The tagged pointer/value trick (using the fact that allocations are aligned to boundaries thus allowing for 2-3 bits of information you can use to tell which is

The tagged pointer/value trick (using the fact that allocations are aligned to boundaries thus allowing for 2-3 bits of information you can use to tell which is which) is a trick we used in my first MacOS app 30 years ago, though we used bits at other end since you couldn't allocate the full address space on those old Macs.