performance in 64 bits machines.

   From: "P.U.Kruppa" <ulrich at pukruppa.net>
   
   So this is what I did, but how can I find out if it uses 64-bit 
   instructions?
   
Try (integer-length most-positive-fixnum)

This will provide a valuable hint about the implementation native word
size, although there is of course nothing in the standard whose
purpose is to tell you the details of data representation.  The intent
portable language definition is to insulate you from these things.