When comparing the architectures in Westmere and Sandy Bridge, you can see the significant changes Intel has made with the new Sandy Bridge architecture. As you can see from these illustrations, Westmere included a separate northbridge, which contained the graphics and other components. With Sandy Bridge, though, the GPU is on the same die as the CPU, removing one of the key components of the northbridge. In Sandy Bridge, the System Agent serves as the equivalent to the northbridge (what Intel in the past sometimes called the �uncore�). The System Agent contains the display engine, DMI (desktop management interface), memory controller, PCI-E 2.0, and PCU (power control unit). The PCU handles all of the power and thermal management for the chip. It also controls the turbo technology built into Sandy Bridge. The System Agent is connected to the ring bus (the black line and red blocks in the illustration that connect the LLC, cores, graphics, and System Agent), another new addition in Sandy Bridge. Using a ring bus in a quad-core Sandy Bridge chip provides four times the LLC (last level cache, a.k.a. L3 cache) bandwidth than was available in the previous generation.Source of Information : Computer Power User (CPU) December 2010
