The actual Lisp interpreter is rather tiny compared to the other stuff. Just do a 'ls -l *.c | sort -gk 5' in the src directory. By far the largest file is xdisp.c, which is the redisplay engine, and only a handful of people in the world dare to touch it. Next comes input handling (keyboard.c), then then dealing with coding systems, terminals, win32-API, image handling, windows, processes, buffers, and much more. AFAICS the first file to actually deal with Lisp is lread.c and comes in at #21.