On amd64 (and IIRC also Alpha) hardware checks that unused bits of address are all equal to highest used bit (or IIRC all zero on Alpha) to discourage this kind

On amd64 (and IIRC also Alpha) hardware checks that unused bits of address are all equal to highest used bit (or IIRC all zero on Alpha) to discourage this kind of tricks. And I vaguely remember that ESA/390 is actually 32bit address architecture in hardware and it's 31bitness comes from using highest bit of address as a flag that such tricks are taking place.