XS: Lisp on Lego MindStorms

We present a Lisp system XS which is designed to control RCX blocks of the Lego MindStorms Robotics Invention System (RIS). Unlike previous Lisp/Scheme implementations for the MindStorms, the evaluator of XS runs autonomously on the RCX, with its own runtime stacks and garbage-collected heap. It communicates with the front-end subsystem on a PC, to provide an interactive programming environment with features such as backtrace, function trace, and terminal interrupt. The evaluator supports a language based on Lisp/Scheme, extended with functionality for interfacing with RIS devices such as motors and various kinds of sensors. It also supports mechanisms such as event waiting and asynchronous event handlers for controlling robots built with RIS.