The UCSD P-system was my first operating system. When I used it (1981-1987), I didn’t even know the term, “operating system.” I just knew I had files, a compiler, an editor, and a way to execute programs. The mental image I have of the P-system is not that of a desktop (like modern systems) but rather that of a series of rooms. In some ways, this seems like a richer experience than the desktop metaphor. The mind fills in imaginary graphics the same way it creates pictures when one is reading a book.