To be fair v6 was able to be ported, it just wasn't as easy (I know because I ported it to native VAX) the compiler was the hard part, along with the evil context switch primitives
That comment is only partially related to those primitives - the primitives are used in a bunch of other places with more obvious functionality - it's the doing something strange because the SSWAP is set that you're not supposed to understand - where that bit is set is miles away and not obviously connected (plus there's that "return(1)" from a routine that no one who appears to call it uses the result).
V7's save()/restore() are more portable and easier to understand
