Hardware Reference
In-Depth Information
•
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) information 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 guidance)
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 capacity 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 make the
formatting more efficient by minimizing the amount of overhead so that as much of each sector as
possible can be used for storing user data. The ratio of the user data in a sector to the total number of
bytes physically used by the sector on the disk denotes the sector efficiency. The way tracks and
sectors have been formatted over the years has allowed efficiency to improve, but only up to a point.
The sector format on a given drive is dictated by the controller, which was originally a separate
entity from the drive. One of the first hard disk controllers used in PCs was the Xebec 1210, the HDD
controller used in the vintage 1983-1987 IBM XT.
Table 9.2
shows the sector format created by the
Xebec 1210/1220 controllers. (The 1220 had an integrated floppy controller.)
Table 9.2. Xebec 1210/1220 (IBM XT) 512-Byte Sector Format
As you can see from
Table 9.2
,
drives formatted using those controllers had sectors that stored 512
bytes of user data yet consumed 605 total bytes on the disk, for a sector efficiency of 84.6%. Later
controllers were able to increase format efficiency by reducing the size of the gap and other
information in the header. For example,
Table 9.3
shows the sector format created by the Western
Digital WD1003 and WD1006 controllers used in the vintage 1984-1987 IBM AT.
Table 9.3. Western Digital WD1003/WD1006 (IBM AT) 512-Byte Sector Format
