+;;;
+;;; Another way to look at lowtags is that there is no one lowtag
+;;; length. On 32-bit platforms, fixnums and other-immediates have a
+;;; lowtag length of two bits, and pointers have a lowtag length of
+;;; three bits. On 64-bit platforms, fixnums and pointers gain an
+;;; extra bit, and six "pad" lowtags waste the extra encoding space so
+;;; obtained.
+;;;
+;;; x00 -- fixnum
+;;; x10 -- other-immediate
+;;; 001 -- instance-pointer
+;;; 011 -- list-pointer
+;;; 101 -- fun-pointer
+;;; 111 -- other-pointer
+;;;
+;;; If you change the tag layout, check the various functions in
+;;; src/runtime/runtime.h to see if they need to be updated, along
+;;; with print_obj() in src/runtime/print.c, possibly gc_init_tables()
+;;; in src/runtime/gc-common-c and possibly the code in src/code/room.