But then those accesses have to be protected by a lock in order to enforce invariants between "start" and "end". The whole point of representing both with a single atomic value is that this makes it trivial to preserve those required invariants.
Because the queue grows only in one direction and it is consumed in the other direction, reading a stale value from the other side is still safe, so two-word atomicity is not needed.
In fact the other side value is normally cached in a local memory location so to amortize the cost of roundtrips over multiple reads and writes.
