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New North Bridge – System Agent

The only functional unit of the new Sandy Bridge processors left to discuss is the so-called System Agent that contains all external interface controllers: PCI Express, DMI, memory and display interfaces. In fact, this System Agent is very similar to what we know as Uncore in Nehalem processors. However, System Agent in Sandy Bridge is nevertheless not sully identical to Uncore. It doesn’t contain the L3 cache, which is an individual functional unit working at the processor frequency in the new microarchitecture. Another distinguishing feature of this System Agent is that it also uses the ring bus to exchange data with the processor and graphics cores as well as with the L3 cache.

Speaking of the innovations in the System Agent we would first of all like to mention the long-anticipated improvement of the memory controller. The memory controller in Westmere (Clarkdale) processors was combined with the graphics core and this solution didn’t prove successful. They have finally eliminated this issue in Sandy Bridge: the new memory controller works at least as fast as the controller in Lynnfield CPUs. Note that it supports dual-channel DDR3 SDRAM: DDR3-1067 or DDR3-1333 according to the formal specifications. But in reality Sandy Bridge processors support multipliers that also allow clocking the memory at 1600, 1866 and 2133 MHz.

You can get an idea of how fast the memory controller in Sandy Bridge processors actually is from the following results of Aida64 benchmark:

The results courtesy of
They tested Core i7-2400 processor with
dual-channel DDR3-1600 memory (7-7-7-21-1T timings).

The latency of the memory sub-system in a computer with a Sandy Bridge CPU inside is comparable with the latency of a similar LGA1156 platform with a Core i7 processor. However, the new processors have indisputably higher memory sub-system bandwidth.

The PCIE bus controller in Sandy Bridge is similar to the same controller in LGA1156 processors. It supports 16 PCI Express 2.0 lanes that can either form a single PCIE x16 bus, or two PCIE 8x busses. Therefore, the old LGA1366 platform will still remain demanded after the launch of the new LGA1155 systems: it will be the only system supporting full-speed video sub-systems including multiple GPUs connected via PCIE bus with the maximum bandwidth.

An important change has been made to the display interfaces as well. The graphics core in the new processors will support HDMI 1.4, which key peculiarity is 3D support.

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