Computer programs are portable to the extent that they can be moved to new computing environments with much less effort than it would take to rewrite them. In the limit, a program is perfectly portable if it can be moved at will with no change whatsoever. Recent C language extensions have made it easier to write portable programs. Some tools have also been developed that aid in the detection of nonportable constructions. With these tools many programs have been moved from the PDP-11 on which they were developed to other machines. In particular, the UNIX operating system and most of its software have been transported to the Interdata 8/32. The source-language representation of most of the code involved is identical in all environments.