What's difference between memory_order_relaxed and no memory order in c++0x?

What's difference between memory_order_relaxed and no memory order in c
++0x?

/*
The following code sample is from
http://www.justsoftwaresolutions.co.uk/threading/memory_models_and_synchronization.html
*/

std::atomic<int> x(0),y(0);

void thread1()
{
    x.store(1,std::memory_order_relaxed);
    y.store(1,std::memory_order_relaxed);
}

void thread2()
{
    int a=y.load(std::memory_order_relaxed);
    int b=x.load(std::memory_order_relaxed);
    if(a==1)
        assert(b==1); // no guarantee that b==1 when a==1
}

std::thread t1(thread1);
std::thread t2(thread2);

--------------------------------------------------------------------------------------------
If I replace the atomic<int> with int ( as the following code). what
will happen??

int x(0),y(0);

void thread1(){
    x=1;
    y=1;
}

void thread2(){
    int a=y;
    int b=x;
    if(a==1)
        assert(b==1);
}

std::thread t1(thread1);
std::thread t2(thread2);

------------------------------------------------

In the above two samples, there is no guarantee that b==1 when a==1
in thread2.

Therefor, what's difference between memory_order_relaxed and no memory
order in c++0x?

Does memory_order_relaxed prevent c++ compiler from code reordering or
anything else ??