[gpvos]

It's more like Wine: the code runs directly on the hardware, but there is an extra layer that emulates the system calls. Except that in this case, the emulation has a lot of supporting code in a kernel driver, something that Wine doesn't have. (Actually, for Win32 programs there is also such a layer, however that layer and the NT kernel were designed together from the start, so it can be pretty thin.)

[WorldMaker]

It's an interesting fuzzy gray boundary between the Ubuntu on Windows and "emulation". It's still built as an NT sub-system (like Interix was), and it's still the NT kernel ultimately in charge of everything. The difference seems to be the NT kernel implementing the generic POSIX standard versus the NT kernel implementing the specific system calls of the Linux kernel. So on one hand it is an emulation because there are specific real world binary behaviors (and quirks and bugs) it's trying to replicate, versus implementing standards from a specification document, but on the other hand, it still seems to be the NT Kernel doing NT Kernel things.

[quietplatypus]

I guess the line is a bit blurry, especially when you factor in fast virtualization techniques that involve running emulated instructions directly :)

[quietplatypus]

Gotcha. Thanks for the explanation! Which would you say was harder, Wine or this? It seems like Wine should have been harder since the NT kernel was designed with multiple userspaces in mind...

[gpvos]

Wine is basically reverse engineering a badly documented black box. On the other hand, even though the Linux source is available, Microsoft programmers probably aren't allowed to look at it to prevent their code from becoming a derivative GPL'ed work.