How to use Interrupt in PIC16F877A in C..??

#include<pic.h>
__CONFIG(LVP_OFF & BOREN_OFF & PWRTE_ON & WDTE_OFF & FOSC_HS);
#define _XTAL_FREQ 20000000
#define DAD PORTB
#define RST RD7
#define CLK RC3
#define DATA RC4
 
 
 
char displayPointer=0;
extern const char  font[];
 
unsigned char buffer[32]; // buffer for screen
 
void interrupt ISR()		// This just swaps the buffer to the display
	{
	if(TMR2IF)				// make sure its the timer interrupt.
		{
		PORTB = 0;					// clear old data first
		if(displayPointer == 0 )  	// 1st frame..
			DATA = 1;				// Data = 1 on the first clock
		CLK = 1;
		__delay_us(10);				// Clock the shift registers
		CLK = 0;
     	DATA = 0;
		PORTB = buffer[displayPointer];	 // Move budffer line by line		
		if(++displayPointer==31) 		//  24 LED columns
			displayPointer = 0;			// back to first..
		}
	TMR2IF = 0;
	}
 
void display(const char* str)
	{
	int addr;
	char x, y;
	while(*str != 0)					// while there is still characters
		{
		addr = (int) *str++ - 0x20;		// get character
	//	DAD=addr;
 addr *=5;
//DAD=addr;
		for(y=0;y<5;y++)				// 5x8 font so 5 columns
			{
			buffer[31] = font[addr++];  // put it in buffer	
 
 	for(x=0;x<32;x++)
				{
				buffer[x] = buffer[x+1];	// shift all the columns <--
				//DAD=buffer[x];
__delay_ms(4);				// so you can see it
				}
			}
		buffer[31] = 0;						// This is the scpace between 
		for(x=0;x<32;x++)					// the characters.. Or they will be too close
			{
			buffer[x] = buffer[x+1];		//	shift the space in
			__delay_ms(5);
			}							// NOTE!!! your array is 28 not 24 like mine
		}	
	}
 
 
void main(void)
	{
	ADCON1 = 0x6;
	T2CON = 0x1e;	
	PR2 = 50;							// timer preload value
	TMR2IE = 1;							// enable timer 2 interrupt
	PEIE = 1;							// enable peripheral interrupt
	GIE = 1;							// enableglobal interrupt
	TRISB = 0;							// Port B as output...
	TRISC = 0;	
RST=1;						// Port C as ouput...
	while(1)
		{
		display("WELCOME     ");   // Here's the message
		}
	}	// End main

for(y = 0;y < 300 ;y++)

void displaystring(void)				// this routine prints through the screen buffer
	{									// moving one pixel at a time
	signed char x=32,y=0;				// I made these signed so I could print 
	for(y = 0;y  < 96 ;y++)				// to nowhere so I could produce the scrolling effect
		{
		clr();							// Clear the display buffer
		strput("HELLO WORLD!!",x--,0);	// adjust the scrolling string
		Blit();							// pass to screen buffer
		__delay_ms(80);					// time to view
		}
	}