[kryptiskt]

Traditionally it has been done because the last three bits in an object pointer typically are always zero because of alignment, so you could just put a tag there and mask it off (or load it with lea and an offset, especially useful if you have a data structure where you'd use an offset anyway like pairs or vectors). In 64-bit architectures there are two bytes at the top that aren't used (one byte with five-level paging), but they must be masked, since they must be 0x00 or 0xff when used for pointers. In 32-bit archs the high bits were used and unsuitable for tags. All in all, I think the low bits still are the most useful for tags, even if 32-bit is not an important consideration anymore.