This is a LISP interpreter that fits in a single 512-byte floppy disk sector (and can thus be directly booted by x86 BIOS!)
The authors, Justine Tunney with some collaborators in Scott Wolchok, Ilya Kurdyukov, the pseudonymous Moonchild, and Alain Greppin, pared a LISP down to an absolutely minimal implementation that had the critical property of being able to interpret the Meta-circular Evaluator from John McCarthy’s LISP 1.5 paper – thus establishing that it is capable of becoming a full LISP. A demonstration program was chosen to establish this fact, and the pruning began. The resulting interpreter does not even have a numeric representation; the claim is made that that’s fine, because we always have Church Numerals or similar encodings if we need them! A bold, but obviously computationally correct, claim!
The description in the above link is very thorough and includes some demo graphics(/videos?) of the interpreter operating under Blinkenlights, Justine’s extremely verbose and fascinating x86 emulator (which is probably worth its own post). As a bonus, it contains the complete listings for both SectorLISP and McCarthy et al.'s LISP 1.5 for comparison’s sake!