How do atomics larger than the CPU's native support work

With current C++ compilers you can have atomic support of atomics that are larger than the actual support of your CPU. With x64 you can have atomics that are 16 bytes, but std::atomic also works with larger tuples. Look at this code:

#include <iostream>
#include <atomic>

using namespace std;

struct S { size_t a, b, c; };

atomic<S> apss;

int main()
{
    auto ref = apss.load( memory_order_relaxed );
    apss.compare_exchange_weak( ref, { 123, 456, 789 } );
    cout << sizeof ::apss << endl;
}

The cout above always prints 32 for my platform. But how do these transactions actually work without a mutex ? I don't get any clue from inspecting the disassembly.