> Correct me if I'm wrong, this isn't emulation right?
> Could this be API substitution? Microsoft intercepting API calls and running a native
> machine equivalent? My knowledge about Win32/64 programming is almost null.
> I recall that's what CXBX was doing, translating DirectX calls from certain SDK
> versions to native calls. But I could be wrong.
If I had to guess I'd say it would end up being built on top of the (now retired) Windows XP mode that Windows 7 used, which in turn was built on Microsoft's Virtual PC code.
Add some kind of code analysis on top of that, looking for common code and applying common optimizations, and yeah, maybe some API call replacement and you might have some level of playable XBox assuming you have decent hardware.