DMI, PCIe, and memory controller

Intel’s designed Nehalem to be completely modular. As such, features can be added or removed from the core architecture to service intended markets. Server CPUs for example can feature more cache, more cores (up to six currently), more QPI links, etc than traditional desktop Nehalem variants like Bloomfield or Lynnfield.

DMI takes center stage

In the case of Lynnfield, to cut costs Intel takes the basic Bloomfield core and removes features like the QPI link and triple-channel memory controller, and replaces it with DMI (Direct Media Interface) and a dual-channel controller.

DMI has actually been in use for years now in previous Intel chipsets. It’s a chip-to-chip interconnect that Intel has used previously to link the North Bridge chip (the IOH) with the South Bridge chip in their chipsets. In the case of the X58 platform, DMI is used to link the X58 IOH with the ICH10R South Bridge chip. You can see DMI in action on X58 in this block diagram:

As you can see, the DMI bus is limited to 2GB/sec of bandwidth. That pales in comparison to QPI’s 25.6GB/sec. However, it turns out that 25.6GB/sec of bandwidth is overkill for most desktop users. Unless you’re running a server with multiple processors that need to communicate with each other, or you’re running multiple graphics cards (2-Way, 3-Way SLI/CrossFire or Quad SLI/Quad CrossFire) most users won’t tap into the potential bandwidth advantages QPI offers.

With one x16 PCIe graphics card potentially drawing up to 16GB/sec of bandwidth though, DMI wasn’t sufficient for handling PCIe graphics duties.

To solve this problem, Intel’s integrated PCI Express functionality directly onto the Lynnfield CPU. You can see it on the right-side of Lynnfield in this die shot:



Now compare the Lynnfield die layout to Bloomfield’s layout:



Lynnfield supports up to 16 PCI Express 2.0 lanes, providing single-card or dual-GPU functionality. The 16 lanes can be configured in x16 (in the case of 1 card) or dual x8 (in the case of two graphics cards) configurations, with full support for CrossFire and SLI. Additional PCIe functionality is handled by the P55 chipset, which supports up to 8 x1 PCIe devices (500MB/sec each). You can see how the Lynnfield/P55 platform is configured in this block diagram:





With just 16 lanes of PCIe functionality present in Lynnfield, support for 3-Way SLI/CrossFire configurations isn’t possible. Users who would like to run 3-Way SLI will have to purchase a P55 motherboard with NVIDIA’s nForce 200 chip built-in. From polling motherboard manufacturers, it doesn’t sound like there’s going to be a ton of boards to choose from for users who would like to go this route; right now we know EVGA will offer their P55 FTW 200 and P55 Classified 200 boards, and ASUS will offer workstation-oriented P55 boards with the nForce 200 chip integrated on the motherboard, but that’s about it.

From what we gather, the motherboard manufacturers generally feel that someone who will want 3-Way SLI is going to want Intel’s X58 platform. After all, X58 can natively drive anywhere from 2x16 PCIe cards to 4x8 cards.

Dual-channel memory controller

As we’ve mentioned previously, for Lynnfield Intel removes one of the three memory channels found in Bloomfield, knocking it down to 2-channels (128-bits wide). However, Lynnfield’s dual-channel memory controller officially supports faster DDR3-1333MHz memory.

Technically, Bloomfield’s triple-channel controller only supports DDR3-1066MHz RAM.

When configured with dual-channel DDR3-1333MHz memory, Lynnfield’s peak memory bandwidth is 21.2GB/sec. That’s still not as high as Bloomfield’s triple-channel 192-bit wide DDR3-1066MHz, which offers up to 25.6GB/sec of peak memory bandwidth, but it’s close.

Of course, both the P55 and X58 chipsets can run with faster DDR3-1600 and higher memory modules, but technically that’s outside the specifications of the chipset. That hasn’t stopped motherboard makers and memory manufacturers from peddling their fastest DDR3-2000MHz memory kits though. Even Intel’s gotten in on this action, with their P55 and X58 motherboards supporting memory speeds outside the official specifications of the chipset.

Like Bloomfield, Intel says DDR3 memory modules must run at 1.65V or lower. Anything higher, and you risk damaging the memory controller integrated inside your CPU.

With the third memory controller and QPI removed, Lynnfield needs fewer pins than Bloomfield – 1156 versus 1366. As such, a completely different socket is required, LGA-1156. After first launching with Prescott Pentium 4 CPUs over 5 years ago, LGA-775 is finally being replaced.