Is there any software package that can simulate the inner workings of a processor? The reason I am looking for this is that I am learning about the lower level concepts and I would like a more interactive environment. I am looking for a system which provides a working processor and a way of seeing it work. Additionally it would be nice if it could be modified.
I realise that the question is a bit vague but I have not found software which provides anything resembling what I want and since I work in higher levels I am not familiar with the tools.