XS: Lisp on Lego MindStorms

Lego MindStorms is a robot development kit which makes it possible to control one's own robot by attaching various sensors and motors to a programmable block, called RCX, with an 8-bit CPU. In this paper, we present a Lisp system XS which runs on an RCX block to control robots. Unlike previous Lisp/Scheme implementations for the MindStorms, the evaluator of XS runs autonomously on the RCX, with its own runtime stack 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 programming language based on Scheme, extended with functionality for interfacing with attached devices such as motors and sensors. It also supports mechanisms such as event waiting and asynchronous event interrupts that are necessary for controlling robots.