I'm pretty sure Lisp OSes (you know, on the bytecoded Lisp Machines from companies such as Symbolics and TI) worked like that, and I know the CISC variants of t

I'm pretty sure Lisp OSes (you know, on the bytecoded Lisp Machines from companies such as Symbolics and TI) worked like that, and I know the CISC variants of the AS/400 architecture worked like that as well. (I don't know the RISC AS/400 or iSeries that well.) An interesting aspect of that is what it does to your security model: In the AS/400 world, where applications are compiled to bytecode and then to machine code, the software that compiles to machine code is trusted to never emit dangerous machine code, as there are no further checks on application behavior. In the Lisp Machine world, anyone who can load microcode is God. In Singularity OS, the .Net runtime is effectively the microcode and the same remarks about Lisp Machines apply.