Lisp Machine Emulator

A scratch-built emulator for a subset of the early-80s Lisp Machine instruction set. The goal was never compatibility — it was to find out what the day-to-day felt like, from cold-boot to a working REPL, on top of a modern Lisp runtime.

Boots a minimal ROM image, exposes a tagged-memory model, and drops the user into a REPL that can LOAD small Lisp files. Garbage collection is a stop-the-world mark-and-sweep; no compaction.

(defun gc ()
  "Walk roots, mark reachable cells, then sweep the unmarked free list."
  (mark-roots)
  (loop for region in *heap-regions* do
        (sweep region)))

I learned what I came for; performance plateaued where reasonable tuning ran out, and adding a JIT would have been a different project. The code still builds and runs on SBCL; it's archived rather than abandoned because the existing surface is intentional and works.