This document contains information about the boot sector of a disk, including: 1. It describes the contents and layout of a disk boot sector, including fields like the jump instruction, system ID, bytes per sector, etc. 2. It provides sample code to read the boot sector and display the values of the different fields. 3. It compares the typical/expected values of the boot sector fields with the values obtained from reading an actual disk boot sector.

1. Contents Of Boot Sector
2
3
4
1
6 5
7
11
8
10
9
12
13 14 15
16
17
18
Boot Parameters
Disk Bootstrap
Program
SSiiddee 00,, TTrraacckk 00

2. Description
Boot Parameters
Jump Instruction 3
360
Kb
1.2
Mb
1.44
Mb
No.
of
bytes
EB3490
MSDOS5.0
System ID 8
No. of bytes/sector 2 512 512 512
No. of sectors/cluster 1 2 1 1
No. of sectors in reserved area 2 1 1 1
No. of copies of FAT 1 2 2 2
Max. no. of root dir. entries 2 112 224 224
Total no. of sectors 2 720 2400 2880
Media descriptor 1 FD F9 F0
No. of sectors/FAT 2 2 7 9
No. of sectors/track 2 9 15 18
No. of sides 2 2 2 2
No. of hidden sectors 2 0 0 0

3. Reading Boot Sector
No. of
Description bytes
Jump Instruction
System ID
Bytes/sector
Sectors/cluster
Sectors in
reserved area
Copies of FAT
Max. root dir. entries
Total no. of
sectors Media
descriptor
Sectors/FAT
Sectors/track
Sides
3821212212222
main( )
{
struct boot
{
char jump[ 3 ] ;
char sysid[ 8 ] ;
int bps ;
c .h. ar spc ;
char rest[ 482 ] ;
} ;
struct boot b ;
printf ( ”Insert disk,
Press any key ) ;
getch( ) ;
.a b. sread ( 0, 1, 0, &b ) ;
}
BBPP
DDBBPP
Side 0,
Trk 0,
Sec1
Side 0,
Trk 0,
Sec1

4. # include ”dos.h”
main( )
{
struct boot
{
char jump[ 3 ] ;
char sysid[ 8 ] ;
int bps ;
char spc ;
. .
} ;
struct boot b ;
absread ( 0, 1, 0, &b ) ;
for ( i = 0 ; i <= 2 ; i++ )
printf ( ”%X”, ) ;
b.jump[ i ]
for ( i = 0 ; i <= 7 ; i++ )
printf ( ”%c”, b.sysid[ i ]
) ;
printf ( ”Bytes/sector = %d”, b.bps ) ;
printf ( ”Sectors/cluster = %d”, b.spc ) ;
}
int i ;
......CCoonntt

5. In General
absread (drive no., no. of sectors to read,
sector from where reading should begin, buffer )
absread ( 0, 1, 0, &b ) ; ;
ROM - BIOS DOS
S0, T0, S1
Every sector referred using
side, track, sector
Drive A - 0
Drive B - 1
Drive C - 128
D. r i v e D . - 1 2 9. . . .
LSN 0
LSN 36
Every sector referred using
LSN
Drive A - 0
Drive B - 1
Drive C - 2
D. r i v e D . - 3. . . .
S0, T1, S1

6. What Is
It? Description
1.44 Mb Disk
Jump Instruction
System ID
Bytes/sector
Sectors/cluster
Sectors in
reserved area
Copies of FAT
Max. root dir. entries
Total no. of
sectors Media
descriptor
Sectors/FAT
Sectors/track
Sides
Obtained
Values
Typical
Values
EB3490
IBM 3.3
512
1
1
2
224
2880
F0
9
18
2
0
102
….
20480
0
1
-24
0
2048
7
-2
0
0
8
BP
DBS
Virus

7. BBPP
DDBBSS
Anti-Viral
LLSSNN 5500
# include ”dos.h”
char a[ 512 ] ;
printf ( ”Insert infected disk, Press any key” ) ;
absread ( 0, 1, 50, a ) ;
0, 1, 0, a ) ;
}
((V ( (
main( )
{
getch( ) ;
abswrite (

8. Better...
#include ”dos.h”
main( )
{
char a[ 512 ] ; char ch = ’y’ ;
char *names[ ] = { ”O Jerusalem”, ”Yankee Doodle”,
”Robinson Cruso”, ”Eddie Murphy”, ... } ;
printf ( ”Insert uninfected disk, Press any key” ) ;
getch( ) ;
absread ( 0, 1, 0, a ) ;
while ( ch = = ’y’ )
{
printf ( ”Insert infected disk, Press any key” ) ;
getch( ) ;
abswrite ( 0, 1, 0, a ) ;
printf ( ”Another disk y/n” ) ;
} ch = getche( ) ;
}

9. Directory Sector
DDiirreeccttoorryy
SSeevveerraall eennttrriieess,, eeaacchh ooff 3322--bbyytteess
Description Size
Filename
Extension
Attribute
Reserved for Future use
Time
Date
Starting Cluster number
Size
8 bytes
3 bytes
1 byte
10 bytes
2 bytes
2 bytes
2 bytes
4 bytes
( (

10. # include ”dos.h”
main( )
{
Printing Directory
struct entry
{
char n[ 8 ] ; char ext[ 3 ] ;
char unused[ 17 ] ;
} ; long int size ;
struct entry e[ 1 6 ] ;
printf ( ”Insert disk, Press any key” ) ; getch( ) ;
absread ( 0, 1, 19, e ) ;
for ( i = 0 ; i <= 15 ; i++ )
{
for ( j = 0 ; j <= 7 ; j++ )
printf ( ”%c”, e[ i ].n[ j ] ) ;
for ( j = 0 ; j <= 2 ; j++ )
printf ( ”%c”, e[ i ].ext[ j ] ) ;
}
}
printf ( ”%ld”, e[ i ].size ) ;