Arduino GCode Interpreter interprets G-Code commands sent from the computer. The official page for Arduino GCode Interpreter is here: Arduino G-Code Interpreter and it's useful to get an idea what it does and how to configure it. However, the release we're interested in is Chris Meighan's rewrite of the interpreter. It implements several nice things that original interpreter doesn't have, for instance hardware timer stepping, acceleration, true arcs, G-code lookahead.

You can get the firmware here: http://contraptor.svn.sourceforge.net/viewvc/contraptor/trunk/firmware/