windows - why can MemoryBarrier be implemented as a call to xchg?

Question

on msdn http://msdn.microsoft.com/en-us/library/windows/desktop/ms684208(v=vs.85).aspx, MemoryBarrier is implemented as a call to xchg.

// x86

FORCEINLINE
VOID
MemoryBarrier (
    VOID
    )
{
    LONG Barrier;
    __asm {
        xchg Barrier, eax
    }
}

I can't find some material in "Software Developer's Manual". please tell me the reason.

Answer 1

Two things happen here:

1. The compiler is given an opaque block to insert into the output instruction stream. As it does not know which data is accessed inside the block, it cannot reorder the other statements around it.

2. The xchg instruction performs an atomic read-modify-write operation, which requires ordering on the memory bus, so the CPU enforces a memory barrier.