When looking at (almost?) all the 64 bits CPUs out there, you see that the MSB of addresses are not really used. 48 bits give you 256 terabytes, which is about the RAM installed into a HPC rack these days. But so much space can't be really used conveniently because access times prevent efficient access across a rack or a datacenter. Message passing often takes over when communication is required across compute nodes or blades.

So we're left with these dangling bits... The addresses are usually "virtual" and require translation to physical space. But you know I hate wasted resources, right ? So I want to use the MSB as an "address space ID" and store some meta-information such as process number. It's easy to check if this address is valid so it adds some sort of protection while allowing the whole addressable space to be linearised.

Of course there is the question of "how many bits should be allocated" because there is always a case that will bump into fixed limitations. The solution is to make a flexible system with not just an ID to compare, but also a bitmask (this is used nowadays for comparing addresses in chipsets, for example). The OS will allocate as many bits as required by the workload, either 56 address bits for 256 tasks, or 40 bits for 16M tasks, or any desired mix (think : unbalanced binary tree)...