CDR-coding might be practical on modern architectures after all. for example, on x86, put these CDR bits at the top of a word: 00 EOL cdr of this cell is NIL 01 NEXT cdr of this cell is next cell 10 ATOM this is not a list cell 11 GONE this cell has moved elsewhere and use the following access code fwd mov eax,[eax*4] ; don't let FWDs into regs cmp eax,$C000000 jae fwd ret car mov edx,[eax*4] ; guaranteed not FWD test eax,eax ; so if MSB set, jc rtn ; must be ATOM mov eax,edx cmp edx,$C000000 jae fwd rtn ret cdr mov edx,[eax*4+4] shl eax,1 jns NIL mov eax,edx cmp edx,$C000000 jae fwd ret by and large, the mov will shadow the cost of testing - so that essentially the latter will be free note that there's no code to denote a CONS cell. i'm not sure when i realised that you don't need one - that it's sufficient to have CDR-NEXT and FWD, because you can make a CONS cell out of [ NEXT FWD ], and as long as you always follow FWDs somewhere (a good strategy, which is implemented above, is to never allow them into CPU registers), they're transparent. it might make sense - and save some space - to put $C0000000 into a register - say, ebp - so comparing it takes two bytes instead of 6. especially when open-coding functions (gods, i wish Intel hadn't removed the conditionall calls and returns from the 8080 instruction set...!)