What exactly does a non-shallow filecmp.cmp do?

Consulting the source filecmp.py reveals that if shallow=False, filecmp.cmp first checks a few select properties of os.stat(), regardless of whether shallow is True or False. If the stat properties that are examined are the same, it returns True. Else, it checks its internal cache to see if the files have already been compared earlier. If it has, it returns True. Else, it reads BUFSIZE = 8*1024 chunks of data from both files and does an exact contents comparison until it reaches the end of the file. It returns True if the two files have exactly the same contents.