Apple is using some of the bits in a normal 64 bit pointer for other purposes, something that's been done in production Lisps for a long time, no later than whe

Apple is using some of the bits in a normal 64 bit pointer for other purposes, something that's been done in production Lisps for a long time, no later than when the Vax appeared with its byte addressed pointers. E.g. a valid 32 bit pointer has the 2 LSB bits as zero, allowing for 3 other values and all sorts of tricks with them. By comparison, the CADR Lisp machine had a 32 bit word size, with 8 bits used for tags, and the remaining 24 bits used for immediate data or as a word addressed pointer. The lowRISC project proposes to add tag bits taken from other regions of memory that are fetched into a tag cache, and promoted as additional bits in the L2 and L1 caches and above. So starting with the L2 cache words will be internally 66 bits long.