That's not actually any better speed-wise. And the CPU would still have to microcode the copy because of caches (think of what involvement the memory controller has in doing a cache to cache copy)

The real gain in being able to have the memory controller do a memcpy() independent of the CPU would be to let the CPU operate on data out of its caches in parallel to the memcpy() being executed. But that only helps for a very specific class of memcpy() and is highly system dependent (you have to worry about the expense of keeping caches coherent among other things.) Anyway, an integrated GPU or other additional block of hardware behind the memory controller is a better candidate for this sort of thing than a user-level CPU instruction.

Also, use a better libc.