Multitasking using interrupt

#include<xc.h>
#include<stdio.h>
#define _XTAL_FREQ 20000000L       // 20 meg crsytal
#pragma config WDTE = OFF, PWRTE = OFF, CP = OFF, BOREN = OFF, DEBUG = OFF, LVP = OFF, CPD = OFF, FOSC = HS           // HS on,  WDT off, BOR on, PWRTon..

char displayPointer=0;           // for interrupt use...
extern const char  fnt[];         // Font in external C file
extern const char  anim[];
extern const char  anim1[];
unsigned char ledstr[7];
unsigned char buffer[64];          // buffer for screen
unsigned char backbuffer[64];       // Spare screen for drawing on
char pow[8]={128,64,32,16,8,4,2,1};
unsigned char digit[] = {0x3f,0x6,0x5b,0x4f,0x66,0x6d,0x7d,0x7,0x7f,0x6f};


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..
       RC4 = 1;           // Data = 1 on the first clock only
     RC3 = 1;
     __delay_us(20);           // Clock the shift registers
     RC3 = 0;
     RC4 = 0;             // Make sure data stays low for the rest of the cycles
     PORTB = buffer[displayPointer];   // Move buffer row by row( 4 row sections per row )     
     if(++displayPointer==64)      // 32 LED row sections in total
       displayPointer = 0;       // Back to first row..
     }
   TMR2IF = 0;               // Clear timer 2 interrupt flag
   }

void pixel(signed char x,signed char y,int cond)
   {
   int tmp;
   char pix,msk;
   if(x<0 || y<0) return;       // outside drawing limits negative
   if(x>63 || y>7) return;       // outside drawing limits positive
   tmp = (y << 3) + (x>>3);     // Linear position
   pix = x%8;             // pixel required
   pix = pow[ pix];
   msk = backbuffer[tmp];       // get exsisting data
   if(cond == 2)
     pix ^= msk;           // XOR data to screen
   if (cond == 1)
     {
     pix = ~pix;
     pix &= msk;           // AND data to screen
     }
   if(cond == 0)
     pix |= msk;           // OR data to screen
   backbuffer[tmp] = pix;       // apply changes
   }

void charput(char ch, signed char x,signed char y)
   {
   signed char x1, y1;         
   const char* addr2;         // pointer to character
   char disp;
   ch -= 0x20;             // characters starts a 0 not 0x20
   addr2 = &fnt[0];         // start of font array
   addr2 = addr2 + ((int)ch * 8);   // start place in font array
   for( y1=0;y1<8;y1++)       // eight rows
     {
     disp = *addr2;
     for (x1 = 0; x1<8; x1++)   // eight pixels
       {       
       if(disp & pow[x1])
         pixel(x+x1,y+y1,0); // OR the pixel to the display buffer
       }
     addr2++;
     }   
   }

void strput(char* ch, signed char x,signed char y)
   {
   int addr;
   
   while (*ch )
     {
     charput(*ch++,x,y);       // write a string to the display buffer
     x+=7;
     }     
   }

void clr()
   {
   int addr;
   for(addr=0;addr<64;addr++)         // Empty display buffer
     backbuffer[addr]= 0;
   }

void Blit()
   {
   int addr=0;
   GIE = 0;
   for(addr=0;addr < 64;addr ++)
     {
     buffer[addr] = backbuffer[addr];   // put all data from display buffer
     }                   // to screen buffer
   GIE = 1;
   }

void number(long numb)
   {
   int x, y;
   unsigned char ch;
   long divisor = 100000;
   for(x=0;x<6;x++)
     {
     ch = digit[numb/divisor % 10];
     for(y=0;y<8;y++)
       {
       RC5 = 0;
       if(ch&0x80) RC5 = 1;
       RC6 = 1;
       NOP();
       RC6 = 0;
       ch<<=1;
       }
     divisor/=10;
     }
   RC7 = 1;
   NOP();
   RC7 = 0;

   }
void displaystring(void)         // this routine prints through the screen buffer
   {                   // moving one pixel at a time

     clr();
     sprintf(ledstr,"SUN OCT24");   // Clear the display buffer
     strput(ledstr,0,0);         // adjust the scrolling string
   
     Blit();               // pass to screen buffer
     __delay_ms(800);           // time to view

   }

void main(void)
   {
   int sx,sy;
   long loop = 0;
   int xdir=1, ydir=1;
   ADCON1 = 0x6;             // ALL digital
   T2CON = 0x1e;             // T2 on, 16:1 pre scale
   PR2 = 60;               // timer preload value ( equates to 1.4mS with 20mhz crystal)
   TMR2IE = 1;               // enable timer 2 interrupt
   PEIE = 1;               // enable peripheral interrupt
   GIE = 1;               // enableglobal interrupt
   TRISB = 0;
   PORTC = 0;               // Port B as output...
   TRISC = 0;               // Port C as ouput...
   displaystring();
   while(1)
     {
     clr();
     sx += xdir; sy += ydir;   
       
  strput((char *)"]",sx,sy);

     if(sx>60) xdir = -1;
     if(sy>5) ydir = -1;
     if(sx<0) xdir = 1;
     if(sy<0) ydir = 1;
     __delay_ms(60);
     if(loop & 0x080)
        Blit();
     else
       {
       clr();
       sprintf(ledstr,"  %06ld",loop);
       strput(ledstr,0,0);
       Blit();
       }
     number(loop++);
     }
   }   // End main