Originally posted by: I4AT
Because MS did a shitty job of emulating them, every Xbox game would run fine on the 360 if time and effort were put into them, but MS did a hack job because they never really gave a shit about backwards compatibility in the first place. They promised 100% BC before the 360 was released to draw sales and have an excuse to fully drop support for the Xbox, they made a few crap releases to keep their customers from bitching too much, then dropped it altogether.
If you've followed the development of many emulators, youll find that they reach a certain point where a huge amount of games run just fine, but theres a number of games that dont quite conform to the norm and dont emulate right, and it takes years if ever to get them working. Various fixes and parameters need to be enabled for these specific cases in order to get them to work right - playstation 1 emulation is a perfect example of this.
Its much more difficult to emulate a system when the architectures differ so wildly. The xbox CPU is single threaded and uses out of order execution. The 360 CPU is capable of running 6 threads between three cores, and uses in-order execution. Its my understanding that you cant use multiple threads to speed up emulation of a single thread, so theyre basically already gimped from the start. And thats not even getting to the instruction set or order of execution differences.
Granted, they dont have to emulate the CPU when they can translate the code much easier, but the performance of some games could be bizarre if they hit architectural bottlenecks that wouldnt have existed on the XB1. The point is, they did a very impressive job in getting it working as fast as it does already. This is far from a hack job, but there are going to be some limitations when youre doing this kind of software BC.
Sony took the easy way out by just leaving in the PS2 hardware in the original PS3 revs, but even they went back on that. At least when you buy a 360, you know itll have some backwards compatibility. I follow console gaming pretty closely, and for the life of me I dont even know whether or not new PS3s actually have ANY BC.