What is leJOS or TinyVM?

Like its predecessor (TinyVM), leJOS is a tiny JavaTM runtime written by Jose Solorzano. It’s function is a replacement firmware for the Lego Mindstorms^TM RCX, a programmable brick which is part of Lego's Robotics Invention System^TM (RIS). The RCX contains a Hitachi H8300 processor and 32 Kb of RAM (of which 28 Kb are safe for the firmware to use).
TinyVM's focus differs from that of leJOS. TinyVM is a low footprint replacement firmware (< 10 Kb), whereas the focus in leJOS is completeness, and performance to some extent. The current footprint of leJOS in the RCX is 17 Kb.
leJOS allows us to experiment without breaking TinyVM. It gives us a chance to try out a different design for the API, a different usability model, and so on. TinyVM gives an idea of how small a reasonable Java runtime can be, while leJOS tells us about the amount of features that can be put in an RCX Java runtime before space runs out.
The following shows what leJOS and TinyVM both have in common:

• Object oriented language (Java).
• Preemtive threads (tasks).
• Arrays, including multi-dimensional ones.
• Recursion.
• Synchronisation.
• Exceptions.
• Well-documented API.

Both JavaTM runtimes offer complex API’s to control the RCX. This includes for example the controlling of motors, sensors, LCD and Buttons and further on a sensorlistener and the communication between several RCX-Bricks or between RCX and PC.
There's a Wired News article about Lego Mindstorms at where Jonathan Knudsen, who has written a couple of programs for leJOS, is quoted as saying that leJOS is "very cool stuff." (Jonathan Knudsen is the author of The Unofficial Guide To Lego MindstormsRobots). (http://www.wired.com/news/culture/0,1284,39504,00.html)