In fact, today already there are CPUs that support larger address spaces. But OSes should be smart and only provide pointers with the top 16 bits free while the

In fact, today already there are CPUs that support larger address spaces. But OSes should be smart and only provide pointers with the top 16 bits free while the application fits - even if some programs in the future end up using >256TB of address space, not all will, and those that don't should be able to take advantage of 48-bit pointers. Moreover, a single program could even take advantage of a "low heap" that stores metadata, and can have 48-bit pointers pointing to it, and a "high heap" of the actual large data (whose addresses take the full 64 bits, but may include tagged pointers to the low heap without doubling the memory footprint of the already humongous 256TB)