Running a diff between SVN working copy and BASE rev

How do you run a diff between a SVN working copy and the BASE rev from the terminal? I tried to acomplish this using chdiff, but that command only accepts <file1> and <file2> as args. How do I tell it to run a diff between my working copy of a file and the BASE rev of that file so I can see all local changes before doing a commit?</file2></file1>