics on the drive to encode/
decode, so these patterns are sometimes used when the manufacturer is testing the drive
during initial formatting. A special test pattern might cause errors to surface that a nor-
maldatapatternwouldnotshow.Thisway,themanufacturercanmoreaccuratelyidentify
marginal sectors during testing.
Each physical sector consists of a number of fields, including the following:
• Gap —A break or slack space in the recording that separates sectors allowing for tim-
ing and rotational speed variations
• ID —Information identifying the physical sector location address and status
• Sync —A timing mark for providing disk controller to drive timing alignment
• Address Mark —A mark indicating where the data field actually begins
• Data —The actual 512 or 4,096 bytes of user data being stored
• ECC —Error correction code, a number calculated via a complex polynomial that can
detect and possibly even correct errors in the data being read
Additional space for each track is normally used for embedded servo (head guidance) in-
formation as well as track index marks and additional gaps for timing purposes. The gaps
are like spacings in the recording and serve basically the same function as having gaps
of no sound between the songs recorded on a CD or tape, helping the player to identify
where one song ends and another begins.
The sector headers and trailers along with additional overhead such as servo (head guid-
ance) information are all created during the low-level format process, which for ATA
drives can only be performed at the factory. This overhead accounts for the lost space
between the unformatted capacity of a disk and the formatted capacity. For example, a
2MB (unformatted) 3 1/2-inch floppy disk has a formatted capacity of 1.44MB, and an
older 38MB unformatted capacity (for instance, a Seagate ST-4038) hard disk has a ca-
pacity of only 32MB when it is formatted. Because the hard drives you purchase today
can only be low-level formatted at the factory, the manufacturers now advertise only the
formatted capacity.
In the quest to create higher and higher capacity drives, drive manufacturers have tried to
maketheformattingmoreefficientbyminimizingtheamountofoverheadsothatasmuch