Hardware Reference
In-Depth Information
miles. At 4,958 miles per second, this imaginary skyscraper-sized head would circle the earth once
every 5 seconds (at only two-tenths of an inch over the surface)! It would also read 231.33MB in one
lap around this equatorial track.
There is also sideways velocity to consider. Because the average seek time of 8.5 milliseconds is
defined as the time it takes to move the heads over one-third of the total tracks (about 9,241 tracks in
this example), the heads could move sideways within a scale distance of 1.71 miles in that short time.
This results in a scale-seek velocity of more than 726,321 mph, or 202 miles per second!
This analogy should give you a new appreciation of the technological marvel that the modern HDD
actually represents. It makes the old Jumbo Jet analogy look rather pathetic (not to mention grossly
inaccurate), doesn't it?
Tracks and Sectors
A
track
is a single ring of data on one side of a disk. A disk track is too large to manage data
effectively as a single storage unit. Individual disk tracks can store more than a megabyte of data,
which would be inefficient for storing small files. For that reason, tracks are divided into several
numbered divisions known as
sectors
. These sectors represent arc-shaped pieces of the track.
Various types of disk drives split their disk tracks into different numbers of sectors, depending on the
density of the tracks. For example, floppy disk formats use 8-36 sectors per track, although hard disks
usually store data at a higher density and today can have up to 2,000 or more physical sectors per
track.
The sectors on a track are numbered starting with 1, unlike the heads or cylinders that are numbered
starting with 0. For example, a 1.44MB floppy disk contains 80 cylinders numbered 0-79 and two
heads numbered 0 and 1, whereas each track on each cylinder has 18 sectors numbered 1-18.
Each physical sector stores 512 or 4,096 bytes of data; however, additional bytes of storage are used
in sector headers and trailers to manage the tracks and sectors. You might find it helpful to think of
each disk sector as being like a page in a book. Each page contains text (data), but the entire page is
not filled with text; rather, each page has margins. Information such as chapter titles and page numbers
are placed in the margins, and the margins provide “gaps” between text that allow you to visually
separate pages of text. These “margins” (headers and trailers) are created during the low-level
formatting process, 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 manufacturer. 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 normal data pattern would not show. This way, the manufacturer can more
accurately identify 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 timing 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
