C言語でレジスタストレージクラスとは何ですか?

#include<stdio.h>
main ( ){
   register int i;
   for (i=1; i<=5; i++)
      printf ("%d ",i);
}

1 2 3 4 5

#include<stdio.h>
int main(){
   register int a;
   printf("%d",a); //prints default value of a =0
}

0

#include<stdio.h>
int main(){
   register int i = 10;
   int *p;
   //int *p = &i; //error occurred ,here we are trying to request address of register    variable
   printf("Value of i: %d", *p);
   printf("Address of i: %u", p);
}

Error:add of reg var?