14.4.1 Native threads on Windows

Each Lisp mp:process has a separate native thread.

Lisp code can only run in one thread at a time and uses a lock to enforce this.

When running a Lisp executable, you can only call back into Lisp in a thread that was created by Lisp.

When running a Lisp DLL, you can call back into Lisp in any thread.

LispWorks User Guide - 8 Apr 2005