From the page: "YARV is Yet Another Ruby VM. But it is the official Ruby VM since Ruby 1.9. It is a stack-based VM which runs the YARV bytecode, or intcode because each element of the instruction sequence is in fact stored as an int.
There is an instruction table for YARV on the original YARV homepage, but it is out-dated now. So I build a new one and make it publicly available in case some one would be interested in."