Same author did the synthesis kernel with Carl Pu, I think? The Synthesis kernel was genius, but relied on self modifying code, so can't work with modern chips well.
update: oh, I just remembered why it _is_ an issue for modern processors, Massalin leaned very heavily on the general purpose double compare-and-swap that was present on some generations of the 68k architecture, and x86-64 only has double contiguous compare and swap