How does Thread.MemoryBarrier() flush cache?

Question

Thread.MemoryBarrier() makes the program to fetch value from main memory instead of using registers, but if we have the following programs:

_a = xxx;
_b = xxx;
Thread.MemoryBarrier();
_c = xxx;
_d = xxx;

does all _a, _b, _c, _d value all fetched from main memory or only the wrapped variables _b and _c are fetched from main memory, or only _c and _d which after the barrier are fetched from main memory?