8051 Atmel programming in C- Need help for one example

TH0=0x3C;   TL0=0xAF;

if (!(--counter)--)
{     
         a = 1 ;
                    }

#include <reg51.h>
#include <math.h>
typedef unsigned char byte;

byte a[16]={1,22,31,4,5,6,7,8,23,3,10,30,73,40,55,93};
byte b[16]={3,14,7,13,17,19,97,1,12,128,127,124,123,122,33,21};
byte c[16]={9,15,4,13,41,49,57,18,74,3,39,59,112,100,91};
byte d[16]={33,58,68,66,21,19,97,1,14,222,56,12,3,9,15,11};

sbit switcher=P0^0;
byte i=0;
byte counter;


byte xdata display _at_ 0x8001;
void Inic(void) {
   EA=1;
   ET0=1;
   TMOD=0x01;
   TH0=0xC3;
   TL0=0x50;
   TR0=1;
   counter=0;
   i=0;

}

void timer0 (void) interrupt 1 using 2 {
   
   TH0=0xC3;
   TL0=0x50;
   
if((++counter==0)) {    counter=20;
   if(switcher) {
   
   if(!(a[i]%3) )
     display = a[i];

     i++;
     if(i==16) i=0;
      if(!(b[i]%3) )
       display = b[i];

     i++;
     if(i==16) i=0;
       if(!(c[i]%3) )
       display = c[i];

     i++;
     if(i==16) i=0;
       if(!(d[i]%3) )
       display = d[i];

     i++;
     if(i==16) i=0;
   }}
   TR0=0;
}

void main(void) {
   Inic();
   while(1){TR0=1;}
   }