There's absolutely no reason it _has_ to be one-way: It's not like the CPU intrinsically speaks x86_64 or is directly attached to memory anyway. When inventing

There's absolutely no reason it _has_ to be one-way: It's not like the CPU intrinsically speaks x86_64 or is directly attached to memory anyway. When inventing a new ISA we can do anything. And if we're talking about memcpy over (small) ranges that are likely still in L1 you're definitely not going to notice the difference.