How the computer handles variables

global_variable.c

#include <stdio.h>

// this is a global variable.
int a = 1;

// Can be used within a function without declaration.
void changeValue() {
  a = 5
}

int main(void) {
  printf("%d\n", a);
  changeValue();
  printf("%d\n", a);
  system("pause");
  return 0;
}