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40:17H

#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)(); }.

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40:17H

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  1. #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(*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(*old)();void interrupt newint9();char far *scr=(char far *) 0x00400017;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 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. 20 21 22 Head = 0x24 23

  8. 0xIE 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 (*old)();void interrupt new1();char for *scr = (char far*) 0x0040001C; void main(){ old = getvect(0x09); setvect(0x09,new1); keep(0,1000);}

  11. void interrupt new1 (){if(inportb(0x60)==83){*((char far*)0x0040000+*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); outportb(0xA0,0x20);

  14. OCW2*OCW3 00 If EOI is to be Initialized 001 FOR Non Specific EOI 01 If other Registers are to be accessed

  15. 01 To read IRR or ISR 10 = IRR 11= In-Service Register

  16. No EOI relevant Don’t Care Other Register to be Accessed

  17. IRR Accessed 01 PIC Notified about reading operation

  18. ISR Accessed 01 PIC Notified about reading operation

  19. #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 TSRelse exit Program But this wont Work

  23. 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) ( ); }

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