This is FreeWPC, an open source operating system for the Williams Pinball Controller (WPC) family of pinball machines. FreeWPC is licensed under the GNU General Public License, Version 2.
The sources for the core routines are located in 'kernel'; other common code is in 'common', and game-specific code can be found in a subdirectory based on the machine name, under 'machine'. There are also a number of tools included with the distribution for generating some of the source code and for building ROM images that are under 'tools'.
You will also need several tools not here in order to build. See the file doc/build.html for more detailed information. Most importantly, you'll need a copy of the gcc6809 cross-compiler. Many standard UNIX utilities are needed as well. FreeWPC has been built successfully on Linux and Cygwin.
Email suggestions/comments/questions to brian@oddchange.com.
Visit the homepage at http://www.oddchange.com/freewpc for more information.