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Northbridge Overclocking question for the masses

post #1 of 2
Thread Starter 
Since the beginning of my career on here a few years ago, there are a few things I remember clearly in my time here. The reaction to the ATI HD 5000 series cards, the first time I found out people used LN2 to cool processors, and "If you have Phenom II processor, try to get that Northbridge to 2800Mhz.

In regards to the last one I just realized I don't think I ever asked why. I just kind of did it (well to 2600 anyway, my 965 hates being reasonable). I know it opens up memory bandwidth and my system certainly feels snappier, but let's pretend I don't know any of that.

Let's pretend I don't know anything about ocing, temps, volts, women, lasers, ice cream, and cats. Using benchmarks and logic can any one explain the real world gains with a OC'd Northbridge?

I'm just in a learning mood today and would like to know why its ingrained in my very DNA to over clock my North bridge every time I'm set back to default.
post #2 of 2
Thanks to AMD, placing the memory controller on the CPU allows faster interaction with memory. However this complicates CPU stability when overclocking. Luckily you have me to help make the complicated simple.

The first thing you need to do is to understand, as with all overclocking, how the Northbridge works. So how does the Northbridge work? It is really quite simple:

Data sent to the CPU is first sent to the Crossbar, which then distributes it to the different cores of the CPU. That data has instructions to either be stored in CPU Cache, go to system RAM to be stored for easy access, or be sent back out to wherever it needs to go. When it is sent to RAM, the EMC (Embedded Memory Controller) or Northbridge takes the data so that it can distribute it within RAM. If any data is to be called from RAM, the CPU requests it from the Northbridge.

How does this data travel along the CPU to RAM and all around? On the HyperTransport bus.

But wait – don’t you increase that to increase the CPU speed? Why yes, of course! But doesn’t the Northbridge take control of the data passed to the RAM? Yes it does, but it is still all on the same bus. The Northbridge takes the data and routes it to where it needs to go between the CPU and the RAM. Since it is its own “section” on the CPU, it has its own multiplier. Similar to the CPU (and even all of its cores), there is a multiplier which sets the speed of this section. This is how you overclock the Northbridge.

But, but… wait!? You just gave me a bunch of mumbo-jumbo! Oh, hush-puppies, I gave you all the information you need to overclock the Northbridge, but since I am so nice I will make it simpler.

In order to overclock the Northbridge, the first thing you need to do is check to make sure that you can access the Northbridge. This is done by going into BIOS, where you should see an option for Northbridge Multiplier (it can also be called CPU->NB Multi or NB Multi). If you do not have this, it is a simple Motherboard problem: Go out and buy a better one, because the manufacturer does not allow the option. If you are able to change the Northbridge, good news! You can help stabilize your CPU overclock and increase your benchmark scores.

For those that have the ability to overclock the Northbridge, I have constructed a table which shows suggested Northbridge speeds relative to CPU speeds. Please note that there is a +/- 200 MHz difference for each step. In order to find your speed without my table, you can use this formula:

Northbridge Frequency (+/-200) = (CPU-Frequency * 2 ) / 3.15
These tables are only suggestions – your settings may vary somewhat, although in some cases these recommended numbers may yield the best results. Actually there is no single way to show which will be best because each benchmark utilizes memory differently. Memory intensive software will require a higher Northbridge, while programs which solely utilize CPU speed rely more on the CPU Cache and CPU speed settings.

Calculated CPU and Northbridge Speeds


Similar to a CPU, the Northbridge’s voltage can also be tuned – it may also have a VDD. Northbridge voltage is a lot simpler to adjust than CPU voltage as it only needs to be bumped up every once in a while. The default voltage is roughly 1.175v.

The max that you should ever go to is 1.3v, and in only high overclocks do you ever need to go to 1.35v (try not to go this high unless you really think it will help). Northbridge voltage is not of concern in most cases – it should only be touched if you really need it.

NOTE: The CPU voltage option should be under CPU-NBv. The option titled (if available) Northbridge or NB voltage is for the Northbridge on the motherboard, not the CPU.

Geekbench Charts (higher is better)

The graphs show an experiment that puts my theory to the test. The highlighted areas show what my calculations indicate as the best results for CPU overclocking. As you can see, my suggestions are not 100% on the money – actually they are a bit opposite to what you might think.

If you look at the benchmarks, you can tell that my suggestions are not 100% right – this is due to a throttling issue. The CPU can be throttled in two ways:

The first is that the CPU has a high overclock and that everything which is connected to the CPU is slow. This causes errata errors, or CPU miscalculations. To fix this you have to increase RAM and Northbridge, and work with the HyperTransport Link (HTT).

The other throttle is that the devices attached are faster than the CPU needs to be. This normally does not cause any instability but can cause a decrease in performance, as with the benchmarks above. Geekbench needs to access the RAM pretty quickly during one of the tests. At the right speed, memory is accessed perfectly, yielding a high score, while at other times it is not being accessed fast enough or is being accessed way too fast. Either way, this yields a lower score.

Final Things to Note Before Finishing Up
If you do find your CPU Overclock is not stable and you have messed around with the Northbridge multiplier, you may want to look at the Northbridge’s voltage. Sometimes an increase will make it stable. When running Prime95 or other of stress-test tools, if a core fails or there is a BSOD, run a small FFT test to check to see if it is the Northbridge. If the small FFT works, then your Northbridge overclock is fine and you need to work on memory settings.

In conclusion, this article should allow you to gain a higher overclock with CPU stability.
-ROG
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