The P55’s new system architecture

Below is a diagram of a standard Core 2 Duo system, and it represents the general layout of an Intel system up until Nehalem or an AMD system up until the Opteron.

A typical pre-Nehalem Intel system.

In the diagram above, the core logic chipset consists of two primary chips:

  • Memory controller hub (MCH): The memory controller hub, also called a “northbridge,” links the CPU and GPU with main memory.
  • I/O controller hub (ICH): The I/O controller hub, also called the “southbridge,” links the MCH to peripherals and mass storage. The ICH typically hosts the USB and other expansion ports, mass storage interfaces, network interfaces, and the like.

As the GPU gained in size and importance, the standard PC system essentially took on a kind of hacked-together non-uniform memory architecture (NUMA) topology, with two main pools of DRAM (main memory and graphics memory) attached to the two main processors (the CPU and the GPU). As the amount of graphics memory increased to the point where the GPU became a second system on a daughtercard, this topology began to get more and more unbalanced and inefficient in its use of memory and bandwidth.

In 2003, AMD made the obvious improvement by moving the memory controller hub up to the CPU socket, so that main memory could attach directly to the CPU the way that GDDR had been directly attached to the GPU for some time. You can see the results below, and, give or take an I/O bridge chip or so, this is basically how AMD single-socket systems have looked since the memory controller went on-die.

A typical AMD single-socket system

You can see from the diagram that, with the memory controller moved onto the processor die, the northbridge has become a kind of “graphics hub”—it hosts the discrete GPU via some PCIe graphics lanes, and it typically has an integrated graphics processor (IGP) along with the requisite display ports. The ICH is still there, doing pretty much the same job it always did.

Today’s P55 launch jumps from the first diagram above to the diagram below in a single move.

Intel’s P55 platform

Intel’s P55 can be seen as an evolution of the AMD topology shown previously, with the graphics hub and memory hub functionality all moved right onto the processor die. Because the northbridge is completely gone, the southbridge/ICH has been rechristened the “platform controller hub,” and it’s now the only chip in the core logic “chipset” (aside from the BIOS, which is also typically included in the chipset count).

The PCH is connected to the processor socket by the relatively low-bandwidth (2GB/s) DMI bus that used to connect the MCH to the ICH. Disk I/O, network traffic, and other types of I/O will have to share this link. This shouldn’t be a problem for single-socket systems, though.

So with the advent of the P55, Intel’s core logic has gone from a two-chip to a one-chip implementation, pushing ahead of the comparable AMD platform. In theory, this very tight, direct coupling of the GPU + GDDR and CPU + DRAM systems should make for a performance boost vs. both earlier topologies.

It’s “in theory,” because Tech Report’s corei5/i7benches show some surprising results that indicate that all isn’t right with the new platform under Windows 7, and that (contrary to the stated conclusion) gamers are better off with the older Core 2 Duo systems.

Take a look at the last benchmark on this page, which shows that the minute you boost the image quality settings to begin stressing the GPU instead of the CPU, Nehalem loses its advantage and actually underperforms the older systems. This effect holds in both the i5 and the i7, so it can’t be an issue with SMT. Something is screwy with Windows 7, the drivers, or something on Intel’s end, but our money is on Windows 7. This is actually the most interesting benchmark result I’ve seen today, and I’m hoping that Scott and others will follow up on it and different combinations of OSes and games to isolate the issue.

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