Frequencies and Multipliers
Although overclocking Core i7 based systems is a fairly new procedure, it is not that complicated at all. We believe that overclocking systems built on new CPUs is not any harder than platforms built around previous generation quad-core Core 2 Quad processors. However, it is important to understand that since one of the major changes brought by the new Nehalem microarchitecture is an absolutely new platform design, Core i7 overclocking requires a totally different approach.
Generally, overclocking old LGA775 systems is performed by raising the processor bus frequency. As the processor bus frequency increases, so do the processor and memory frequencies are proportionally connected with the FSB speed by means of multipliers and dividers. The processor clock frequency multiplier is defined by the nominal CPU frequency, but can also be set lower if necessary. The only exceptions here make Extreme Edition processors that have an unlocked clock multiplier. It allows overclocking by simply setting the clock frequency multiplier at a higher value than the nominal. As for the divider connecting the FSB and memory frequencies is set by the chipset North Bridge, which features the memory controller in LGA775 systems. Contemporary chipsets support wide range of memory frequency dividers, so the memory frequency can be adjusted in a very flexible manner even overclocked independently of the CPU.
The situation is completely different in LGA1366 platforms using new Core i7 CPUs. These processors not only have 8MB shared L3 cache and an integrated memory controller, they use a totally new series interface to connect to the chipset. As a result, new generation systems have no traditional FSB bus that used to be the primary factor shaping up all the system frequencies. Now this key role belongs to the so-called base frequency – BCLK, which has no application whatsoever in its initial form. However, BCLK frequency is used to set the frequencies of all major functional parts of an LGA1366 system. Among them are:
- Processor frequency, which is the actual frequency of the processor cores.
- Frequency of the North Bridge built into the CPU also called Uncore Clock or UCLK. The 8MB processor L3 cache and triple-channel memory controller integrated into the CPU are clocked at this frequency.
- DDR3 memory frequency.
- Frequency of the QPI interface connecting the processor with the chipset.
Core i7 processors have four different multipliers used to obtain these four frequency values. In other words:
- [CPU frequency] = BCLK x [CPU multiplier].
- [Uncore frequency] = BCLK x [Uncore multiplier].
- [Memory frequency] = BCLK x [Memory multiplier].
- [QPI frequency] = BCLK x [QPI multiplier].
All four multipliers mentioned above are completely independent. The only exceptions are the memory frequency multiplier and the multiplier for the North Bridge built into the processor: [Uncore multiplier] should be at least twice as high as the [Memory multiplier].
The nominal BCLK frequency for any Core i7 processors equals 133MHz. However, the derived frequencies may vary depending on the CPU models. The table below describes nominal frequencies for the Core i7 lineup defined by the official specifications. Currently, the lineup includes three models:
Although the specs define all major frequencies for each processor model, Intel in fact offers a little more freedom with coefficients determining these frequencies. In reality, only the processor clock multiplier and QPI multiplier have strictly set maximums. All other multipliers of the mass production CPUs can be changed in pretty wide range. The table below describes value intervals available for different CPU models (the nominal values are marked with bold font):
So, Core i7 processors, except for the very expensive Core i7-965 Extreme Edition CPU, overclock in only one single way: by raising the base BCLK frequency. However, once you set it over the nominal 133MHz, the frequencies of all other system parts will automatically rise over their nominal value, namely the L3 cache, memory and QPI bus frequencies. Unfortunately, setting lower multipliers for secondary frequencies will help to compensate the proportional increase only for DDR3 SDRAM, because all processors except the top model already have the minimal multipliers set for Uncore and QPI. But the good news is that L3 cache and QPI bus have great potential at higher frequencies. Therefore, overclocking will be limited in most cases by the processor cores and not by their “support group”.