I just read someone's feedback over on newegg saying that he had to enable an option in the bios (extreme memory ...) and that got him working without any further adjustments.
As I mentioned, with an i5 750 you cannot run RAM any faster than 1333 without effectively OCing your computer. You can often turn on XMP in BIOS, and many people recommend doing this, but this changes your BCLK which also affects your CPU speed. It might, as a side effect, end up turning off functionality such as Turbo, EIST, sleep states. You can usually turn those back on but you have to specifically do it. This differs from motherboard to motherboard.
Many people who do not OC turn on XMP and they don't realize that they've lost a little bit of speed from their i5 750 and have likely "lost" the chips Turbo functionality as well.
If you're going to make your RAM run faster, I would recommend manually making the changes yourself - not turning on XMP. Regardless, either route you take will mean you have to OC your computer or make trade-offs. If you don't want to OC then leave the frequency at 1333. If they are good RAM chips you can still run them at 1333 and shoot for lower timings and possibly even lower the V as well. There's not much difference between 1333 MHz and 1600 MHz anyway; here's an article you may want to read:
http://www.tomshardware.com/reviews/core-i7-870-1156,2482.html
To provide more detail on why memory multipliers and XMP affect your CPU, we need to discuss how the RAM and CPU speeds are determined. There are 3 important settings: BCLK, Memory Multiplier, and the CPU Clock Ratio.
RAM Speed = BCLK x Memory Multiplier
CPU Speed = BCLK x CPU Clock Ratio
By default, the 1156 socket chips run with a BCLK of 133. The CPU and RAM attain different speeds by using different multipliers.
The Max Memory Multiplier that is available to the i5 750 chip is one that allows RAM to reach a speed of 1333 MHz at default BCLK. Doing the math, this max memory multiplier is 10 because 133 BCLK x 10 Memory Multiplier = 1333 MHz.
The Max Memory Multiplier that is available to the i7 8xx chips is one that allows RAM to reach a speed of 1600 MHz at default BCLK. Doing the math, this is 12 because 133 BCLK x 12 Memory Multiplier = 1600 MHz.
The i5 750 runs with a default CPU Clock Ratio of 20. This gives it it's default CPU speed of 133 BCLK x 20 CPU Clock Ratio = 2.66 GHz. Keep in mind that most of the time an i5 750 CPU will actually run at a +1 CPU clock ratio so usually the chip is at 2.80 GHz and Turbo can take it even further. When XMP is turned on for 1600 MHz RAM, the BCLK is usually changed to 160 (this could differ between boards as well, but in my experience and from what's been reported the BCLK changes to 160). Remember, the max memory multiplier available to the i5 750 is 10, so to hit 1600 MHz the motherboard must change the BCLK to: 1600 / 10 = 160.
As a result of this BCLK change, the CPU speed changes. If the CPU Clock Ratio did not change your CPU would be OCed to 20 x 160 = 3.2 GHz. Most (if not all) motherboards would deem this too dangerous to allow when all a user does is enable the XMP profile. So, the boards drop the CPU Clock Ratio in response to the raised BCLK. Based on what I've seen around the forums, most if not all boards will drop the CPU Clock Ratio to 17. This means your CPU is running at a speed of 17 * 160 = 2.72 GHz.
Many boards (maybe all?) treat this as an OC. If you have left other settings at "Auto", the boards will disable Turbo, EIST, and Sleep States. These functions can be turned back on, but you typically have to go in and flip the settings from "Auto" to "Enabled". "Auto" means the motherboard decides whether or not to allow these functions. By setting it to "enabled" you ensure that they are always available. (These are the values Gigabyte allows, other boards may have different options.)
So where you had a stock speed of 2.66 GHz but typical speed of 2.80 GHz with the ability to go even faster with Turbo (up to 3.2 GHz) - by turning on XMP the i5 750 is now locked at 2.72 GHz, has "lost" Turbo, and EIST and C states are likely disabled if you make no other changes.
Of course once you have XMP turned on and your BLCK gets bumped up to 160, if you do make the changes to re-activate Turbo, your machine will run even faster. When Turbo kicks in, the CPU Clock Ratio changes. For example, with one core of an i5 750 active Turbo might raise the CPU Clock Ratio as high x24. This gives a stock CPU a max speed of 133 BCLK x 24 CPU Clock Ratio = 3.20 GHz. But with XMP on and a BCLK of 160, your new max speed is 160 x 24 = 3.84 GHz. You'll want to test your system for stability running at these settings. Keep an eye on Voltage and heat.
Putting this all together, at default/stock we have:
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 20
RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 20 = 2.66 GHz
When Turbo kicks in (for example, let's say it is at it's max):
BCLK = 133
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24
RAM Speed = BCLK x Memory Multiplier = 133 x 10 = 1333 MHz
CPU Speed = BCLK x CPU Clock Ratio = 133 x 24 = 3.20 GHz
With 1600 RAM and XMP on
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 17
RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 17 = 2.72 GHz
You are locked at this point if you make no other BIOS changes.
With 1600 RAM and XMP on, assuming you make the changes necessary to turn Turbo back on (for this example, let's say Turbo is at its max):
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 24**
RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 24 = 3.84 GHz
**To enable Turbo with XMP on in this scenario you will have to specifically enable Turbo AND set your i5 750's CPU Clock Ratio to 20. This means that your system will be OCed to a pre-Turbo level of:
With 1600 RAM and XMP on and CPU Clock Ratio to 20 when Turbo and EIST are not in effect:
BCLK = 160
Memory Multiplier (effectively) = 10
CPU Clock Ratio = 20
RAM Speed = BCLK x Memory Multiplier = 160 x 10 = 1600 MHz
CPU Speed = BCLK x CPU Clock Ratio = 160 x 20 = 3.20 GHz
As I said before, you're overclocking now and you'll want to test your system for stability running at these settings. Keep an eye on Voltage and heat. Also, although I used XMP as an example you could accomplish the same thing by setting BCLK, the memory multiplier, timings and voltage by hand. IF you're going to go to the trouble of OCing your rig, I would recommend not using XMP but instead make the adjustments yourself.
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