18 Hard Disk Drive

8/11/2019 18 Hard Disk Drive

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Revision no.: PPT/2K403/02Revision no.: PPT/2K605/03

Hard Disk Drive


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Revision no.: PPT/2K605/03

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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Revision no : PPT/2K605/03


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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).




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Interleave ratio 1:1,1:2,1:3 was used in IDE, ESDI, SCSI

respectively




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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.


Integrated Drive Electronics / AT


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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


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


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


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.


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


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.



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.



FAT12 FAT16)


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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.



FAT12 FAT16)


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Table shows the number of sectors per cluster for FAT16.


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.


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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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) (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.


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.


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".


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.


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.


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.


Fdisk startup screen


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Fdisk startup screen


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.

Documents

18 Hard Disk Drive