Years ago The Commander and Bart Locanthi used JIT in the form of some C that write machine code into an array of chars, cast it to a function pointer, and called it. (I appologise if the details are not correct but this is the idea).
I have a need for such a thing again - trying to speed up a compressed video decoder. Anyone done such a thing this millenium? Does the x86 data execution prevention mean you just cannot do this - my targets are desktop OSs other than plan9 (sadly). -Steve