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Revision no.: PPT/2K403/02Revision no.: PPT/2K605/03
Hard Disk Drive
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CMS INSTITUTE, 2006. All rights reserved. No part of this material may be reproduced, stored or emailed without the prior permission of Programme Director, CMS Institute
Introduction
Hard disks allow data to be stored at far denser levels and can
be accessed very quickly.
In a hard disk, the magnetic material is layered on to an
aluminum or glass platter which is polished to mirror
smoothness.
The information on a hard disk is stored in extremely small
regions or magnetic domains through the use of control
mechanisms
Control mechanisms arrange magnetic particles in patterns
that electronically correspond to 0s and 1s.
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Introduction (contd.)
In hard disk the head actually 'flies' microns above the surface
of the platter and is never really allowed to touch the surface of
the hard disk.
In most of the hard disks, the drive platters spin at 5400 RPM,
7200 RPM, 10000 RPM
The arm that controls the head is responsible for moving the
head to the correct location on the disk
Hard disks contain more than one platter, and a corresponding
number of read-write heads that together decide the capacity
of the hard disk.
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Many hard disks can store 80GB per platter.
This implies that each platter holds 40 GB per side
Two read-write heads are used - one for each side of the
platter.
Considering the speeds at which the platters spin, if the heads
come into contact with the platters, there would be severe
damage to the disk surface and consequently to the data
stored.
Introduction (contd.)
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Inside of the Hard disk drive.
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The Platter
The media is the hard metallic disk made of Aluminum and
coated with iron oxide which gives a typical rust brown look.
Unlike the floppy disk drive, the media in the hard disk drive is
permanently fixed to the drive mechanism, hence it is also
called Fixed disk drive.
Both sides of the disk platter is coated with the magnetic
material which provides additional storage space.
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Hard disk surface is formed with concentric circular paths of
data storage called tracks
Each track is sub-divided into sectors
The density of tracks on a Hard disk are 300 - 1024 tracks
(maximum) on one surface
where as a floppy disk can have typically 40/80 tracks on one
surface.
The number of sectors/track is also higher than the floppy
disk, i.e., 17 sectors/tracks against 9 or 15 sectors/tracks.
The Platter (contd.)
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The platters and head arrangement
in the hard disk drive.
The Platter (contd.)
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Read/Write Head
The Hard Disk Drive uses a coil of winding to electrically
induce magnetic flux on the recording surface or medium.
Similar coil is also used to detect the existence of the magnetic
flux on the medium.
These coils form the Write and the Read mechanisms. The
assembly consisting of the R/W coils is called a head.
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One head assembly is provided for every recording surface
The R/W head assembly is mounted on a carriage device
It can move linearly to access any of the track spread over the
entire disk surface.
All the heads are mounted on one carriage assembly.
This assumes the access of same numbered tracks on all
surfaces simultaneously i.e., head 0 on surface 0 accesses the
track 0 (of surface 1) & so on.
Read/Write Head (contd.)
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Track 0 of the surface 0,1,2 & 3 in the same plane are called as
Cylinder 0.
When the entire head carriage is moved from a track to access
another, i.e., from track0 to track1, it is in effect accessing
cylinder 1.
Movement of carriage assembly to move from track to track is
achieved by driving it with a stepper motor or in some cases a
voice coil mechanism. This is called head actuation
Read/Write Head (contd.)
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Sometimes the rack and the pinion gear mechanism is also
used.
Usually each step of the motor moves the R/W head by one
track position.
If the head has to move, for example to track number 300, then
the stepper motor must move 300 steps in the required
direction.
Carriage Actuator (contd.)
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Voice coil actuator(contd.)
Voice coil method of actuation is done usually in large
capacity drives.
The voice coil mechanism moves the head carriage assembly
by pure electro-magnetic force.
Typically in a hard disk, voice coil is mounted on a track and
surrounding a stationary magnet.
Coil mechanism is connected to the head carriage assembly.
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Function Stepper motor Voice coil
Relative speed Slow Fast
Temperature sensitivity Yes No
Position sensitivity Yes No
Auto head park No Yes
Reliability / Accuracy Poor High
Cost Low High
Comparison between Stepper motor and voice coil actuator.
Voice coil actuator (contd.)
It determines and controls the position of the head over each
track thus enabling access to every cylinder on the disk.
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Hard Disk Spindle Motor
Spindle motor, also sometimes called the spindle shaft, is
responsible for turning the hard disk platters, allowing the
hard drive to operate.
All PC hard disks use servo-controlled DC spindle motors.
Servo system is a closed-loop feedback system which is the
same technology as is used in modern voice coil actuators.
Increasing the speed at which the platters spin improves both
positioning and transfer performance.
Rotational latency is the time that the heads must wait for the
correct sector number to come under the head.
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Hard Disk Data Encoding and Decoding
Hard disks store information in the form of magnetic pulses.
Magnetic information on the disk consists of a stream of (very,
very small) magnetic fields.
Information is stored on the hard disk by encoding information
into a series of magnetic fields.
It is conceptually simple to match "0 and 1" digital information
to "N-S and S-N" magnetic fields.
Earliest encoding methods were relatively primitive and
wasted a lot of flux reversals on clock information.
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Frequency Modulation (FM)
First common encoding system for recording digital data on
magnetic media was frequency modulation.
One is recorded as two consecutive flux reversals, and a zero
is recorded as a flux reversal followed by no flux reversal.
FM is is very wasteful wherein each bit requires two flux
reversal positions, with a flux reversal being added for
clocking every bit.
FM requires double (or more) the number of reversals for the
same amount of data.
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Modified Frequency Modulation (MFM)
A refinement of the FM encoding method is modified
frequency modulation, or MFM.
MFM improves on FM by reducing the number of flux reversals
inserted just for the clock.
When a 1 is involved there is already a reversal (in the middleof the bit) so additional clocking reversals are not needed.
When a zero is preceded by a 1, we similarly know there was
recently a reversal and another is not needed.
MFM encoding was used on the earliest hard disks, and also
on floppy disks.
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Run Length Limited (RLL)
Improvement on the MFM encoding technique used in earlier
hard disks and used on all floppies is run length limited or
RLL.
Two primary parameters define how RLL works, and therefore,
there are several different variations.
RLL considers groups of several bits instead of encoding one
bit at a time. Two parameters that define RLL are
Run length
It is the minimum spacing between flux reversals
Run limitIt is the maximum spacing between flux reversals.
RLL used on a drive is expressed as "RLL (X,Y)" or "X,Y RLL"
where X is the run length and Y is the run limit.
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Partial Response, Maximum Likelihood (PRML)
Traditional method of reading and interpreting hard disk data
is called peak detection.
As data density increases, the flux reversals are packed more
tightly and the signal becomes much more difficult to analyze
which can potentially cause bits to be misread from the disk.
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Partial Response, Maximum Likelihood (PRML)(contd.)
It becomes very hard for the circuitry to actually tell where the
flux reversals are and to combat this problem a new method
was developed to solve the data interpretation problem and
this technology, called partial response, maximum likelihood
or PRML.
PRML employs sophisticated digital signal sampling,
processing and detection algorithms to manipulate the analogdata stream coming from the disk and then determine the most
likely sequence of bits this represents.
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Extended PRML (EPRML)
Improvement on the PRML design has been developed called
extended partial response, maximum likelihood, extended
PRML or just EPRML.
EPRML are still based on analyzing the analog data stream
coming from the read/write head to determine the correct data
sequence.
Use better algorithms and signal-processing circuits to enable
to accurately interpret the information coming from the disk.
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Hard Disk Geometry
Geometry determines where data is stored on the surface of
each platter, and maximum storage capacity of the drive.
There are five numerical values that describe geometry:
Heads
Cylinders
Sectors per track
Write precompensation
Landing zone
Note:- All hard disk drives have geometry factors that must be
known by the BIOS to read and write to the drive.
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Heads
The number of heads is relative to the total number of sides of
all the platters used to store data.
The maximum number of heads limited by BIOS is 16.
Some hard disk drive manufacturers use a technology called
sector translation.
This allows some hard drives to have more than two heads per
platter.
It is possible for a drive to have up to 12 heads but only one
platter. Max 16
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Drive heads
Heads (contd.)
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Cylinders
Data is stored in circular paths on the surface of each platter.
Each path is called a track.
A set of tracks (all of the same diameter) in each platter is
called a cylinder.
Number of tracks per surface is identical to the number of
cylinders.
The number of cylinders is a measurement of drive geometry;
the number of tracks is not a measurement of drive geometry.
BIOS limitations set the maximum number of cylinders at 1024.
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Cylinders (contd.)
Cylinders
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Sectors per Track
A hard disk drive is cut (figuratively) into tens of thousands of
small arcs, like a pie. Each arc is called a sector and holds 512
bytes of data
BIOS limitations set the maximum number of sectors per track
at 63.
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Write Precompensation Cylinder
Older hard drives had a real problem with the fact that sectors
towards the inside of the drives were much smaller than
sectors toward the outside.
An older drive would write data a little further apart once it got
to a particular cylinder and this cylinder was called the Write
Precompensation (write precomp) cylinder.
Hard drives no longer have this problem, making the write
precomp setting obsolete.
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Landing Zone
Old stepper motor hard drives needed to have the read/write
heads parked before being moved in order to avoid accidental
damage.
Landing zone value designated an unused cylinder as a
"parking place" for the read/write heads.
Today's voice coil drives park themselves whenever they're
not accessing data, automatically placing the read/write headson the landing zone.
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Interleaving
Platters of the hard drive are rotating with a very high speed;
typically 3600 rpm and above
While recording the data one or two sectors may skip till the
write signal is received, if they are numbered one after the
other.
If this happens a second sector will be read after completing a
full rotation and the speed of the hard disk will slow down
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Interleaving (contd.)
To avoid this problem the sectors are numbered separating
them physically from each other.
This process is named as INTERLEAVING PROCESS
The factor by which it is separated is known as INTERLEAVE
FACTOR.
INTERLEAVE RATIO is the ratio by which the sectors are
separated from each other.
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For IDE drives generally the interleave (optimum value) is 1,
Due to the integrated logic the IDE drives use 1:1 interleave
ratio
Note Due to advance BIOS chipset we can implement 1:3 or more
for IDE interface hard disk.
This can be done even by the utility called as DM (Disk
Manager).
Interleaving (contd.)
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Interleave ratio 1:1,1:2,1:3 was used in IDE, ESDI, SCSI
respectively
Interleaving (contd.)
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Physical Geometry
Physical geometry of a hard disk is the actual physical number
of heads, cylinders and sectors used by the disk.
Original setup parameters in the system BIOS are designed to
support the geometries of these older drives.
There are three figures that describe the geometry of a drive:
the number of cylinders on the drive ("C"), the number of
heads on the drive ("H") and the number of sectors per track("S") and together they comprise the "CHS" method of
addressing the hard disk.
Today's drives do not have simple geometries and therefore
do not have the same number of sectors for each track, and asa result drives must be accessed using logical geometry
figures, with the physical geometry hidden behind routines
inside the drive controller.
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Logical Geometry
When you perform a drive parameter autodetection in your
system BIOS setup or look in your new IDE/ATA hard disk's
setup manual, you are seeing the logical geometry values that
the hard disk manufacturer has specified for the drive.
Since newer drives use zoned bit recording it is not possible toset up the disk in the BIOS using the physical geometry.
BIOS routines for the original AT command set allowed a hard
drive size only upto 504 MB wherein a drive could have no
more than 1024 cylinders, 16 heads, and 63 sectors/track.
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Logical Geometry (contd.)
Older hard disks that had simple structures and low capacity
did not need special logical geometry.
Newer drives cannot have their true geometries expressed
using three simple numbers and thus BIOS is given bogus
parameters that give the approximate capacity of the disk, and
hard disk controller is given intelligence so that it can do
automatic translation between the logical and physical
geometry.
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LBA (Logical Block Addressing)
Most modern drives can be accessed using logical block
addressing (LBA) instead of using the logical geometry
numbers directly.
In this method a totally different form of logical "geometry" is
used wherein the sectors are just given a numerical sequence
starting with 0 and the drive just internally translates these
sequential numbers into physical sector locations.
Largest logical geometry numbers for IDE/ATA drives are16,383 cylinders, 16 heads and 63 sectors which yields a
maximum capacity of 8.4 GB.
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INT13 Extensions
In 1994, Phoenix Technologies (the BIOS manufacturer) came
up with a new set of BIOS commands called Interrupt 13
extensions (INT13) by feeding the LBA a stream of
"addressable sectors".
Drives larger than 8.4 GB can no longer be accessed using
regular BIOS routines, and require extended Int 13h
capabilities. System with INT13 extensions can handle drives upto 137GB.
Hard drives must be accessed directly by an operating system
supporting Int 13h BIOS extensions to see the whole drive, ordrive overlay software used and if the drive is addressed using
conventional geometry parameters, it will be limited in capacity
to only 8.4 GB.
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Autodetection
Manual installation process was always a bit of a problem.
Today, all PCs can set the CMOS properly by using
autodetection.
Autodetection simply means that the CMOS asks the drive for
those stored values and automatically updates the CMOS.
Most CMOS setup utilities have a hard-drive type called "Auto
and by setting the hard-drive type to Auto, the CMOS
automatically updates itself every time the computer is started.
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Hard Disk Interfaces and Configuration
The interface is the communication channel over which all the
data flows that is read from or written to the hard disk.
Nowadays there are really only two main interfaces used for
hard disks: IDE/ATA and its variants, and SCSI and its
variants.
Presence of IDE/ATA controllers on all modern motherboards
makes this interface less expensive for most people than
going with SCSI, which would require the addition of a SCSI
host adapter.
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ST-506 / ST-412 Interface
Developed in 1980 by Seagate Technologies, to work with the
company's 5 MB ST-506 hard disk and later revised to support
the 10 MB ST-412.
In hard disks of these type, there was no built in logic board as
modern drives have.
This interface is recognized in older systems by the use of two
ribbon cables wherein one of the cables is 20 pins wide andcarries data, and the other is 34 pins and carries control
signals.
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Enhanced Small Device Interface (ESDI)
ESDI was developed in the mid-1980s by a consortium of hard
disk manufacturers led by Maxtor.
It had a maximum theoretical bandwidth of 24 Mbits/second.
ESDI suffered under competition from IDE/ATA which offered
simpler configuration, lower cost and improved performance.
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g
Attachment (IDE/ATA) Interface
Most popular interface used in modern hard disks, commonly
known as IDE.
IDE drives were the first ones to popularize integrating thelogic controller onto the hard disk itself.
First hard disks to have integrated controllers weren't
technically using the IDE/ATA interface but were in fact so-called "hardcards", which were designed and sold by the "Plus
Development" division of Quantum.
Connection to the system bus was maintained through the use
of a cable that ran either directly to a system bus slot, or to a
small interfacing card that plugged into a system bus slot.
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g
Attachment (IDE/ATA) Interface (contd.)
Later, chipset manufacturers began integrating IDE/ATA hard
disk controllers into their chipsets, so that instead of
connecting the drives to a controller card, they were
connected directly to the motherboard.
Connection between the system and the hard disks is 16 bits
wide, so two bytes of data are passed at a time between the
system and any hard disk.
Two drives are supported on each IDE/ATA channel, withspecial signalling used to ensure that commands sent for one
drive don't interfere with the other.
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g
Attachment (IDE/ATA) Interface (contd.)
First formal standard defining the AT Attachment interface was
submitted to ANSI for approval in 1990.
Western Digital, created "Enhanced IDE" or "EIDE", a
somewhat different ATA feature set expansion which included
powerful features such as higher capacities, support of non-
hard drive storage devices, for a maximum of four ATA
devices and substantially improved throughput.
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SFF-8020 / ATA Packet Interface (ATAPI)
IDE/ATA interface, originally was designed to work only with
hard disks.
A special protocol was developed called the AT Attachment
Packet Interface or ATAPI which is used for devices like
optical, tape and removable storage drives.
It enables CDROMs etc. to plug into the standard IDE cable
used by IDE/ATA hard disks, and be configured as master orslave, etc. just like a hard disk would be.
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SFF-8020 / ATA Packet Interface (ATAPI) (contd.)
ATAPI driver is used to communicate with ATAPI devices
which must be loaded into memory before the device can be
accessed.
ATAPI devices will coexist with IDE/ATA devices and behave
as if they are regular IDE/ATA hard disks and will even allow
booting from it.
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Small Computer Systems Interface (SCSI)
Small Computer Systems Interface, abbreviated SCSI and
pronounced "skuzzy". is a much more advanced interface than
IDE/ATA, and is preferable for many situations, usually in
higher-end machines.
It is less commonly used due to its higher cost and the fact
that its advantages are not useful for the typical home or
business desktop user.
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Programmed I/O (PIO) Modes (contd.)
Table below shows the five different PIO modes
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Direct Memory Access (DMA) Modes and
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Bus Mastering DMA
PIO method requires a fair bit of overhead, as well as the care
and attention of the system's CPU.
Direct memory access or DMA is the generic term used to refer
to a transfer protocol where a peripheral device transfers
information directly to or from memory, without the systemprocessor being required to perform the transaction.
There are two different ways of doing DMA transfers:
Conventional DMA / Third party DMA
Bus Mastering DMA / First party DMA
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Direct Memory Access (DMA) Modes
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and Bus Mastering DMA (contd.)
Conventional DMA / Third party DMA
In this method the DMA controllers on the motherboard
coordinate the DMA transfers.
Bus Mastering DMA / First party DMA
In this method the peripheral device itself does the work of
transferring data to and from memory, with no external DMA
controller involved which is also called bus mastering because
when such transfers are occurring the device becomes the
"master of the bus".
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Ultra DMA (UDMA) Modes
In Ultra DMA, data is transferred on both the rising and falling
edges of the clock.
Double transition clocking, along with some other minor
changes made to the signalling technique to improve
efficiency, allowed the data throughput of the interface to be
doubled for any given clock speed.
First implementation included three Ultra DMA modes,
providing up to 33 MB/s of throughput.
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Ultra DMA (UDMA) Modes (contd.)
Table below shows all of the current Ultra DMA modes, along
with their cycle times and maximum transfer rates:
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IDE/ATA Controllers
A device that resides within the system and interfaces with a
peripheral device is often commonly called a "controller
IDE/ATA controller acts as the middleman between the hard
disk's internal controller and the rest of the system.
Data pathway over which information flows in the IDE/ATA
interface is called a channel.
Each IDE channel is capable of communicating with up to two
IDE/ATA devices however theoretically it is possible to
configure and use as many as four (or even more) different
IDE/ATA interface channels on a modern PC.
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IDE/EIDE Id tifi ti
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IDE/EIDE Identification
Master and slave jumper settings
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H d Di k L i l St t d Fil S t
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Hard Disk Logical Structures and File Systems
File system is the general name given to the logical structures
and software routines used to control access to the storage on
a hard disk system.
Operating systems use different ways of organizing and
controlling access to data on the hard disk, and this choice is
basically independent of the specific hardware being used-the
same hard disk can be arranged in many different ways, and
even multiple ways in different areas of the same disk.
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PC Fil S t
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PC File Systems
There are many different types of file systems in use by
different operating systems for PC hardware which are as
follows:
FAT16
Virtual FAT (VFAT)
FAT32 (32-bit FAT)
NTFS
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File Allocation Table File System (FAT,
FAT12 FAT16)
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FAT12, FAT16)
It is the file system that was used by DOS on the first IBM PCs,
and it became the standard for the PCs that followed.
FAT in Concept
Base storage area for hard drives is a sector, with each sector
storing upto 512 bytes of data.
MS-DOS version 2.1 first supported hard drives using a special
data structure to keep track of stored data on the hard drive, and
Microsoft called this structure the FAT.
FAT is a data structure but it is more like a two-column
spreadsheet.
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File Allocation Table File System (FAT,
FAT12 FAT16)
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FAT12, FAT16) (contd.)
Left column gives each sector a number, from 0000 to FFFF which
means there are 65,536 (64K) sectors and thus this type of FAT is
called a "16-bit FAT".
Right-hand side of the FAT contains information on the status of
sectors.
16-bit FAT addresses a maximum of 64K (216) locations and
therefore, the size of a hard-drive partition should be limited to
64K x 512 bytes per sector, or 32MB.
One needed an improvement to the 16-bit FAT, a new and
improved FAT16 that would enable larger drives which led to the
development of a dramatic improvement in FAT16, called
clustering.
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File Allocation Table File System (FAT,
FAT12 FAT16)
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FAT12, FAT16) (contd.)
Clustering simply means to combine a set of contiguous sectors
and treat them as a single unit in the FAT and these units are
called file allocation units or clusters.
FAT16 could support partitions up to 2GB since FAT 16 still only
contained 64K storage areas.
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File Allocation Table File System (FAT,
FAT12 FAT16)
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FAT12, FAT16) (contd.)
Table shows the number of sectors per cluster for FAT16.
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Virt al FAT (VFAT)
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Virtual FAT (VFAT)
When Microsoft introduced Windows 95 in,1995, a new
variation of FAT was introduced called Virtual FAT or VFAT for
short.
VFAT has several key features and improvements compared to
FAT16 which are
Long File Name Support
Improved Performance
Better Management Capabilities
Only significant change in terms of actual structures is the
addition of long file names.
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FAT32 (32 bit FAT)
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FAT32 (32-bit FAT)
Hard disk manufacturers started to create drives so large that
FAT16 could not be used to format a whole drive in a single
partition thus to correct this situation, Microsoft created
FAT32.
FAT32 uses 32-bit numbers to represent clusters, instead of
the 16-bit numbers used by FAT16.
It allows single partitions of very large size to be created,
where FAT16 was limited to partitions of about 2 GB and
supports partitions up to 2 terabytes.
FAT32 was first introduced in Windows 95's OEM Service
Release2 was later included in Windows 98, Windows ME and
Windows 2000, and Windows 2003 as well.
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Master Boot Record (MBR)
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Master Boot Record (MBR)
Every hard disk must have a consistent "starting point" where
key information is stored about the disk, such as how many
partitions it has, what sort of partitions they are, etc.
Place where this information is stored is called the master boot
record (MBR) which is always located at cylinder 0, head 0, and
sector 1, the first sector on the disk.
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Master Boot Record (MBR)
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Master Boot Record (MBR) (contd.)
Master boot record contains the following structures:
Master Partition Table
This small table contains the descriptions of the partitions that are
contained on the hard disk.
Master Boot Code
The master boot record contains the small initial boot program
that the BIOS loads and executes to start the boot process.
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Partition Types
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Partition Types
A hard drive may have up to four partitions. These partitions
divide into one of two types: primary and extended.
Primary Partitions
Primary partitions store the OS(s) and if you want to boot from a
hard drive, it must have a primary partition.
In Windows 9x and 2000, the primary partition is always C:, and
you cannot change.
A hard drive can have up to four primary partitions.
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Active Partition
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Active Partition
Active partition comes into play when a hard drive stores
multiple primary partitions, each with a valid operating system.
For a primary partition to boot, you must set it as the active
partition.
MBR looks for a primary partition set to "active".
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Boot Managers
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Boot Managers
Programs specifically designed for the task of booting and
they are usually called Boot Managers or boot loaders.
It analyzes the primary partitions on the disk and then presents
a menu to you and asks which operating system you want to
use.
Boot managers are in many ways indispensable when working
with multiple operating systems.
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Extended Partition
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Extended Partition
Extended partitions are not bootable and one hard drive can
only have one extended partition.
Extended partitions are completely optional.
When you create an extended partition, it does not
automatically get a drive letter instead, you divide theextended partition into "logical drives".
An extended partition may have as many logical drives as you
wish, limited only by the letters of the alphabet for Windows 9x
systems.
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Partitioning
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Partitioning
Partitioning the hard disk is the act of dividing it into pieces.
Partitions are one of the major disk structures that define how
the disk is laid out.
Rules that govern partition setup are as follows:
A maximum of four primary partitions can be placed on any hard
disk.
Only one partition may be designated, at any given time, as active.
DOS will only recognize the active primary partition.
One of the four partitions may be designated as an extended DOS
partition.
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Fdisk startup screen
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Fdisk startup screen
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Troubleshooting Tips
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Troubleshooting Tips
Some of the tips are as follows:
If your computer won't boot from your hard drive
Run FDISK again and check whether the partition is set to active.
If FDISK reports a disk size that isn't true
BIOS may be incorrectly identifying your hard drive and thus run your
BIOS setup program and confirm the size.
No access to a particular hard disk.
Simply boot from your Windows CD and let the SETUP program
format the disk for you or
If you've started the PC with a startup floppy made with Windows
95/98, use the /S switch with the FORMAT command (FORMAT C: /S)
to copy the system files.