More in-depth on the backwards compatibilty:
In an
interview with Bit-Tech.Net, Richard Harris of ATI has revealed the very first details on how backward compatibility will work on the Xbox 360. Explaining the role ATI hardware will play in the process, Harris states:
“They have implemented compatibility purely through emulation (at the CPU level). It looks like emulation profiles for each game are going to be stored on the hard drive, and I imagine that a certain number will ship with the system. They already have the infrastructure to distribute more profiles via Live, and more and more can be made available online periodically.” The Director of European Developer Relations at ATI and former nVIDIA employee explained that the most difficult part of making backward compatibility possible will be with the graphics:
“Emulating the CPU isn’t really a difficult task. They have three 3GHz cores, so emulating one 733MHz chip is pretty easy. The real bottlenecks in the emulation are GPU calls – calls made specifically by games to the nVIDIA hardware in a certain way. General GPU instructions are easy to convert – an instruction to draw a triangle in a certain way will be pretty generic. However, it’s the odd cases, the proprietary routines that will cause hassle.” 
Harris also explained that contrary to what most people think, the RSX (featured in the PlayStation 3) is not more powerful than the
Xbox 360 graphics chip ATI has developed for Microsoft. So, although the RSX has a higher clock speed, the different nature in both consoles’ shader architecture makes a direct comparison absurd.
“That mere 10% clock speed that RSX has on Xenos is easily countered by the unified shader architecture that we’ve implemented,” explains Harris.
“Rather than separate pixel and vertex pipelines, we’ve created a single unified pipeline that can do both. Providing developers throw instructions at our architecture in the right way, Xenos can run at 100% efficiency all the time, rather than having some pipeline instructions waiting for others. For comparison, most high-end PC chips run at 50-60% typical efficiency. The super cool point is that ‘in the right way’ just means ‘give us plenty of work to do’. The hardware manages itself.” Harris also discussed other interesting topics and gave his own word on the much discussed superiority of the PlayStation 3 over the Xbox 360. To read the complete interview, click
here.