In the first pass, the team collects GPU shaders, and what Stillwell describes as 'Enlightenments', a term we hadn't come across before. "Enlightenments are our starting point for making game transcompilation better," he says. "This is more a CPU requirement than a GPU one. Enlightenments tell us what instructions wrote to hardware, function entry points, and so on. Basically, the set of information we need to know upfront when we first build."
Helping the task immensely is the fact that certain aspects of the Xbox 360 hardware design are indeed built into the Xbox One processor - specifically, support for texture formats and audio. "It's what makes this sort of possible for us, because then we can take all of those shaders that we collect and we can package them and all the Enlightenments, and then we just go through and we do actual performance playthroughs to determine that the emulator is executing everything right."
It's not an easy task because fundamentally, the Xbox 360's PowerPC processor is worlds apart from Xbox One's x86 foundation. Floating point calculations need to be adapted from 40-bit to 32-bit, with potential implications for aspects like collision detection, but Microsoft's aim here is clear - to be able to host game code on their virtualised Xbox 360 and for it to run as close as possible to original hardware.
But fundamentally, Xbox 360 back-compat works on the principle of an emulation layer. There is some hardware assistance and - yes - some 'secret sauce' (Microsoft didn't want to be drawn on how the emulator supports Xbox 360's VMX128 vector units, for example), but the team is now at the point now where everything an Xbox 360 can do, its emulator can mirror in software.