|Inside the Intel Sandy Bridge Microarchitecture|
|The Ring Architecture|
Sandy Bridge-based processors will have a ring architecture for the internal components of the CPU to talk to each other. When a component wants to “talk” with another component, it puts the information in the ring and the ring will move this information until it reaches its destination. Components don’t talk to each other directly, they have to use the ring. Components that use the ring include the CPU cores, each L3 memory cache (which is now called Last Level Cache, or LLC, and is not unified, see Figure 5), the system agent (integrated memory controller, PCI Express controller, power control unit, and display), and the graphics controller.
In Figure 5 you can see the ring (black line) with its “stops” (red boxes). It is important to understand that the ring is physically located over the memory caches (imagine a ski lift where each red box is a stop) – since the illustration is bi-dimensional, you may have the impression that the ring wires run inside the cache, which is not the case.
Also, each last level cache isn’t tied to a particular CPU core. Any core can use any of the caches. For example, in Figure 5, we have a quad-core CPU with four last level caches. Core 1 isn’t linked to cache 1; it can use any of the caches. This also means that any CPU core can access data that is stored in any of the caches.
click to enlarge
Figure 5: The ring architecture (black line with red “stops”)
There are actually four rings: data ring, request ring, acknowledge ring and snoop ring. They run at the same clock rate as the CPU internal clock. It is based on the QPI (QuickPath Interconnect) protocol, the same one used by socket LGA1366 CPUs to talk to the chipset.
Each component decides when to use the ring, if empty, and the ring always choose the shortest path to the destination.
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