If you take a look at the block diagram above, you’ll notice that most of the blocks are fairly obvious … But the purpose of the “FSB replacement block” may not be obvious. This particular block essentially implements a kind of on-die “frontside bus” with the exact same latency and bandwidth characteristics as the older bus that connected the CPU and GPU when they were discrete parts.

The FSB replacement essentially lowers the performance characteristics of the SoC in order to retain the performance level of older Xbox 360 machines.