Pointer Tagging for x86 Systems
Years ago, someone said "32 bit addresses!  Thats huge!  Lets use the top few bits for other stuff, like gate A20 and select lines of hardware".That came to bite people in the form of the "3GB hole", the "PCI hole", etc.    And those hacks were painful for lots of people.I feel that by reusing address bits for other purposes in a non-flexible way like this, we're just repeating the mistakes of history.   After all, there are 44 zettabytes of data in the world (in  2020), and addressing that is already beyond what a 64 bit number can do!And one day, we'll have that much storage in your hand - your phone has about 2^80 atoms in it, so storing 2^64 bytes in there is totally physically possible.