#include <reg51.h>

int a=0;
sbit datas = P1^0;

void int_ex0(void) interrupt 0
{
 a++;  //breakpoint here
}

void main(void)
{
 EA=1;   //enables all interrupts
 EX0=1;  //enables int0

 INT0=1;
 datas=1; //both to high, so keyboard can send

 while(1);
}