"a lot of old Lisp is probably reasonably close to parseable" I think you would be hard-pressed to find any old Lisp code that a modern READ cannot turn into a

"a lot of old Lisp is probably reasonably close to parseable" I think you would be hard-pressed to find any old Lisp code that a modern READ cannot turn into a list which could trivially be transformed into a valid s-expr in that dialect. The reverse is much less true, since modern Lisp-like languages allow many more characters, are often case-independent, and often have standard "decorations" (e.g. &REST or #:optional). Common Lisp is so kitchen-sinky that you'd be hard pressed to find old Lisp code that was then reasonably portable that will be outright hard to port to a modern CL implementation. Most of it will probably run as-is, or with a thin macro package doing mechanical translations. Unportable code is unportable code. If it was so tied to e.g. Allegro that it wouldn't run in any other CL environment, that's hardly the language's fault. However, really early Lisp stressed the "S" part, and rarely did anything strongly system-specific.