1. #include <dos. h> void interrupt(
#include <dos.h> void interrupt(*old)(); void interrupt newint9(); void main() { old = getvect(0x09); setvect(0x09,newint9); keep(0,1000); } void interrupt newint9() { if (inportb(0x60)==0x1F) { outportb(0x20,0x20); return; } (*old)(); }

2. #include <dos. h> void interrupt(
#include <dos.h> void interrupt(*old)(); void interrupt newint9(); int i=1; void main() { old=getvect(0x09); setvect(0x09,newint9); keep(0,1000); }
void interrupt newint9() { if (inportb(0x60)==0x1F) { i++; if (i<=5){ outportb(0x20,0x20); return; } i=1; } (*old)(); }

3. #include <dos. h> void interrupt(
#include <dos.h> void interrupt(*old)(); void interrupt newint9(); char far *scr=(char far *) 0x ; void main() { old=getvect(0x09); setvect(0x09,newint9); keep(0,1000); } void interrupt newint9() { if((inportb(0x60)==83)&&((*scr)&12==12)) { outportb(0x20,0x20); return; } (*old)(); }

4. 40:17H 1
Alt Pressed
Ctrl Pressed
83 = Scan Code for Del

5. Keyboard Buffer Keyboard Buffer is located in BIOS Data Area.
Starts at 40: IEH
Ends at 40 : 3DH
Has 32 byes of memory 2 bytes for each character.
Head points is located at address 40 : 1A to 40 : IBH
Tail points located at address 40 : IC to 40 : IDH

6. 40:1AH
40:1CH
Head Tail
40:1EH
40:3DH

7. ‘A’ 0’
‘B’ 83 20 21 22
Head = 0x24 23

8. 0xIE
‘B’ 83
0x20
Head = 24

9. So KBD buffer acts as a circular buffer.
The taut value should be examination to get to the start of the buffer.

10. #include <dos. h> void interrupt (
#include <dos.h> void interrupt (*old)(); void interrupt new1(); char for *scr = (char far*) 0x C; void main() { old = getvect(0x09); setvect(0x09,new1); keep(0,1000); }

11. void interrupt new1 () { if(inportb(0x60)==83) {. ((char far
void interrupt new1 () { if(inportb(0x60)==83) { *((char far*)0x *scr)=25; if((*scr)==60) *scr=30; else *scr+=2; outportb(0x20,0x20); return; }}

12. EOI Code for Slave IRQ

13. For Master. outportb(0x20,0x20); For Slave. outportb(0x20,0x20);
For Master outportb(0x20,0x20); For Slave outportb(0x20,0x20); outportb(0xA0,0x20);

14. OCW2*OCW3 7 6 5 4 3 2 1 X 00 If EOI is to be InitializedX
00 If EOI is to be Initialized
001 FOR Non Specific EOI
01 If other Registers are to be accessed

15. 7 6 5 4 3 2 1
01 To read IRR or ISR
10 = IRR
11= In-Service Register

16. 7 6 5 4 3 2 1
No EOI relevant
Don’t Care
Other Register to be Accessed

17. 7 6 5 4 3 2 1
IRR Accessed
01 PIC Notified about reading operation

18. 7 6 5 4 3 2 1
ISR Accessed
01 PIC Notified about reading operation

19. #include <stdio. h> #include <dos. h> #include <bios
#include <stdio.h> #include <dos.h> #include <bios.h> void main ( ) { char a; outport(0x20,8); outport(0x20,0x0A); a=inport(0x20); printf (“value of IRR is %x”, a); outport(0x20,0x08); outport(0x20,0x0B); a=inport(0x20); printf (“value of ISR is %x”, a); }

20. More about TSR Programs
A TSR need to be loaded once in memory
Multiple loading will leave redundant copies in memory
So we need to have some check which will load the program only once

21. Using a Global Variable as a flag

22. int flag; flag =1; Keep(0,1000); if (flag==1). Make TSR else
int flag; flag =1; Keep(0,1000); if (flag==1) Make TSR else exit Program
But this
wont
Work

23. Answers is to use a memory area as flag that is global to all programs
Answers is to use a memory area as flag that is global to all programs. i.e. IVT

24. int 65 is empty, we can use its vector as a flag.
Address of vector
seg = 0
offset = 65H * 4

25. #include<stdio.h>
#include<BIOS.H>
#include<DOS.H>
unsigned int far * int65vec = (unsigned far *) MK_FP(0,0x65*4)
void interrupt (*oldint) ( );
void interrupt newfunc ( );
void main() {
if((*int65vec) = = 0xF00F)
oldint =getvect (0x08);
setvect(0x08, newint);
(*int65vec) = 0xF00F;
keep (0,1000);
}else {
puts (“Program Already Resident”); }}
void interrupt newfunc ()
{ :::::::
:::::::
(*oldint) ( ); }

26. But what if another program is resident or using this vector.
Another Method
Service # 0xFF usually does not exist for ISR’s.
Key is to create another service # 0xFF for the ISR interrupt besides other processing.

27. #include<stdio.h>
#include<BIOS.H>
#include<DOS.H>
void interrupt (*oldint) ( );
void interrupt newfunc ( unsigned int BP,..…,flags);
void main() {
_DI = 0;
_AH = 0xFF;
geninterrupt (0x13);
if (_DI = = 0xF00F)
puts (“Program Already Resident”);
exit (0); }

28. else
{ oldint = getvect (0x13);
setvect (0x13, newint);
Keep (0, 1000); }}
void interrupt newint ( )
{ if (_AH == 0xFF){
DI = 0xF00F;
return;
}else {
::::::: }
(*oldint) ( );