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Just like your CPU, the Northbridge on your motherboard (i865 and newer) has its own internal frequency and latencies which affect overall system stability. This is referred to as the NBCC (North Bridge Core Clock). The NBCC directly affects the performance and stability of your memory and CPU because Intel system used a NB based memory controller.
It has been recently discovered that the NBCC varies with your systems FSB and multiplier settings. The NBCC can be calculated by dividing your CPU current multiplier by its default multiplier and then multiplier the sum by your FSB.
For Example:
E6600 @ 500Mhz and a 7 multiplier:
(9 / 7) x 500 = 642Mhz NBCC
So it can be seen that lowering your multiplier, even though offering addition headway for FSB on the CPU, will increase the NBCC, reduce NB stability and thus cause the overall system stability to decrease.
XE (Extreme Edition) and ES (Engineering Sample) processors have the unique ability to adjust their multipliers up (All XE, not all ES) and down (all chips) while maintaining its multiplier status as default.
For Example:
X6800 @ 500Mhz and a 7 multiplier (just like above)
(7 / 7) x 500 = 500Mhz NBCC
As you can see, the X6800 has the exact same settings as the E6600, however, the NBCC is lower, resulting in increased system stability.
Moving along, the NBCC has a series of latencies at which it operates. These latencies have considerable effect on overall system performance. The latencies within the NB increase when your NBCC hits specific values, thus increasing stability, but decreasing performance. A range of latencies that operate in specific NBCC values are referred to as straps. There is a 1066Mhz strap, 1333Mhz strap, and so on. However, the name is misleading, because motherboard engineers change the frequency at which specific straps set in. The trigger NBCC`s for each range of latencies is different for all motherboards. However, you can manually test many different NBCC values by using Super Pi or a memory bandwidth test to find where the latencies within your system change.
What does this mean for our overclocking?
The classical response to an instable overclock was:
1. Increase CPU voltage.
2. Increase Memory voltage.
However, one must now consider NB voltage (stability) when system instability arises. This is especially true with Core 2 systems which are capable of far higher FSB than what the NB can handle.
So with this in mind, an Intel overclocker must be aware of:
⢠what strap they are in
⢠what part of the strap they are in
If you are at the limit of a specific strap, you will most likely find your system to be less than stable. However, increase your NBCC to the next strap and your system suddenly becomes more stable, but not as fast.
For example. The P5B Deluxe changes from the 1066Mhz strap to the 1333Mhz strap after 400Mhz. Therefore, 400Mhz on a P5B is faster, but less stable than 401Mhz.
Some motherboards, like the Intel D975XBX2 and the Abit AB9 QuadGT allow the user to manually adjust what strap they are in. This unique feature allows overclockers to maximize their experience. If you find the limit of your CPU to be around 412Mhz, you can still maintain the 1066Mhz strap and maximize your CPU`s potential.
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