Articles: Memory

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LGA1156 Processors: Memory Controller Peculiarities

Before we get to the actual benchmark results, we decided to remind you what the major operational principles of the memory controller in LGA1156 systems are. Core i7 and Core i5 processors (as well as the upcoming Core i3 and Pentium) manufactured in LGA1156 form-factor feature a built-in dual-channel DDR3 SDRAM controller located on the same die as the CPU cores. Like the on-die L3 cache, this controller works at its own frequency, which is different from that of the CPU. This frequency varies depending on the processor model and together with other parameters has certain influence on the overall memory sub-system performance.

Another feature of the LGA1156 processor memory controller is the support of different memory speeds depending on the processor. More expensive models with faster memory controller support DDR3-1600 SDRAM, mainstream solutions can only work with DDR3-1333 SDRAM being the fastest, while low-cost models can only clock the memory as DDR3-1067.

Here is the summary of DDR3 SDRAM frequencies supported by LGA1156 processors:

In other words, systems equipped with less expensive CPUs can’t accommodate fast DDR3-1600 SDRAM, at least without base clock frequency (BCLK) increase beyond the nominal value. However, the owners of Core i5 and Core i3 based systems can get access to faster memory modes during overclocking, although they will have to overclock not only the memory but also the CPU.

The thing is that LGA1156 processors use the same 133 MHz clock generator (BCLK) and a number of independent multipliers:

  • CPU clock frequency multiplier. This multiplier is determined by the default processor frequency and the user cannot increase it beyond the nominal. However, it can change automatically during work due to Turbo Mode technology and Enhanced Intel SpeedStep.
  • Memory frequency multiplier. The list of supported multipliers for Core i7-800 processors includes 6x, 8x, 10x and 12x, which implies that you can use these processors with DDR3-800, DDR3-1067, DDR3-1333 and DDR3-1600 SDRAM. Unfortunately, Core i5 processors do not support 12x memory frequency multiplier that is why the maximum memory frequency for them is 1333 MHz. The junior LGA1156 processor model from the Pentium series only offers two lowest multipliers for the memory frequency: 6x and 8x.
  • Uncore multiplier that sets the memory controller and L3 cache frequencies. All LGA1156 processors have this multiplier locked. It equals 18x for Core i7 CPUs and 16x – for Core i5.

As we see, the owners of the most expensive Core i7 processors have the most flexibility in configuring their memory sub-system. Therefore, we used this particular processor in our today’s tests. However, all conclusions we are going to make today will also be true for other LGA1156 CPUs. The only difference for the owners of less expensive processors is that they have fewer opportunities to have their memory working at higher speeds. However, all junior processor models still have all the same tools for manipulating the memory timings.

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