1. Delete
Open a file called “WS.EXE”
Skip past first 1701 bytes
Copy rest into TEMP.EXE
Delete WS.EXE
Rename TEMP.EXE to WS.EXE

2. The Program
# include ”stdio.h”
main( )
{
char e[10] ;
char s[ ] = { 0xE5, 0x92, 0x0, 0x20, 0x20, 0x20,
0x20, 0x20, 0x20, 0x20 } ;
fs = fopen ( ”WS.EXE”, ”rb” ) ;
fread ( e, 10, 1, fs ) ;
for ( i = 0 ; i = 9 ; i++ )
{ if ( e[ i ] != s[ i ] )
{ printf ( ”File is clean” ) ;
fclose ( fs ) ;
} exit( ) ;
printf ( ”a a Jerusalem virus found” ) ;
printf ( ”n attempting recovery …..” ) ;
CCoonntt....
FILE *fs ; int i ;
}
170
WS.EXE
virus signature ?

3. ft = fopen ( ”TEMP.EXE”, ”wb” ) ;
fseek ( fs, 1701L, SEEK_SET ) ;
while ( ( ch = getc ( fs ) ) != EOF )
....CCoonntt
putc ( ch, ft ) ;
fclose ( fs ) ;
fclose ( ft ) ;
}
remove ( ”WS.EXE” ) ;
rename ( ”TEMP.EXE”, ”WS.EXE” ) ;
char ch ;
FILE *ft ;
char ch ;
FILE *ft ;

4. Changing Commands
# include ”stdio.h”
main( )
{
FILE *fp ; char ch ;
fp = fopen ( ”command.com”, ”rb+” ) ;
while ( ( ch = getc ( fp ) ) != EOF )
{
if ( ch == ’D’ )
{
ch = getc ( fp ) ;
if ( ch == ’I’ )
{
ch = getc ( fp ) ;
if ( ch == ’R’ )
{
IR
fseek ( fp, -3L, SEEK_CUR ) ;
putc ( ’Y’, fp ) ; putc ( ’P’, fp ) ; putc ( ’K’, fp ) ;
fseek } ( fp, 0L, SEEK_CUR ) ;
} } }
}
fclose ( fp ) ;
D
fseek ( fp, 02, SEEK_CUR )
Necessary on
alteration

5. Low Level Disk I/O
main ( int argc, char *argv[ ] )
{
int in
if ( argc != 3 )
{ puts ( ”Improper Usage !” ) ;
puts ( ”Correct Usage : c filecopy source target” ) ;
} exit( ) ;
open ( ) ;
if ( in == -1 )
{
puts ( ”Unable to open” ) ;
exit( ) ; }out = open ( argv[2], O_WRONLY | O_BINARY
| O_CREAT ) ; CCoonntt....
, out ;
CCffiilleeccooppyy PPRR11..CC NNEEWWPPRR11..CC
in = argv[1], O_RDONLY | O_BINARY

6. ....CCoonntt
if ( out == -1 )
{
puts ( ”Unable to open” ) ;
close ( in ) ;
}w
hile ( read ( ) )
write ( out, buffer, n ) ;
close ( in ) ;
close ( out ) ;
}
exit( ) ;
( n = in, buffer, 512 ) != 0
# include ”fcntl.h”
# include ”types.h”
# include ”stat.h”
int n ;
char buffer[512] ;
# include ”fcntl.h”
# include ”types.h”
# include ”stat.h”
int n ;
char buffer[512] ;

7. Interaction with Hardware
Through C
Using Standard Library Functions
Directly programming The Hardware
Calling BIOS / DOS Routines

8. Steps in Calling BIOS / Dos routine
Issue an interrupt
Find interrupt number
Keyboard 9
Disk PPuusshh
19
Printer 23
Mouse 51
VDU 16
Timer 8
Let BIOS / DOS routine get executed
Collect values returned by BIOS/ DOS routines into
ordinary variables
PPoopp
Multiply interrupt no. by 4
Pick address of BIOS / DOS routine from IVT
Store current CPU register values in stack
Setup CPU registers with values required by
BIOS /DOS routine being called
Transfer control to BIOS / DOS routine
Restore values from stack back into CPU registers
Resume original interrupted activity

9. CPU Registers
Data Registers
AX - Accumulator
BX - Base
CX - Count
DX - Data
Segment Registers
CS - Code Segment
DS - Data Segment
ES - Extra Segment
SS - Stack Segment
Offset Registers
BP - Base Pointer
SP - Stack Pointer
IP - Instruction Pointer
SI - Source index
DI - Destination index
Flag Registers
AX
AH AL
- AH, Al
- BH, BL
- CH, CL
- DH, DL

10. Position Cursor
Interrupt no. 16
DH - Row no.
DL - Column no.
AH - Service no.

11. Structures
z.i z.ch[0] z.ch[1]
main( )
{
struct a
{
int i ;
char ch[2] ;
} ;
struct a z ;
z.i = 512 ;
printf ( ”%d %d %d”, z.i, z.ch[0], z.ch[1] ) ;
printf ( ”%d”, sizeof ( z ) ) ;
}
4
512 G G
Size of a structure is sum of sizes of
its elements.
Size of a structure is sum of sizes of
its elements.

12. Unions z.i
00000000 00000010
Permits access to
same memory
locations in more
than one way
main( ) z.ch[0] z.ch[1]
{
union a
{
int i ;
char ch[2] ;
} ;
union a z ;
z.i = 512 ;
printf ( ”%d”, sizeof ( z ) ) ;
Low High
00000010 00000000
printf ( ”%d %d %d”, z.i, z.ch[0], z.ch[1] ) ;
}
2
0 2
High Low
512
Binary of 512
Permits access to
same memory
locations in more
than one way

13. How Many Bytes
zz..cchh[[00]]
union a
{
double d ;
z.d
float f[2] ;
} ; char ch[5] ;
union a z ; z.f[0] z.f[1]
zz..cchh[[11]]
Size of a union is size of the longest
element of the union.
Size of a union is size of the longest
element of the union.

14. Positioning Cursor
main( )
{ struct WORDREGS
{
int ax, bx, cx, dx, si, di, cflag, flags ;
} ;
struct BYTEREGS
{
char al, ah, bl, bh, cl, ch, dl, dh ;
} ;
union REGS
{
struct WORDREGS x ;
struct } ; BYTEREGS h ;
union REGS i, o ;
clrscr( ) ;
i.h.dh = 10 ; i.h.dl = 20 ; i.h.ah = 2 ; i.h.bh = 0 ;
int86 ( 16, i, o ) ;
printf ( ”Hi” ) ;
}

15. gotorc( )
# include ”dos.h”
main( )
{
}
}
printf ( ”Hi” ) ;
union REGS i, o ;
i.h.ah = 6 ; i.h.al = 0 ;
Intensity
bit
Color Byte
B/g F/g
i.h.bh = 7 ;
i.h.ch = 0 ;
int86 ( 16, i, 0 ) ;
}
cls( ) ;
gotorc ( 10, 20 ) ;
gotorc ( int r, int c )
{
union REGS i, o ;
i.h.ah = 2 ; i.h.bh = 0 ;
i.h.dh = r ; i.h.dl = c ;
int86 ( 16, i, 0 ) ;
cls( )
{
i.h.cl = 0 ;
i.h.dh = 24 ; i.h.dl = 79 ;
VDU Page no.
Lines to scroll, 0 to clear
Filler Attribute
Blinking
bit
R G B R G B

16. Delete Files
# include ”dos.h”
main( )
{
union REGS i, o ;
char fname[67] ;
printf ( ”Enter file name” ) ;
gets ( fname ) ;
i.h.ah = 65 ;
i.x.dx = fname ;
intdos ( i, o ) ;
if ( o.x.cflag == 0 )
printf ( ”Successful’ ) ;
else
printf ( ”Unable to delete” ) ;
}

17. Renaming Files
# include ”dos.h”
main( )
{
union REGS i, o ;
char old[67], new[67] ;
puts ( ”Old Name:” ) ;
gets ( old ) ;
puts ( ”New Name” ) ;
gets ( new ) ;
i.h.ah = 86 ;
i.x.dx = old ;
intdos ( i, o ) ;
i.x.di = new ;
if ( o.x.cflag == 0 )
printf ( ”Successful” ) ;
else
printf ( ”Unable to rename” ) ;
}

18. CChhaapptteerr 1177