New Details Emerge On Microsoft's XBox 360 "Fusion" Processor
The XBox 360 is an infamous example of the problems manufacturers can encounter when pushing the performance envelope—the initial XBox consoles weren't designed with sufficient thermal tolerances which lead to the infamous RROD (Red Rings of Death). It's been several years since Microsoft patched that particular problem, but the new XBox 360 S marks the first time the console's Xenon CPU and ATI-built GPU have been built on the same die. What makes this particular fused processor so interesting is the inherent difficulty of its creation. When AMD and Intel set out to design their single-chip hybrids, they control all the IP for both parts and have experience in designing modular components. For IBM and Microsoft, collaborating on the new XBox 360 processor wasn't so simple. In order to build the two designs on a single die, IBM had to first flip its core design 90 degrees become intimately familiar with the ATI-designed GPU core, and then build a fused core that exhibited exactly the same latencies, intra-chip communication speed, and real-world performance.
That's where the "FSB Replacement" block in the diagram above comes in. Ordinarily, a CPU designer would crow over the latency advantage inherent to moving additional components on-die. In this case, however, the introduction of any change—even a change that should improve performance—could be detrimental to a game. Unlike in computing, games are often coded 'to the metal,' which means they're written to take specific advantage of unique functions (and often errata) of a particular type of processor. Ironically, this means that preserving the <i>bugs</i> of a chip when moving from one process technology to another is critical. To that end, there's the FSB replacement, whose job it is to introduce the appropriate latencies. Note that while the CPU and GPU are built into a single chip, the core's 10MB of EDRAM actually sits on package. This almost certainly reduces cost; building 10MB of on-package cache would've significantly increased die size and forced IBM to throw out an entire core if the 10MB chip was flawed.
Compared to the original XBox 360, the new chip uses 60 percent less power and 50 percent less space. Given the fact that MS and Sony have both implied their consoles have a multi-year future ahead of them, we could see at least one more die shrink to 28nm technology before this generation runs out of life. For an unrelated (but timely) bit of news on how subtle differences between consoles can impact game performance, Shacknews has a recent story on how gamers using certain older XBox 360's ran into stuttering problems and slow cutscenes when watching the intro to <i>Shank</i>. According to the game developers:
"The first is that it appears that older Xbox 360's have slower disc access rates, so the opening cinematic chugs, and loading between levels takes longer - when we tested this on development kits, this never came up so that took us completely by surprise, and we're looking into it. Note that the actual game experience is not affected in any way."