Correct. x86-64 mandates “canonical” addresses where all the upper bits must be the same in order to dereference a pointer.[0] So on a processor with 48 physica

Correct. x86-64 mandates “canonical” addresses where all the upper bits must be the same in order to dereference a pointer.[0] So on a processor with 48 physical address lines, the upper 16 must be either all set or all unset. This is obviously to prevent people using those upper bits to store extra data. Of course, this doesn’t stop you from being able to[a], it just makes it harder. [a]: The processor doesn’t know the “type” of a register’s value; It’s just bits. It could be a pointer, integer, floating point, etc. until you tell the processor to do something with it. So one could obviously use those extra bits to store data, then when a dereference is needed, store those bits in another register, dereference, then put the bits back. But that’s a lot of work to save a byte or two. [0]: Intel SDM Volume 1 §3.3.7.1: > In 64-bit mode, an address is considered to be in canonical form if address bits 63 through to the most-significant implemented bit by the microarchitecture are set to either all ones or all zeros.