순환형태의 fib함수 질문

int fib(int n)
{
printf("fib(%d) is called.\n", n);
if(n==0) return 0;
if(n==1) return 1;
return (fib(n-1) + fib(n-2));

이 코드의 출력 결과가 이렇게 나오는데요.. 어떠한 과정으로 출력이 되는건지 이해가 안됩니다. 설명해주실 수 있나요?

fib(6) is called
fib(5) is called
fib(4) is called
fib(3) is called
fib(2) is called
fib(1) is called
fib(0) is called
fib(1) is called
fib(2) is called
fib(1) is called
fib(0) is called
fib(3) is called
fib(2) is called
fib(1) is called
fib(0) is called
fib(1) is called
fib(4) is called
fib(3) is called
fib(2) is called
fib(1) is called
fib(0) is called
fib(1) is called
fib(2) is called
fib(1) is called
fib(0) is called