Note: Most Sandy Bridge-E information is currently under embargo, but the information below comes from Bit-tech's article detailing SNB-E overclocking.

Sandy Bridge-E release is getting closer and closer all the time, but there has been one concern: overclocking. Right now, it looks like the initial i7-lineup will consist of three CPUs: i7-3820, i7-3930K, i7-3960X. We covered most of the SB-E specs in our Additional info on Sandy Bridge-E Processors, X79 and LGA2011 article. The two latter ones, i7-3930K and i7-3960X, will have an unlocked CPU multiplier, which allows easy CPU overclocking by just changing the multiplier. However, the cheapest model, i7-3820, does not have an unlocked CPU multiplier.

With LGA 1155 based CPUs, this means you are limited to adjusting the BCLK (base clock). The BCLK is 100MHz by default but unlike in previous microarchitectures, it can only changed by a few MHz until you run into problems. This means you must buy a CPU with unlocked multiplier (i.e. a K CPU) in order to overclock effectively. There was concern that SNB-E would follow a similar path.

Fortunately, Sandy Bridge-E isn't as limited as regular Sandy Bridge. On top of BCLK and CPU multipliers, Intel has introduced something called reference clock ratio (RCR). This is a multiplier that affects the CPU and memory frequency, but does not affect things such as SATA and USB buses like the BCLK does. Hence it can be changed more dramatically than the BCLK without causing instability. This isn't an open multiplier though; Intel has limited it to 1.00x, 1.25x and 1.66x.

Compared to regular CPU multiplier overclocking, playing around with the RCR is a bit more complicated due to only two possible multipliers. You will need three equations to figure out the three values needed for accurate overclock. Lets break down the math:

  1. Frequency / Reference clock ratio = X
  2. X / 100 = CPU multiplier
  3. X / CPU multiplier = BCLK

To show how these equations work in real world, lets see the math for 4.2GHz overclock.

1) 4200 / 1.25 = 3360

The frequency must be entered in MHz (multiply GHz by 1000). The first step is to figure out which reference clock ratio you need to use. The 1.25 multiplier allows a maximum overclock of around 4.7GHz, so that should be enough for most people. You could, of course, use the 1.66 and simply lower the CPU multiplier enough but then you may face issues with your RAM (more on this later).

2) 3360 / 100 = 33.6 ~ 34

In the second step, you simply plug in the result you got from the previous equation and divide that by 100. You will likely end up getting some decimals, but the CPU multiplier must be a whole number, which means you must round the number. In this case, the closest integer to 33.6 is 34. Remember that your maximum CPU multiplier is 36, so if you get a higher number, you might want to use a bigger RCR.

3) 3360 / 34 = 98.82

In the final step, you need to divide the result you got from the first equation by the CPU multiplier (the rounded one) from the second equation. You will again end up with a decimal number, so round that so that you have only two decimals (BCLK can be adjusted in increments of 0.01MHz). So using the equations, for a 4.2GHz overclock you would set:

Reference clock ratio = 1.25x
CPU multiplier = 34
BCLK = 98.82MHz
1.25 * 34 * 98.82MHz = 4199.85MHz ~ 4.2GHz

And voilá, you are done! Using the above information, you can figure out the three values needed for overclocking your CPU to certain frequency. These three numbers are the ones you will be entering into the BIOS. Before applying these numbers, you can confirm them by multiplying them all together and you should end up with the frequency you entered in step 1. Of course, a voltage change will most likely be required as well but that is part of overclocking basics so we won't get into that now.

There is one bit hidden into the reference clock ratio though - it affects the memory as well. The total memory frequency is:

BCLK * reference clock ratio * memory multiplier

The problem is that we don't know what memory multipliers Intel will support in SB-E platform. Intel claims memory frequency changes in increments of 266MHz, which with 100MHz BCLK suggests at least multipliers of 13.33x, 16.00x and 18.66x. With 1.25x RCR you shouldn't get into troubles with memory, but big overclocks with 1.66x RCR may face some memory limitations, especially if there are no smaller memory multipliers. For example, if the lowest memory multiplier ends up being 13.33x, then your memory would run at 2213MHz with 100MHz BCLK and 1.66x RCR. That might be too much for a stable setup, and you'd certainly want higher quality memory.

Again, a big thanks to Bit-tech for their detailed article on SB-E overclocking!

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  • Filiprino - Wednesday, September 21, 2011 - link

    This means overclocking similar to past times.
  • ajp_anton - Wednesday, September 21, 2011 - link

    Seems like a waste of memory channels if they also force you to overclock it.
    2213MHz quad channel is like 4426MHz dual, overkill anyone?
  • DanNeely - Wednesday, September 21, 2011 - link

    Rule of thumb for intel is 1 channel/2 physical cores; so while it's massive overkill for the downbinned quadcore part and a bit more than than is needed for this years hexcore, if they launch an 8 core IB-E chip next year it will need all 4 channels. This would be the same thing they did when making LGA1366 triple channel despite only launching with quads.
  • Assimilator87 - Wednesday, September 21, 2011 - link

    So you basically need a PhD in math to overclock this thing.
  • pewter77 - Wednesday, September 21, 2011 - link

    i wouldn't say three arithmetic operations constitutes needing a PhD in math....
    more like kindergarten math or 1st grade.
  • erple2 - Thursday, September 22, 2011 - link

    True, but I remember when overclocking was stupid simple (Celery days) - multiplier x bus clock speed = how fast you're now running. I also remember burning out that CPU in 2 years running at 450 MHz rather than the 300 MHz it was "specced" for.

    I suppose as things get more complicated as bus speed kind of disappeared and was no longer tied with RAM speed, and things became more independent of your PCI bus speed (which was a percentage of your bus speed).

    Either way, overclocking kind of passed me by a while ago. It just doesn't matter to me anymore. I'm looking more for "fast enough" with more stability.
  • MrSpadge - Thursday, September 22, 2011 - link

    I hope for everyone you don't have a math PhD..
  • dfghdfshhh - Friday, September 23, 2011 - link

    Come go and see, will not regret it Oh look

  • zhangqq - Monday, October 31, 2011 - link
  • fcento - Monday, October 31, 2011 - link

    I made an application to calculate the overclock so you don't have to do all the calculations every time.

    The only tweak i did was to round-down step 2 so if you try 4200MHz the multiplier will be lower (x33) and the BCLK slightly higher (101.82) which should give better performance.
    I might change this though with future releases and when i'm 100% sure about the available memory multipliers (i assumed x10.6 x8 x13.33, x16 and x1866 since it's the only way to get ddr3 800,1060,1333,1600 and 1866 MHz support at x1 RCR and stock BCLK)
    With the program the aim was to get the best overclock without having the ram overclock to crazy speeds because of the RCR.

    Here is the link for the program, i hope you like it
    Here is a screenshot

    Follow me on twitter @fcento for updates

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