![]() In fact, the Xbox One has a thread scheduler - it's similar to Windows in that it apportions based on need whereas the 360 was locked, it was very consistent," Stillwell explains. "What we found was it's not just the threads, the different cores. In theory, the Xbox 360's six hardware threads could be mapped to specific cores in Xbox One, but issues cropped up as the team took on more challenging games. ![]() The ambition in adapting code designed for a tri-core PowerPC CPU running at 3.2GHz on low-power, low-frequency x86 cores is remarkable. "So, there's a multiple playthrough session where we have an army of over 100 testers who go through, collect thousands of hours of video data and then from there we'll get back data that lets us tune, and so the emulator gets a little bit more robust every cycle that we go through with game releases." "The way we talk about it offhand is that effectively we try to make sure that the game never knows that it's not running on an actual 360 console, and so whatever value the game presents, we return the value that it expects to get from the 360 hardware and we just do all that in software," Stillwell continues. How Does Xbox 360 Backwards Compatibility on Xbox One Actually Work? Rich presents a video overview of Xbox 360 compatibility for Xbox One, and how the 4K and HDR-enabled X-enhanced 360 titles work. 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. It's not an easy task because fundamentally, the Xbox 360's PowerPC processor is worlds apart from Xbox One's x86 foundation. ![]() "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." 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. Basically, the set of information we need to know upfront when we first build." Enlightenments tell us what instructions wrote to hardware, function entry points, and so on. "This is more a CPU requirement than a GPU one. "Enlightenments are our starting point for making game transcompilation better," he says. In the first pass, the team collects GPU shaders, and what Stillwell describes as 'Enlightenments', a term we hadn't come across before. "We take each game, we recompile it so that it runs, but basically we're running it still in a 360, and the team goes through the game with multiple passes." "Basically, we have a VGPU - or an Xbox 360 GPU that we've recompiled into x86 - and we run the entire 360 OS stack," explains Bill Stillwell, Xbox Platform Lead. There's also been talk of some level of hardware compatibility integrated into the Xbox One processor to make the job easier, but beyond that, technical details on the process are thin on the ground - until now. So how was this all achieved? How does back-compat actually work?Īcross the years, we've only picked up a few titbits on the process - essentially that the original Xbox 360 PowerPC executables are reverse-engineered into an intermediate, then recompiled into x86. Following an impressive early access period, the backwards compatibility program rolled out in full force, and hundreds of Xbox 360 titles are now available to play on Xbox One, with a clutch of enhanced 4K games now available for Xbox One X. Two-and-a-half years ago, Phil Spencer took to the stage at Los Angeles' Galen Centre to showcase the original Xbox 360 version of Mass Effect running on Xbox One hardware. One of the most memorable of E3 reveals also happens to be one of the greatest technological achievements of the current-gen era.
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