Читаем этот абзац и... Понимаем, что микрокод

Finally, it has been asked, whether it would be possible to create your own version of microcode, which could in example be optimized, or even add more functionality, eg. new instructions. This question can be answered with a no. To create such a microcode some would need information about the interna of the target microprocessor, which exceed the level of all known documentation. The only possible way is, if those, who originally created the microprocessor, do create the microcode updates.