1. Interrupt Mechanism Interrupt Compared With Procedures Call MyProc
A= Addition(4,5);
Printf(“hello world”);

2. Int 21h
Int 10h
Int3

3. Main
Call proc1()
Int 21h
Proc1()
Int 10h
Proc2()

4. Interrupt Vector Table
0000:0000
INT1
0000:0004
INTFF
0000:03FFH

5. LO(0) LO(1) HI(0) HI(1) 0000:0001 INT 0 0000:0003 0000:0004 INT 1
0000:0007

6. ISR’s are Routines Resident in Memory
IO.SYS
Device Driver
Command. COM
USER PROGRAM
Rom Bios
F000:0000

7. 004CB0 COMMAND 000410 Environment 0050D0 MEM 000350 Environment
Address Name Size Type
Interrupt Vector
ROM Communication Area
DOS Communication Area
IO System Data
CON System Device Driver
AUX System Device Driver
PRN System Device Driver
CLOCK$ System Device Driver
COM System Device Driver
LPT System Device Driver
LPT System Device Driver
LPT System Device Driver
COM System Device Driver
COM System Device Driver
COM System Device Driver
000A MSDOS System Data
0020E IO System Data
KBD CE System Program
HIMEM E DEVICE=
XMSXXXX Installed Device Driver
FILES=
FCBS=
LASTDRIVE=
0007D STACKS=
COMMAND A Program
004C MSDOS Free --
004CB COMMAND Environment
0050D MEM Environment
MEM E0 Program
01C MSDOS C Free --
09FFF SYSTEM System Program
0C IO System Data
MOUSE F System Program
0CC MSDOS Free --
0CC MSCDEXNT D0 Program
0CC REDIR A Program
0CD0B DOSX A0 Program
0D DOSX Data
0D58F MSDOS A Free --
bytes total conventional memory
bytes available to MS-DOS
largest executable program size
bytes total contiguous extended memory
0 bytes available contiguous extended memory
bytes available XMS memory
MS-DOS resident in High Memory Area

8. Interrupt Invocation
Push Flags, CS, IP Registers, Clear Interrupt Flag
Use (INT#)*4 as Offset and Zero as Segment
This is the address of interrupt Vector and not the ISR
Use lower two bytes of interrupt Vector as offset and move into IP
Use the higher two bytes of Vector as Segment Address and move it into CS=0:[offset+2]
Branch to ISR and Perform I/O Operation
Return to Point of Interruption by Popping the 6 bytes i.e. Flags CS, IP.

9. -d 0:84
0000: C 10 A7 00-4F A |...O.U...U.
0000: A A7 00 9A 10 A U
0000:00A0 B8 10 A F CC B8 10 A T.p...t.....
0000:00B0 B8 10 A7 00 B8 10 A AB D4
0000:00C0 EA AE 10 A7 00 E8 00 F0-B8 10 A7 00 C C #..
0000:00D0 B8 10 A7 00 B8 10 A7 00-B8 10 A7 00 B8 10 A
0000:00E0 B8 10 A7 00 B8 10 A7 00-B8 10 A7 00 B8 10 A
0000:00F0 B8 10 A7 00 B8 10 A7 00-B8 10 A7 00 B8 10 A
0000: A -q

10. -a
0AF1:0100 int 21
0AF1:0102 -r
AX=0000 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000
DS=0AF1 ES=0AF1 SS=0AF1 CS=0AF1 IP= NV UP EI PL NZ NA PO NC
0AF1:0100 CD INT 21 -t
AX=0000 BX=0000 CX=0000 DX=0000 SP=FFE8 BP=0000 SI=0000 DI=0000
DS=0AF1 ES=0AF1 SS=0AF1 CS=00A7 IP=107C NV UP DI PL NZ NA PO NC
00A7:107C NOP
-d ss:ffe8
0AF1:FFE F1 0A 02 F
0AF1:FFF

11. Parameter Passing to S/W interrupts

12. S/W Interrupts Invocation
Psuedo Variables
AX, BX, CX, DX
_AX = 0x1234H
a = _BX

13. geninterrupt ( 0 x int# )

14. Interrupt 21/09 Description
Int # Service # 9
Inputs
AH = 0x09
DS = Segment Address of string
DX = Offset Address of string
Output
Display a String

15. Example: #include<stdio.h> #include<BIOS.H>
#include<DOS.H>
char st[80] ={"Hello World$"};
void main() {
_DX = (unsigned int) st;
_AH = 0x09;
geninterrupt(0x21); }

16. Union REGS struct half struct full { { unsigned char al;
unsigned char ah;
unsigned char bl;
unsigned char bh;
unsigned char cl;
unsigned char ch;
unsigned char dl;
unsigned char dh; }
struct full {
unsigned int ax;
unsigned int bx;
unsigned int cx;
unsigned int dx; }
typedef union tagREGS
struct full x;
struct half h;
}REGS;

17. Union Description output #include<DOS.H> REGS regs;
regs.h.al = 0x55;
regs.h.ah = 0x99;
printf (“%x”,regs.x.ax);
output
9955

18. Int86
#include<DOS.H>
int86 (0x21, &regs, &regs)

19. Example: #include<stdio.h> #include<BIOS.H>
#include<DOS.H>
char st[80] ={"Hello World$"};
void main() {
REGS regs;
regs.x.dx = (unsigned int) st;
regs.h.ah = 0x09;
int86(0x21,&regs,&regs); }

20. Interrupt 21/42H Description
Int # Service # 42H
Inputs
AL = Move Technique
BX = File Handle
CX-DX = No of Bytes File to be moved
AH = Service # = 42H
Output
DX-AX = No of Bytes File pointer actually moved

21. Animation
BOF cp EOF