Concurrency is an interesting topic. I hadn't thought of that when reading about SubX, which mostly covered the part of assembly/machine language I'm familiar with. I always wished, though, that my assembly class (or an advanced sequel) had covered some of the more advanced topics for modern processors instead of the 8080, like how to set up memory management (the boot process on newer CPUs is rather involved...) and how multicore computation works. I know all about the higher-level implications, and can use threads etc., but have never actually learned how those things are handled at the bottom. Or syscalls etc.; I've only ever used them, not implemented them. That's a bit out of scope for the discussion, but something I look forward to "seeing-through" your future work on accessible internals. :)
I guess part of my curiosity was targetted at how you would build Mu up to handle performance and portability. Although the way they are usually implemented does obscure the internals, it seems possible that some compromise might be found. Any thoughts?
Perhaps the answer is at the higher lisp (AST / intermediate representation) level. Mu and SubX stay perfectly aligned to the final binary, but a code generator could optimize the AST and generate code in Mu for the target platform.
On that note, have you thought about giving them a more accessible API for such purposes? Or even just an s-expression syntax? Otherwise the target format for generation is limited to text, which would lose a lot of semantics in translation. Also, how are you thinking of maintaining introspection continuity across levels as you add more layers? Maybe something simple like source map support?
At the moment my ideas on all these areas are extremely hand-wavy. Ranging from "you don't really need it," to "somebody else's problem" :)
I feel fairly strongly that I don't want to add very many more layers. Ideally just one more. As more high-level languages get added, I want the lower levels to be integrated into Mu, so that they can in turn become the new HLL. But that's just an aspiration at this point.
One thing somebody else suggested was Plan9's assembly syntax. It may provide some inspiration for making SubX portable.