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Home » CPU
Inside Intel Nehalem Microarchitecture
Author: Gabriel Torres 107,264 views
Type: Tutorials Last Updated: August 26, 2008
Page: 2 of 7
Integrated Memory Controller

Since the beginning of times Intel CPUs use an external bus called Front Side Bus or simply FSB that is shared between memory and I/O requests. Nehalem-based CPUs have an embedded memory controller and thus will provide two external busses: a memory bus for connecting the CPU to the memory and an I/O bus to connect the CPU to the external world.

This change improves a lot the system performance for two main reasons. First, now we have separated datapaths for I/O and memory accesses. Second, memory access is faster as the CPU doesn’t need to communicate first with an external controller anymore.

On Figures 1 and 2 we are comparing the traditional architecture used by Intel CPUs and the new architecture that will be used by Intel CPUs with an integrated memory controller.

Front Side Bus Architecture
click to enlarge
Figure 1: Architecture used by current Intel CPUs.

Nehalem Core i7
click to enlarge
Figure 2: Architecture used by Intel CPUs with embedded memory controller.

This new external bus is called QuickPath Interconnect (QPI) and it provides two separated datapaths (one for transmitting data and another for receiving data) for the CPU to communicate with the chipset or with other CPUs, in the case of servers with more than one CPU. As you can see, this bus is the equivalent of the HyperTransport bus used on AMD CPUs. The first generation of QuickPath Interconnect will run at 3.2 GHz transferring two 16-bit data per clock tick, which equals to a maximum theoretical transfer rate of 12.6 GB/s on each direction. For a more detailed explanation about this new bus and also a comparison between it, HyperTransport and Front Side Bus, please read our Everything You Need to Know About the QuickPath Interconnect (QPI) tutorial.

Desktop CPUs will have only one QuickPath Interconnect, while server CPUs will have two independent busses to allow them to be connected together on SMP (Symmetric Multiprocessing) environments.

The memory controller integrated on Nehalem-based processors provides three memory channels, i.e., it is capable of accessing three memory modules at the same time, in parallel, in order to improve performance – in theory triple-channel architecture provides a 50% increase of available bandwidth compared to a dual-channel architecture running at the same clock rate.

So in order to achieve the best possible performance with a Nehalem-based CPU such as Core i7, you need to install three or six (if your motherboard supports six memory sockets, of course) memory modules. You will have to pay close attention to this change, because most people today are used to have a PC with 2 GB or 4 GB (two or four memory modules in order to match the system’s two memory channels) while with Core i7 and other Nehalem-based CPUs you need to have a PC with 1.5 GB, 3 GB or 6 GB for best performance (three or six memory modules in order to match the system’s three memory channel).

Another thing you have to be very careful about is the fact that some motherboards targeted to Core i7 will have four memory sockets, like Intel "Smackover", which is based on Intel X58 chipset. If you install four memory modules you will have more memory available, but you will decrease system performance. For example, if you install 4 GB (four 1 GB memory modules), the system will access the first 3 GB at triple-channel performance, but the memory area between 3 GB and 4 GB will be accessed at single-channel performance. So unless you really need more RAM memory, stick with 1.5 GB, 3 GB or 6 GB. Other manufacturers have already announced that they will produce motherboards with six sockets, so on this boards you must install memories in triplets in order to achieve the maximum possible performance.

With three memory channels available, the CPU will access the memory at 192 bits per time (3x 64 bits), if you have three or six memory modules installed, of course. This gives a maximum theoretical transfer rate of 25.58 GB/s if DDR3-1066 memories are used.

The memory controller embedded on Nehalem-based CPUs accepts only DDR3 memories – no support for DDR2 is given.

Due to the integration of the memory controller, Intel has to change the CPU socket to a new socket using 1,366 pins. So you won’t be able to upgrade your current Intel-based system to a Core i7 by simply changing the CPU; you will have to also replace the motherboard and probably the memories, if you don’t have DDR3 memories. If you do have DDR3 memories, you will probably need to buy one extra module in order to enable the triple-channel mode, if you have only two modules.

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