1. Computer Programming Spring-2007
Union in C language
Accessing mouse in C language

2. Union
As structures, unions are also used to group a number of different variables together.
The difference between union and structure is that, structure treat each of its member as a different memory location store in the main memory.
While union treat each of its member as a single memory location store in the main memory.
i.e. all of the members of union shares a common memory of union member.

3. Union Example union searchOption { int SearchByRollNumber;
char SearchByName[90];
char SearchByAddress[90];
char SearchByPhoneNumber[90]; };
searchOption sv;
void main (void)
int option = 0;
switch (option)
case 0: FunSearchRoll (sv.SearchByRollNumber); break;
case 1: FunSearchName(sv.SearchByName); break;
case 2: FunSearchByAddress(sv.SearchByAddress); break;
case 3: FunSearchByPhone(sv.SearchByPhoneNumber); break;
} }
90 Bytes

4. Accessing Mouse in C/C++
union REGS in,out;      
void callmouse() {        in.x.ax=1;        int86(51,&in,&out); }
int86 (…) is declared in the “dos.h” file, so include this file before calling the function.

6. Accessing Mouse in C/C++
union REGS {
struct WORDREGS x;
struct BYTEREGS h; }
struct WORDREGS {
unsigned int ax, bx, cx, dx; };
struct BYTEREGS {
unsigned char al, ah, bl, bh;
unsigned char cl, ch, dl, dh;

7. Accessing Mouse in C/C++
Interrupt
Service
Purpose 51 1
Show mouse pointer  Call with AX = 1   Returns: Nothing 2
 Hide mouse pointer  Call with AX = 2  Returns: Nothing
The various mouse functions can be accessed by setting up the AX register with different values (service number) and issuing interrupt number 51.

8. Accessing Mouse in C/C++
Interrupt
Service
Purpose 51 3
Get mouse position and button status  Call with AX = 3  Returns: BX = mouse button status  Bit   Significance   0     button not pressed   1     left button is pressed   2     right button is pressed   3     center button is pressed        CX = x coordinate  DX = y coordinate 4
Set mouse pointer position   Call with AX = 4  CX = x coordinate  DX = y coordinate  Returns: Nothing

9. Accessing Mouse in C/C++
Interrupt
Service
Purpose 51 7
Set horizontal limits for pointer  Call with AX = 7  CX = minimum x coordinate  DX = maximum x coordinate  Returns: Nothing 8
Set vertical limits for pointer  Call with AX = 8  CX = minimum y coordinate  DX = maximum y coordinate  Returns: Nothing

10. Setting Mouse Position
     void setposi(int xpos,int ypos)       {              in.x.ax=4;              in.x.cx=xpos;              in.x.dx=ypos;              int86(51,&in,&out);       }

11. Receiving Mouse Position and Button Status
       void mouseposi(int *xpos,int *ypos,int *click)        {               in.x.ax=3;               int86(51,&in,&out);               *click=out.x.bx;               *xpos=out.x.cx;               *ypos=out.x.dx;         }

12. Hiding Mouse
       void mousehide()        {               in.x.ax=2;               int86(51,&in,&out);        }

13. Final Code void main (void ) {
            int xCor,yCor,click,a,b;              a=40*8;  b=12*8;              setposi(a,b);              callmouse();              while (1)              {                     mouseposi(&xCor,&yCor,&click);                     gotoxy(10,9);                     printf("\n\tMouse Position is: %d,%d",xCor/8,yCor/8);                     printf("\n\tClick: %d",click); if ( click == 2 )
break;              }              mousehide(); }