This is why, by default on 32-bit architectures, fixnums in GNU Emacs are 30 bits:

http://stackoverflow.com/questions/106597/why-are-fixnums-in...

http://www.gnu.org/software/emacs/manual/html_node/elisp/Int...

SBCL has a more elaborate tag bit system:

http://sbcl-internals.cliki.net/tag%20bit