How Data is Organized on a Hard
Disk Drive
The surface of the drive platter is organized
with coordinates, much like a map. Data is
stored in concentric tracks on the surfaces of
each platter. (A platter has two sides, and
thus, two data recording surfaces.) A typical
disk drive can have more than 2,000 tracks
per inch (TPI) on its recording surface. A
cylinder describes the group of all tracks
located at a given head position across all
platters. To allow for easier access to data,
each track is divided into individually
addressable sectors.
The process of organizing the disk surface
into tracks and sectors is called formatting,
and almost all hard disk drives today come
preformatted by the manufacturer. The
process of formatting a hard drive applies
addressing data to the platter's surface. In
almost all systems, including PCs and
Macintoshes, sectors typically contain 512
bytes of user data plus addressing information used by the drive electronics (although
some proprietary systems use other sector lengths). The disk drive controller, which
resides on the drive's PCB, uses the formatting information and addresses - much like a
tourist uses a city map - to guide data into and out of a specific location on the hard drive.
Without formatting instructions, neither the controller nor the operating system would
know where to store data or how to retrieve it.
In earlier hard drive designs, the number of sectors per track was fixed and, because the
outer tracks on a platter have a larger circumference than the inner tracks, space on the
outer tracks was wasted. The number of sectors that would fit on the innermost track
constrained the number of sectors per track for the entire platter. However, many of
today's advanced drives use a formatting technique called Multiple Zone Recording to
pack more data onto the surface of the disk. Multiple Zone Recording allows the number
of sectors per track to be adjusted so more sectors are stored on the larger, outer tracks.
By dividing the outer tracks into more sectors, data can be packed uniformly throughout
the surface of a platter, disk surface is used more efficiently, and higher capacities can be
achieved with fewer platters. The number of sectors per track on a typical 3.5-inch disk
ranges from 60 to 120 under a Multiple Zone Recording scheme. Not only is effective
storage capacity increased by as much as 25 percent with Multiple Zone Recording, but
the disk-to-buffer transfer rate also is boosted. With more bytes per track, data in the
outer zones is read at a faster rate. Quantum Corporation is a pioneer in Multiple Zone
Recording, and was the first manufacturer to implement Multiple Zone Recording on 2.5-
inch disk drive products.
Read/Write Heads:Skimming the Surface
Read/write heads are the single most costly component of a hard disk drive, and their
characteristics have a great impact on drive design and performance. Despite their
expense, the head's basic design and objective are relatively simple: a head is a piece of
magnetic material, formed almost in the shape of a "C" with a small opening or gap. A
coil of wire is wound around this core to construct an electromagnet. In writing to the
disk, current flowing through the coil creates a magnetic field across the gap that
magnetizes the disk coating layer under the head. In reading from the disk, the read/write
head senses an electronic current pulse through the coil when the gap passes over a flux
reversal on the disk.
As technology increases, bits are packed more densely, and the space required to store a
bit shrinks. At the same time, the tiny size of the stored data bit causes the signal
produced by the head when reading the bit to become weaker and harder to read. As a
result, the fundamental challenge in packing bits closer together is finding a way to fly
the heads closer to the media to increase the amplitude of the signal. The hard disk drive
industry has made great strides on this front. In 1973, flying heights averaged 17
microinches. Today's heads fly at just three microinches, with 2- to 2.5-microinch flying
heights expected soon. And, in the not too distant future, read/write heads might even
make contact with the media, enabling data to be packed even more densely on the platter
surface but offering the additional challenge of eliminating added wear on the disk media
and read/write heads. (For more information, see "Alternative Disk Media and Contact
Recording" in chapter 4.)
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Areal Density: The Measure of Disk Capacity
The overall capacity of a drive depends on how densely information (i.e., bits) can be
recorded on the disk media. The bits per square inch (BPSI) number is called the areal
density. Areal density is calculated by taking the number of bits per inch (BPI) that can
be written to and read from each track, and multiplying that number by the number of
tracks per inch (TPI) that can be packed onto the disk. BPI depends on the read/write
head, recording media, disk rpm, and the speed at which the electronics can accept bits.
TPI depends on the read/write head, recording media, the mechanical accuracy with
which the head can be positioned on its actuator arm, and the ability of the disk to spin in
a perfect circle. Increasing areal density can come by increasing either or both of these
factors.
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Storing and Retrieving Data: The User's View
Let's take a look at what happens when you retrieve data from a hard disk drive. When
you issue a command to open an existing file, the application program you're running
prompts you to enter the name of the file to open. It then passes the file name to the
operating system, which determines where the file is located on the disk drive - the head
number, cylinder, and sector identification. The operating system transfers this
information to the disk controller, which drives an actuator motor connected to the
actuator arm to position the heads over the right track. As the disk rotates, the appropriate
head reads the address of each sector on the track. When the desired sector appears under
the read/write head, the entire contents of the sector containing the necessary data are
read into a special, ultra-fast memory, called cache, on the drive's PCB. Then, the disk
drive interface chip sends the necessary information to the computer's main memory.
(For more information on hard drive cache memory, see "Improving Performance with a
Cache Buffer" in chapter 5. For more on how hard drives and the host computers in
which they reside communicate, see "Making the Connection: Buses and Hard Disk
Drive Interfaces" later in this chapter.) Storing data on a hard drive is a similar process to
retrieving data, only reversed. The host computer operating system is responsible for
remembering the addresses for each file on the disk and which sectors are available for
new data. If the file you want to store is large - for example, a 10 MB CAD/CAM
drawing - the operating system instructs the controller where to begin writing information
to the disk. The controller moves the read/ write heads to the appropriate track and
writing begins. When the first track is full, the heads write to the same track on
successive platter surfaces. If still more track capacity is required to store all the data, the
head moves to the next available track with sufficient contiguous space and writes the
data there.
Although an extraordinary amount of care and effort goes into making the platters for
hard disk drives, it is not economically feasible to manufacture 100 percent defect-free
media. Therefore, all modern hard drives have a defect management strategy built into
the disk controller to provide defect-free operation in the field. Defect management
involves setting aside some spare sectors on each disk surface to replace a limited number
of defective sectors. At the end of the manufacturing process, the entire disk surface is
scanned for defects and the disk controller stores a map of their locations. When the
operating system requests that information be written to one of the bad sectors, the disk
controller transparently maps it to one of the spares. The disk controller continuously
updates the defect map, to map out any new bad sectors that might occur.
Tricky Business: Head Positioning
Given the tolerances involved in drive electronics and the speed at which drives operate,
the ability to locate data and move the heads accurately to read or write data is an
astounding accomplishment. Since the tracks on a platter are located about 300
microinches apart and the heads are flying three microinches above the surface of the
disk, accurately positioning the read/write heads is comparable to flying a jet plane one
foot off the ground while maintaining a course of flight directly over the center divider of
a freeway.
A number of variables work against the process of accurate head positioning. These
include temperature variations, which cause minute expansions and contractions of the
disk platter, as well as shock and vibration. To counter the effects of these variables and
ensure precise head alignment and positioning, hard drives incorporate an
electromechanical technique called servo positioning, which provides feedback to the
drive electronics to control the position of the head.
There are two primary servo head positioning techniques. Most hard drive manufacturers
have abandoned the older method, dedicated servo, for the newer method, embedded
servo technology.
• Dedicated servo uses positioning information residing on a single dedicated
platter surface accessed by a single dedicated head - a mechanism to which the
other heads are slaved. Dedicated servos require one entire side of a platter, a
significant percentage of the total disk space - particularly in a hard drive with
few platters.
• Embedded servo is emerging as the leading servo implementation for the next
several generations of hard disk drive products. Embedded servo overcomes the
limitations of dedicated servos by interspersing the servo information with the
data in the form of prerecorded servo burst wedges special data patterns recorded
on each of the tracks. When the heads arrive at the intended track location, they
read the servo bursts and send back the information to the drive electronics. Then,
the drive electronics adjust the position of the actuator motor, which positions the
heads so that they receive the maximum signal from the bursts. (The maximum
signal only occurs when the head is exactly over the center of the track.) They
provide the most accurate, error-free, and cost-effective head positioning
technique for small form factor drives.
Embedded servo technology for hard disk drive head positioning has been difficult to
implement in drives which use Multiple Zone Recording techniques. The varying number
of sectors on tracks in different zones vastly complicates the task of reading interspersed
servo data. Quantum was the first hard drive manufacturer to effectively address this
problem by developing specialized servo feedback and controller ASICs that efficiently
handle the task of separating the servo information from the user data.
Making the Connection: Buses and Hard Disk
Drive Interfaces
By definition, an interface is anything that allows two separate or dissimilar systems to
work together or communicate. One of the most critical but least appreciated parts of a
hard disk drive is the interface that allows the drive to communicate with the host system.
In peripherals such as hard disk drives, this communication is achieved through adapter
boards or chips on the motherboard that plug into the host system's bus. So, to understand
drive interfaces, you must first understand a little about the main information corridor
inside a computer: the computer bus.
Introducing the Computer Bus Acting much like a human central nervous system, the bus
serves as a common conduit for carrying signals to and from various computer
components: CPU, video controller, keyboard, storage, and other peripherals. Many types
of bus architectures have been offered by manufacturers - both "open" and "closed."
Open architectures, most notably the Industry Standard Architecture (ISA) of the early
IBM and MS-DOS compatible PCs readily enables outside vendors to design, build, and
market general and special purpose add-on cards. Today's main personal computer bus
architectures include:
• ISA, a low-cost, relatively unsophisticated bus, available in either 8- or 16-bit
versions
• Extended ISA (EISA), a much more sophisticated and powerful 32-bit superset of
ISA designed for 386- and 486-based PCs
• The local bus, originally developed for graphics-intensive applications, used now
as a high speed disk attachment for IBM-compatible computers
• MicroChannel Architecture (MCA), a proprietary 32-bit bus developed by IBM
for its PS/2 line of personal computers
• NuBus, Apple Computer's proprietary 32-bit bus featured in its Macintosh
computers
• The Personal Computer Memory Card International Association (PCMCIA)
standard 16-bit interface
Efficient use of the bus is a key factor to increasing computer system performance. The
speed of data transfer along the bus, which is the product of the bus width and cycle time,
has the greatest effect on overall system performance. (The bus cycle time is proportional
to the number of words transferred per second. The width determines the width of the
transfers.) The bus, which was once 4- or 8-bits wide, typically transferred data at rates of
only up to 1 MB/s. Now, the bus is commonly 16- or 32-bits wide and transfers data at
speeds of 10, 20, and up to 132 MB/s in Peripheral Components Interface (PCI). The next
logical development for the bus is a 64-bit wide interface, which will allow drives and
other peripherals to reach even higher data transfer speeds.
(While many of the bus architectures in the list above are commonplace today, two of
them, the local bus and PCMCIA, are newer bus technologies for connecting storage
devices to the CPU and represent significant technological advancements. Therefore, they
are discussed in detail in chapter 4, "Recent Technological Developments").
Understanding Drive Interfaces
A drive interface is required to provide communication between the computer bus and the
hard drive. Among the information specified by drive interfaces is how fast the disk and
controller should talk to one another, what kinds of commands they can pass back and
forth, the location of data and control lines along the connecting cable, and what level of
voltage they should use for data transfer. Thus, a drive interface is a standardized
combination of connector configuration signal levels and functions, commands, and data
transfer protocols.
As with all other high technology developments, drive interface technology has evolved
significantly in the past two decades. There are two interfaces commonly in use in today's
personal computers. Most IBM-compatible PCs use the Intelligent Disk Electronics
(IDE) interface. Also called the AT or ATA (Advanced Technology Attachment)
interface, for the PC/AT I/O bus it was designed to work with, IDE can transfer data at a
rate of up to 4 MB/s. When connected to a bridge to a faster bus (e.g., local bus), AT can
go up to 13 MB/s. The second most common interface is the Small Computer System
Interface, or SCSI (pronounced "scuzzy"). This higher performance interface, which is
used on Apple Macintosh computers as well as high- performance workstations and
servers, can transfer data at a rate of up to 10 MB/s in 8-bit mode and up to 20 MB/s in
16-bit mode.
Quantum was the first company to introduce hard drive products with embedded interface
controllers. In the late 1980s, usually drives and interface controllers were sold
separately. Today, most hard drives feature an embedded disk drive controller, which
includes an interface controller designed onto a single chip and incorporated into the
single circuit board that contains the drive electronics. By the early 1990s - in less than a
5-year time span - drive manufacturers have reduced the disk drive controller and drive
electronics from two full circuit cards to just five integrated circuit chips that would
easily fit in the palm of your hand.
It stores data/saves because it has memory e.g. gigs, kb and so on.
data in hard disk is stored in binary form(0's & 1's).hard disk is made up of ferromagnetic material(it can be magnetized and demagnetized).
the read/write head present in hard disk magnetizes the platter.thus data is stored in it.deletion is done by demagnetizing the particular sector and track.
Directories. Tiny files that are generally located at the MFT of the Hard drive. they are stored on the RAM temporarily. Directories tell where the file is located.
its a hard drive
THANKS
Are you asking "How is data/files" stored on a spinning magnetic disk within a hard drive, or are you asking how is data/files stored within the flash memory cells of a Solid State [hard] Drive (SSD)? Or, are you asking, "what is a hard disk drive" or "What is inside a hard drive?" Thanks!
in a hard drive
Are you asking "How is data/files" stored on a spinning magnetic disk within a hard drive, or are you asking how is data/files stored within the flash memory cells of a Solid State [hard] Drive (SSD)? Or, are you asking, "what is a hard disk drive" or "What is inside a hard drive?" Thanks!
No, replacing a hard drive will.
A hard drive is one of the main components of a computer where all data for the computer is stored from files essential to run the computer to personal files and documents
The internal hard drive of a computer is an example of a drive on which data (files, folders, etc) can be stored.
The hard drive is the main, and usually largest, data storage device in a computer. The operating system, software titles and most other files are stored in the hard drive.
Portable Software is used.
Portable Software is used.
No, changing your RAM will not affect your files. Your files are stored on your hard drive, which is a separate piece of hardware from the RAM. Once you shut down your system, your files are safely stored on the hard drive, so adding new RAM or replacing old RAM will have no effect on your files.