ok There's an operating system on your SUN and/or Macintosh systems that you've probably never heard of even though it runs before the regular operating system is loaded. That mystery operating system is FORTH.
FORTH is the programming language used to write the FORTH operating system and it compiles itself. FORTH is an odd beast in the programming world. Due to its odd characteristics which include compactness and simplicity, it's great for firmware programming and embedded systems.
If you're tired of not quite understanding OPENBOOT or the firmware process, there are many great and free resources to help you (thanks to the friendly FORTH community) :
Starting FORTH http://www.forth.com/starting-forth/index.html
A great introduction to FORTH, stack programming concepts, and post-fix notation.
Writing FCode Programs http://docs.sun.com/app/docs/doc/801-7050
SUN's guide for firmware programming.
Thinking FORTH http://sourceforge.net/projects/thinking-forth/files/reprint/rel-1.0/thinking-forth-color.pdf/download
Discusses broader topics such as the software development cycle and experienced programmers/developers' views of FORTH.
More information and resources: http://wiki.forthfreak.net/