That's what I was thinking of too; glad you mentioned it. jonesforth consists of 2 files: an assembly language file which implements the minimal commands to be able to load the rest of the language (in forth) from the second file. It is written in literate programming style so that the comments are the documentation which teaches you all about it.

Since then I've often daydreamed about implementing a forth OS on something like the Raspberry Pi (just another project I might not get around to). I'm looking forward to reading the linked article as well.