Disk Basics Kanetkar

8/12/2019 Disk Basics Kanetkar

1/31

13 Disk Basics

The Disk Structure

The Boot Sector

The Root DirectoryLong Filenames

The Data Space

The File Allocation Table

How Fixed Disks Dier

!ne Last Thing about Disks

"ompact Disc

#rimary $olume Descriptor

The Directory Structure

The #ath Table

%xercise

447


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192 Let Us C

y ar the most widely used storage mediums are the loppy

disks and ixed disks &hard disks'( Floppy disks and hard

disks come in )arious si*es and capacities but they all

work basically in the same way+inormation is magnetically

encoded on their surace in patterns( These patterns are determined

by the disk dri)e and the sotware that controls the dri)e(

B

Although the type o storage de)ice is important, it is the way the

stored inormation is laid out and managed that concerns

programmers most( -n this chapter thereore we would ocus our

attention on how inormation is organi*ed and stored on the disk(

The Disk Structure

As most o us know, the disk dri)es in D!S and .indows are

organi*ed as *ero/based dri)es( That is, dri)e A is dri)e number 0,

dri)e B is dri)e number 1, dri)e " is dri)e number 2, etc( The hard

disk dri)e can be urther partitioned into logical partitions( %ach

dri)e consists o our logical parts+Boot Sector, File AllocationTable &FAT', Directory and Data space( ! these, the Boot Sector

contains inormation about how the disk is organi*ed( That is, how

many sides does it contain, how many tracks are there on each

side, how many sectors are there per track, how many bytes are

there per sector, etc( The iles and the directories are stored in the

Data Space( The Directory contains inormation about the iles like

its attributes, name, si*e, etc( The FAT contains inormation about

where the iles and directories are stored in the data space( Figure

13(1 shows the our logical parts o a 1(44 5B disk(


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Chapter 13: Disk Basics 193

Figure 13(1

.hen a ile6directory is created on the disk, instead o allocating a

sector or it, a group o sectors is allocated( This group o sectors

is oten known as a cluster(How many sectors together orm one

cluster depends upon the capacity o the disk( As the capacity goes

on increasing, so also does the maximum cluster number(

Accordingly, we ha)e 12/bit, 17/bit or 32/bit FAT( -n a 12/bit

FAT each entry is o 12 bits( Since each entry in FAT represents a

cluster number, the maximum cluster number possible in a 12/bit

FAT is 212&4087'( Similarly, in case o a 17/bit FAT the maximum

cluster number is 217 &79937'( Also, or a 32/bit FAT the

maximum cluster number is 22:&27:439497( !nly 2: o the 32 bits

are used in this FAT'( All FAT systems are not supported by all)ersions o D!S and .indows( For example, the 32/bit FAT

system is supported only in .in 89 !SR2 )ersion or later( There

are dierences in the organi*ation o contents o Boot Sector, FAT

and Directory in FAT126FAT17 system on one hand and FAT32

on the other( Some o these dierences are discussed below(

Side 0, Track 1Side 0, Track 0

10

9

2

34

1

7;

:

8

11

1213 14 19

17

1;

1:

F1F1

F1F1

F1

F1F1

F1

F2F2

F2

F1 F2

F2

F2F2F2

BS

DS

2

3

1

17

1;

1:

D

F2

D

10

947

;

:

8

11

1213 14 19

DDD

DDD

D

DD

D

D

DSD

DS

DSDS

BS / Boot Sector F1 / First copy o FATF2 / Second copy o FAT D / Root directory structureDS / Data space


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194 Let Us C

The Boot Sector

The boot sector contains two parts< =Boot #arameters> and =Disk

Bootstrap #rogram>( The Boot #arameters are useul while

perorming read6write operations on the disk( Figures 13(2 and

13(3 shows the break up o the boot parameters or the 12/bit and

the17/bit FAT systems along with their typical )alues(

As you can obser)e rom Figures 13(2 and 13(3, the boot

parameters basically contain inormation indicating how the disk

has been organi*ed( Apart rom the irst 11 bytes in the list o boot

parameters, the rest orm the =B-!S #arameter Block> &B#B'(

The entry =5aximum root directory entries> indicates how many

maximum iles can we create in the root directory o the disk( -

one o these entries is a sub/directory name then there can be

se)eral more iles and sub/sub/directories within it( The =5edia

descriptor> byte indicates how the operating system reers to a

particular type o disk internally( For example, a 1(44 5B disk has

a media descriptor F0, whereas, a hard disk has a media descriptorF:(

Description Length Typical Values

?ump instruction 3 %B3"80

!%5 name : 5S.-@4(1

Bytes per sector 2 912

Sectors per cluster 1 1

Reser)ed sectors 2 1

@umber o FAT copies 1 2

5ax( Root directory entries 2 224

Total sectors 2 2::0

Continued


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Chapter 13: Disk Basics 195

continued

5edia descriptor 1 F0

Sectors per FAT 2 8

Sectors per track 2 1:

@o( ! sides 2 2

Hidden sectors 4 0

Huge sectors 4 0

B-!S dri)e number 1 0


Boot signature 1 41

$olume -D 4 348;;:922

$olume label 11 -"-T

File system type : FAT12

Figure 13(2


?ump instruction 3 %B3"80

!%5 name : 5S.-@4(1


Sectors per cluster 1 74



5ax( Root directory entries 2 912Total sectors 2 0

5edia descriptor 1 F:

Sectors per FAT 2 297


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196 Let Us C

Continued

continued

Sectors per track 2 73

@o( o sides 2 299

Hidden sectors 4 73

Huge sectors 4 4182802B-!S dri)e number 1 12:


Boot signature 1 41

$olume -D 4 40:47;;9;4



Figure 13(3

%arlier, the B#B comprised o entries only up to the entry =Hidden

sectors> in Figure 13(2( For example, it didn>t contain the =$olume

label> o the disk( This used to get stored as a directory entry in the

directory sectors( Howe)er, now the B#B has been extended to

include entries like =$olume label, =$olume -D>, =File system

type>, etc( The B#B is now called =%xtended B#B>( .hether the

boot sector contains B#B or %xtended B#B is indicated by the

)alue o the entry =Boot signature>( - it contains a )alue 28h then

the disk has an %xtended B#B(

The =$olume -D> entry is the serial number that is recorded on the

disk while ormatting it( This is the same number that is shown

when we run the =Dir> command on the disk(

A 1(44 5B disk supports a 12/bit FAT system( The total number

o sectors on this disk is 2::0( This )alue is stored in the entry

=Total Sectors>( There is another entry in the boot parameters

called =Huge Sectors>( For a 12/bit FAT System, this entry has a


7/31

Chapter 13: Disk Basics 197)alue 0.For bigger capacity hard disks the )alue o total number

o sectors would be much more than what can be accommodated

in the 2/byte entry called =Total Sectors>( Hence or such disks this

entry contains a )alue 0 and the total number o sectors )alue is

stored in the entry =Huge Sectors>(

Let us now take a look at the 32/bit FAT system>s boot sector

contents( These are shown in Figure 13(4(


?ump instruction 3 %B9:80

!%5 name : 5S.-@4(1


Sectors per cluster 1 :



Root directory entries 2 0Total sectors 2 0

5edia descriptor 1 F:

Sectors per FAT 2 0

Sectors per track 2 73

@o( o sides 2 299

Hidden sectors 2 73

High word o hidden sectors 4 73

Huge sectors 4 4182802

High word o huge sectors 2 4182802

Sectors per FAT 2 4089

High word o sectors per FAT 2 4089

Dri)e description lag 2 0

File system )ersion 2 0


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19 Let Us C

continued

continued

Root directory starting cluster 2 2

High word o root directory

starting cluster

2 2

File system inormation sector 2 1

Back up boot sector 2 7

Reser)ed 7 0

B-!S dri)e number 1 12:

Reser)ed 1 0

Boot signature 1 41

$olume -D 4 748:29317



Figure 13(4

There are signiicant changes in the contents o the boo t sector o

a 32/bit FAT system( The entries =@umber o hidden sectors> and

=Huge sectors> ha)e now been made 4/byte entries( The irst two

bytes contain the low word o the )alue, whereas, the next two

bytes contain the high word )alue( The actual )alue o the entry

can be obtained by using bitwise operators that would be discussed

in "hapter 1:(

The number o sectors per FAT in a 32/bit ile system is likely toexceed what can be accommodated in two bytes( Hence the entry

=Sectors per FAT> or a disk with a 32/bit ile system would

typically ha)e a )alue 0( The )alue o =Sectors per FAT> is now

stored as a 4/byte entity, with the similar arrangement o low word

and high word as discussed earlier(


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Chapter 13: Disk Basics 199The boot sector o a 32/bit FAT system also has new entries like

=Dri)e description lag>, =File system )ersion> =Starting cluster

number o the root directory>, =Sector number o the ile system

inormation sector>, and the sector number o the =Backup boot

sector>(

The =Dri)e description lag> is a two/byte entity( Bit : o this lag

indicates whether or not the inormation written to the acti)e FAT

will be written to all copies o the FAT( The low our bits o this

entry contains the 0/based FAT number o the acti)e FAT( These

bits are meaningul only i bit : is set(

-n the entry =File system )ersion number> the high byte contains

the maor )ersion number, whereas, the low byte contains the

minor )ersion number(

The entry =File system inormation sector> contains a )alue

indicating the sector number where the ile system inormation is

present( This ile system inormation consists o the ields shown

in Figure 13(9(

Description Length

File system signature 4

Total number o ree clusters 4

Sector number o the next ree cluster 4

Reser)ed 7

Figure 13(9

The entry =File inormation sector> contains a )alue !FFFFh i

there is no such sector( The entry =Backup boot sector> contains a

)alue 0FFFFh is there is no backup boot sector( !therwise this

)alue is any non/*ero )alue less than the reser)ed sector count(


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2!! Let Us C

The "oot Directory

The root directory o a 1(44 5B disk containing a 12/bit FAT

system occupies 14 sectors( For other FAT systems this number

may )ary( The directory sectors contain 32/byte entries or )arious

iles6sub/directories present in the root directory( Since each entryis o 32 bytes, one directory sector can accommodate 17 such

entries( As there are 14 directory sectors on a 1(44 5B disk, there

can be a maximum o 224 entries &17 14' in the root directory(

This )alue is stored in one o the boot parameters(

%ach 32/byte entry contains either a ilename or a sub/directory

name( - it is a ile entry then it contains inormation about ile>s

name, its si*e, attributes, starting location on the disk, etc( This

inormation is organi*ed as shown is Figure 13(7(

Description Length

Filename :

%xtension 3

Attributes 1

Reser)ed 1

@umber o 10 msec inter)als in 2 seconds 1

"reation time 2

"reation date 2

Last access date 2

High word o starting cluster number 2

Last modiication time 2

Last modiication date 2

Starting cluster o ile6directory 2

File si*e 4


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Chapter 13: Disk Basics 2!1

Figure 13(7

@ote the ollowing points(

&a' File name can be less than or eCual to : characters( - it is

less than : characters long then it is padded with blanks

on the right( Since .indows 89 we are permitted to use

long ile names( How these are accommodated is a 32/byte entry is discussed in the next section(

&b' %xtension or the amily name can be maximum 3

characters long( - it less than 3 characters it is padded

with blanks( .hile a ilename must ha)e at least one

character, the extension can be all blanks(

&c' -n the attribute byte each bit represents either the type o

the ile or whether the entry is a sub/directory entry( The

meaning o each bit is gi)en in Figure 13(;(

Figure 13(;

- bit 0 is set to 1 then the ile can only be read, it cannot be

modiied or deleted(

#eaningBit nu$%ers

Read !nly

; 7 9 4 3 2 1 0

( ( ( ( ( ( ( 1

( ( ( ( ( ( 1 (

( ( ( ( ( 1 ( (

( ( ( ( 1 ( ( (

( ( ( 1 ( ( ( (

( ( 1 ( ( ( ( (

( 1 ( ( ( ( ( (

1 ( ( ( ( ( ( (

Hidden

System

$olume label entry

Sub/directory entry

Archi)e bit

nused

nused


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2!2 Let Us C

- bit 1 is 1 then the ile is hidden and will not be shown in the

directory( $ice )ersa, i bit 1 is 0, then the ile will be shown

in the directory as a normal ile(

- bit 2 is 1, it means the ile is an operating system ile( Bit 4

identiies the entry as a sub/directory name(

Bit 9 is called an archi)e bit( -t helps a)oid taking backups o

iles that ha)e already been backed up( !S sets the archi)e bitto 1 whene)er a ile is created or modiied( !n backing up this

ile using the BA"E# command, its archi)e bit is set to 0(

Suppose we do not modiy this ile beore we take the next

backup( So its archi)e bit would remain 0( - we now take a

backup, this ile won>t be backed up &sa)ing precious time'

since its archi)e bit is still 0( Howe)er, i we modiy the ile

beore the next backup, its archi)e bit would be set to 1 and

would now Cualiy or backing up(

&d' Following the attribute byte, there is 1 unused byte set

aside or possible uture use( All six bytes are usually setto 0(

&e' %ach directory entry maintains three dates+date o

creation, date o modiication and date o last access(

There are also entries showing time o creation or last

modiication( All these entries occupy 2 bytes each(

Suppose the date o creation o a ile is 10612688 and time

o creation is 10


13/31

Chapter 13: Disk Basics 2!3&' The starting cluster number indicates the place where the

ile begins in the data space o the disk( This ield

occupies 2 bytes in the 32/byte directory entry( -n a 32/

bit FAT system the starting cluster number is likely to be

bigger than what can be accommodated in a 2/byte entry(

Hence or 32/byte FAT system the starting cluster

number entry is broken up into two parts o two bytes

each( !ne part is stored in the =Starting cluster number>

ield( The other part is stored in the 2/byte ield

immediately ollowing the ield =Last access date>( This

ield contains the high word o the starting cluster

number(

&g' The ile si*e ield contains the exact ile si*e in bytes(

Long &ilena$es

Since the ad)ent o .indows 89 the barrier o :(3 based ilenames

has been lited( Hence now we can use ilenames like =This is my

seminar ile>( This is all right rom the point o )iew o

con)enience, but how do we accommodate such long ilenames in

a 32/byte entry .hile storing long ilenames, the name is

distributed o)er multiple 32/bytes entries( The last o these entries

contains a short ilename &alias'( For example, i the long ilename

is =AB"D%FGH-?EL5@!#RST$.IJK123497;:80(TIT>,

then out o the se)eral 32/byte entries that would be reCuired to

accommodate such a name the last entry would contain

=AB"D%F1(TIT>( Let us now understand what is present in

other entries except the last( The breakup o these entries is shown

in Figure 13(:(

Description Length

SeCuence byte 1

nicode characters o name 10


14/31

Alias or longilename

32/byte entry

2!4 Let Us C

Attributes 1

Long entry type 1

"hecksum or matching short name 1

5ore nicode characters o name 12

Reser)ed 2

5ore nicode characters o name 4

Figure 13(:

As can be seen rom Figure 13(: e)ery 32/byte entry reCuired or a

long ilename can accommodate 13 characters & &10 M 12 M 4' 6 2 '(

These characters are stored as nicode characters &in nicode

each character occupies two bytes' and not as AS"-- characters(

Thus, to accommodate ilename =AB"D%FGH-?EL5@!#

RST$.IJK123497;:80(TIT> we would need 40613, i(e( 4

entries &one more entry would be reCuired or a short ile name

&alias o the ilename'( The contents o these entries are shown in

Figure 13(8(

T FFFF FFFF

12349 7;:80' TI

@!#R ST$.I JK

AB"D% FGH-?E L5

AB"D%F1 TIT


15/31

Chapter 13: Disk Basics 2!5Figure 13(8

.hile displaying the directory containing a long ilename how

would the !S come to know o)er how many 32/bytes entries is the

ilename distributed This is simple to determine( All the entries

belonging to a long ilename would ha)e in their attribute byte a

)alue Fh( Also, all the entries belonging to this ilename would

ha)e the same check sum( -n our case, the seCuence byte o the

our directory entries reCuired or the ilename would ha)e the

)alues D, 3, 2 and 1 respecti)ely( @ote that the irst o these )alues

is obtained by bitwise !Ring it with 74(

The Data Space

All iles and sub/directories are stored in this area( -t occupies the

last and largest part o each disk( !S allocates space to iles one

cluster at a time( Remember that a cluster is nothing but a group o

sectors that !S allocates to a ile at a time( How many sectors,

orm a cluster depends upon how the disk has been ormatted(

As a ile is being created, or an existing ile is extended, the ile>s

allocated space grows( .hen more space is needed, !S allocates

another cluster or the ile(

nder ideal conditions a ile is stored under one contiguous block

o space( Howe)er, a ile might be broken into se)eral non/

contiguous blocks( This happens i inormation is added to an

existing ile or a new ile is stored in the space let by an erased

ile( So don>t be surprised i one ile is scattered throughout the

disk( This ile ragmentation slows down the access to the ile>s

contents to some degree(

The &ile (llocation Ta%le

The File Allocation Table &FAT' maps the usage o the data space

o the disk( -t contains inormation about the space used by each


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2!6 Let Us C

indi)idual ile, the unused disk space and the space that is

unusable due to deects in the disk( Since FAT contains )ital

inormation, two copies o FAT are usually stored on the disk( -n

case one gets destroyed, the other can be used( A typical FAT

entry can contain any o the ollowingt mark an unused, reser)ed or

deecti)e cluster, then the cluster corresponding to the FAT entry

is part o a ile, and the )alue in the FAT entry would indicate the

next cluster in the ile(

This means that the space that belongs to a gi)en ile is mapped by

a chain o FAT entries( %ach FAT entry points to the next entry in

the chain( The irst cluster number in the chain is the starting

cluster number in the ile>s directory entry( .hen a ile is created

or extended, a cluster is allocated to the ile by searching the FAT

or unused clusters and adding them to the chain( $ice )ersa, when

a ile is deleted, the cluster that has been allocated to the ile is

reed by clearing corresponding FAT entries &by setting them to

0'( The FAT chain or a ile ends with an entry FFFFh in the FAT(

Figure 13(10 shows a FAT chain or a ile called -"-T(#RG(


17/31

Chapter 13: Disk Basics 2!7

Figure 13(10

This ile occupies cluster number 3, 9, 7 and : on the disk( Hence

the starting cluster number in the directory entry or the ile is 3(

Suppose this ile is to be loaded into memory then !S would irst

load starting cluster number+3>s contents into memory( To indout the next cluster belonging to this ile !S looks at entry number

3 in FAT where it inds a )alue 9( Thereore, now it loads the

contents o cluster number 9 into memory( !nce again !S looks at

the FAT and inds in entry number 9 a )alue 7, hence it loads the

contents o cluster 7 into memory( This process goes on till the !S

inds an entry FFFFh in FAT, which indicates that there are no

more clusters belonging to the ile( Hence the process stops(

@ow that we ha)e understood how the FAT chain is tra)ersed,

let>s dig a little deeper into the FAT( The entries present in FAT

are 12, 17 or 32 bits long depending on the storage capacity o the

disk( Though a 12/bit FAT can handle 4087 clusters only 40;:

clusters are a)ailable or use since some )alues are reser)ed(

Similarly, or a 17/bit FAT out o the possible 79937 clusters that

it can handle only 7991: are a)ailable or use(

32)%yte *irectory entry

-"-T #RG 0003

starting cluster no(

0 1 2 3 4 9 7 ; : 8 10 11

0000 0009 0000 0007 000: 0000 FFFF 0000

&(T +hain


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2! Let Us C

-n a 12/bit FAT three bytes orm two entries( The irst two entries

&0 and 1' in the FAT are reser)ed or use by the !S( This means

that irst 3 bytes in a 12/bit FAT, irst 4 bytes in 17/bit FAT and

irst : bytes in a 32/bit FAT are not used or storing cluster

numbers( !ut o these 3 &or 4, or :' bytes, the irst byte is the

media descriptor byte and the balance contain the )alue FFh(

These balance bytes remain unused( The media descriptor byte

speciies the type o the disk( -t typically has a )alue FDh, F8h,

F0h, F:h or a 370 EB loppy disk, 1(2 5B loppy disk, 1(44 5B

loppy disk and a hard disk respecti)ely( The contents o a FAT

entry are interpreted as shown in Figure13(11(

Values #eaning

12)%it 16)%it 32)%it

000h 0000h 0000000h "luster a)ailable

FF0hNFF7h FFFFhNFFFF7h

FFFFFFFhNFFFFFF7h

Reser)ed cluster

FF;h FFF;h FFFFFF;h Bad cluster i not part

o chain

FF:hNFFFh FFF:hN

FFFFh

FFFFFF:hN

FFFFFFFh

Last cluster o ile

xxx xxxx xxxxxxx @ext cluster in ile

Figure 13(11


19/31

Chapter 13: Disk Basics 2!9As we saw earlier, two identical copies o FAT are maintained on

the disk( All copies are updated simultaneously whene)er iles are

modiied( - access to a FAT ails due to a read error, the !S tries

the other copy( Thus, i one copy o the FAT becomes unreadable

due to wear or a sotware accident, the other copy may still make it

possible to sal)age the iles6directories on the disk(

,o- &i.e* Disks Di//er#hysically, there is more than one platter in a ixed disk( Still, data

is organi*ed on ixed disks by track &cylinder', head, and sector

number, ust as it is on loppy disks(

Since hard disks are typically o high capacity the number o iles

stored on it are )ery high( - all these iles were stored in the root

directory then to manage these iles would become Cuite tedious(

Thereore, the iles on a hard disk are usually split into )arious

sub/directories( For example, in the root directory there could be a

sub/directory containing all "MM programs, another sub/directorycontaining all " programs, still another containing all 5S/.ord

iles, and so on( .ithin a sub/directory there can be still more sub/

directories( %ach sub/directory is linked to its parent directory,

which can be a root directory or another sub/directory, thus

orming a tree like structure, as shown in Figure 13(12

"MM programssub/directory

" programssub/directory

5S/.ord

sub/directory

Letterssub/directory

Documentssub/directory

Root directory


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21! Let Us C

Figure 13(12

A sub/directory is stored in the disk>s data space, ust like any

other ile( The entries in the sub/directory are identical to those in

the root directory, except that a sub/directory is not limited in si*e(

Like an ordinary ile, a sub/directory can grow without bounds as

long as there is disk space a)ailable( Sub/directories can be created

with any type o disk( Howe)er, since sub/directories take up

precious data space, they are primarily intended or use with high/

capacity hard disksO their use with loppy disks are usually

a)oided(

Since the storage capacity o a ixed disk is relati)ely large, some

users preer to split the disk into logical parts and then store

dierent !perating Systems in dierent logical parts( This is

called partitioning o ixed disk( As many as our partitions are

allowed on a hard disk( %ach partition>s data can be kept

completely separate rom the data in the other partitions( %ach

partition can contain its own boot sector and operating system(

nlike loppy diskettes, side 0, track 0, sector 1 o the ixed disk

contains a partition table and a master boot program( The partition

table is 74 bytes long( -t contains inormation about where each

partition is located on the disk( The partition table also indicates

which is the bootable partition( - the hard disk is di)ided into two

partitions+one or D!S6.indows and another or Linux6nix(

The D!S 6 .indows partition is di)ided into Boot Sector, FAT,

Directory and Data Space, whereas, the nix partition is di)ided

into our logical parts, namely, Boot Block, Super Block, -node

Table and Data Blocks( Depending upon which partition has been

marked as bootable in the partition table that partition Boot Sectorcontents are used to boot under that partitions operating system(

0ne Last Thing a%out Disks


21/31

Chapter 13: Disk Basics 211?ust to make our lie that much more diicult, the way R!5/

B-!S reers indi)idual sector on a disk is dierent than the way

D!S6.indows reers it( The R!5/B-!S unctions reer sectors

by their three dimensional locations &side number, track number

and sector number', whereas, D!S6.indows reers them by their

seCuential logical sector numbers(

.hile using B-!S ser)ices remember that,

Tracks are numbered rom outermost track, increasing

towards the innermost track, starting with 0(

Sides are also numbered rom 0 onwards(

Sectors are numbered rom 1 onwards(

Thus any location on the disk can be reerred to by a uniCue

combination o side, track and sector number(

D!S6.indows howe)er doesn>t recogni*e sides, tracks and

sectors( -nstead, they see a disk as a linear seCuence o logical

sectors( The seCuence o logical sectors begins with the irst sector

on the disk+side 0, track 0, sector 1 &boot sector' isD!S6.indows logical sector 0( Similarly, side 0, track 0, sector 2

is D!S6.indows logical sector 1 and so on( @ote that, the last

sector in side 0, track 0 is ollowed by irst sector in side 1, track 0

and so on(

"on)ersion o a sector number rom B-!S to D!S6.indows or

)ice )ersa can be done using ollowing ormulae(

DOS/Win sector number = ( BIOS sector no - 1 ) + ( BIOS side * sectors / track )

+ ( BIOS track * sectors per track *sides per disk )

BIOS sector = ( 1 + DOS/Win sector number ) %( sectors per track )


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212 Let Us C

BIOS track = ( DOS/Win sector no / ( sectors per track* sides per disk )BIOS side = ( DOS/Win sector no / sectors per track ) %( sides per disk )

+o$pact Disc

Almost e)ery concei)able orm o data is making its way to a i)e/

inch disk called compact Disc, "D in short( !ne "D is capable ostoring data eCui)alent to almost 900 loppy diskettes( A "D can

also be used to store data, photographs, music, mo)ies, etc(

There exit dierent types o "Ds( They are