- Apr 12, 2010
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I just posted my first draft of an overclocking guide on my blog. This guide is not meant to be "the one guide", but rather just a lot of useful information, tips, and personal opinions on how to overclock. It is also meant to be generic to any CPU, not just a specific model of Intel or AMD.
I also would greatly appreciate any and all feedback. I'm not the best writer, so I think there's probably a lot of overlapping and confusing paragraphs. I'm also not very familiar with AMD OCing, so if I made some big mistakes with that, please let me know and I'll make the changes.
Also, I will keep the blog posts the most up to date, particularly when I add testing reports and such. There are also links and images that aren't incorporated in this post at this time.
Guide to Overclocking - Part 1
Preface
As I already wrote about, a stable PC is very important. However, today I'm going to try and teach the basics behind how to destabilize your PC with faster clocks - and then, hopefully, get it stable again with safe voltage and temperatures.
There are many reasons to overclock, but for the current generation of CPUs it's more a matter of "why not?" than "why should I?" I'm not here to convince anyone to overclock, but if it's something you're interested in then I hope I can help you understand it a little better and get you on your way. Overclocking can be tedious, but also fun once you get the hang of it and achieve a successful overclock.
There are some differences between how Intel and AMD CPUs are overclocked. A lot of it, though, is nomenclature. Most of my experience is with Intel chips, so that is what I'm going to focus on. However, there are some terms that are, for all intents and purposes, interchangeable with what you see on AMD chips. More on that coming up.
One more thing, for the absolute beginner. If you are totally unfamiliar with overclocking, this is the gist of it: You increase frequencies and/or multipliers or ratios in order to increase the speed at which your hardware runs above and beyond the stock speeds. Once you reach a certain speed, the default voltages are no longer enough to keep it running stable so you need to increase them. The question about how much and which ones are what I hope to help answer by the end of this guide.
Prerequisites
There's a few things that you absolutely need before you can consider overclocking. The first is a motherboard that supports overclocking. Unfortunately, if you own any "big name" PCs like Dell, Gateway, HP, etc., you will not have the option to overclock. Some PC makers like Alienware and CyberPower will use motherboards like Asus, Gigabyte, and MSI which generally do allow overclocking. Just go into your BIOS at startup (typically DEL key during the POST/boot) and see what options you have.
The second thing you need, and this is very important, is good cooling. The case itself needs to have a decent amount of air flow. Typically most "aftermarket" cases will be sufficient, but if you upgraded the motherboard using one of the smaller HP/Dell/etc towers, you may need to make some changes. I've seen many that only have 1 exhaust fan, on the bottom. Ideally you would have at least 1 intake and 1 exhaust. On top of good case cooling, you also will need good CPU cooling. Typically the stock cooler that comes with a CPU is not going to cut it for overclocking. You might get a very small performance bump out of it, but it will quickly run out of thermal headroom. Ideally you need an aftermarket tower cooler, even a cheaper one like the Hyper 212 Plus.
Guide to Overclocking - Part 2
Some Terminology
There are a few things you need to know first. This can be rather tedious and, for the beginner, daunting. I assure that no matter how complicated it looks now, once you enter the BIOS and have a look for yourself, and start to change some values, it should all start making a lot more sense.
Example of BIOS voltages (img on blog)
Voltages
The first thing I'll be addressing is Vcore. This is the CPU core voltage, and is one of the biggest influences on CPU stability. All CPUs have manufacturer specifications on what the highest Vcore the chip is rated to handle, so make sure to check the documentation online before you start. As a general rule for Intel CPUs, 1.4V is the max you want to go, but some high end chips are ok up to 1.5V.
Secondly, Intel has what is sometimes called IMC voltage (Integrated Memory Controller), other times called QPI (QuickPath Interconnect), and yet other times VTT. On AMD rigs, you have a Northbridge Voltage - the NB is pretty similar to the IMC (and in a way the QPI) in that they both handle a lot of information flowing into and out of the CPU and back and forth to other components on the motherboard. Increasing this voltage along with Vcore will help stability. On Intel rigs, I generally keep this around 0.1 to 0.2V below the Vcore value, but that is not a "hard and fast" rule, just a preference. There is one hard rule for IMC/VTT/QPI voltage, though: it must be within 0.5V of your DRAM voltage.
DRAM Voltage is directly related to your RAM. RAM should always be set to the recommended manufacturer speeds. For example, you might have bought a 2x2gb 1333mhz CL8 1.5V RAM set. The 1.5V designates the DRAM voltage needed for that speed (1333mhz) and timings (CL8). I'll explain more on this later in the nitty gritty of OCing.
There are a few other voltages you can probably tweak. In the example picture, that is my BIOS. PLL means "phase-locked loop", and PCH "platform controller hub". The PCH refers to the "south bridge" which is where many information sources have to go through - for example most PCIe 1x slots will go through there as well as USB and SATA hubs. The PLL voltage rarely has to be changed, maybe 0.1V increase at high base clocks. Same goes for the PCH - although in my case I have a PCIe 4x slot that runs through it, with a GT 240 installed to run PhysX. I found that by increasing my PCH voltage to 1.15 helpled stabilize the GPU.
Clocks
Base Clock (Intel) and Front Side Bus (AMD) are quite similar. The BCLK is what all other speeds are multiplied from to get their final speed. So your CPU might run at 22x multiplier - this means 22xBclk. RAM, too, is multiplied by the BCLK. Many Intel CPUs have a 133mhz base clock - so 22x133=2.9ghz. 10x133=1333mhz RAM. The QPI is also a multiple of the base clock. Front Side Bus, or FSB, works in much the same way although usually they use ratios instead of multipliers. This can be a little confusing, but when I say "change the multiplier" consider this to also mean the ratio if you have an AMD rig.
The PCIe clock is a bit special. I have a first gen "i" core - an i5 750 - in which the PCIe bus is unrelated to the base clock. However, as I understand it, this is not the case for the current Sandy Bridge intel cores like the i5 2500k. I have mentioned PCIe clock before in a previous blog post, but suffice it to say you shouldn't touch this right now. I will delve into this deeper at a later time (links will be provided at the end).
CPU Frequency is, of course, the speed your CPU runs and this is the value you are trying to increase. Only a few years ago, 4ghz was considered "the top" and going beyond that was "insane". On any current generation CPU, 4ghz is actually fairly easy to achieve. That isn't to say all CPUs will make that number, though - bear in mind that all hardware is slightly different and has different potential for overclocking. You need to be slow and cautious to get the most out of your hardware.
Feature Terms
There's a lot of features present in most BIOSes. Many of them you don't need to worry about, some you do. Some are completely optional or unrelated to overclocking. For example, the ASUS RoG series of motherboards (extremely top end) have all sorts of features you can access - like being able to overclock from a smart phone. I'm not going to touch on these. Instead I'll focus on what is primarily related to CPU stability when overclocking.
What not to do with your PCIe frequency (img on blog)
On the previous picture, you'll see one very important term - Load Line Calibration (or LLC). What this means, is that the motherboard will try to counter act the nature drop in Vcore once a load is applied to the CPU. This drop actually has a term - called VDroop - and it is in fact a specification made by Intel. I'm not sure if AMD has this or not. One important thing about VDroop is that when an Intel CPU is running with Turbo Boost, it can modify it's multiplier on the fly based on how many cores are needed. If only 2 cores out of 4 are required, the CPU will run at a higher frequency. This can also require a higher voltage, so with LLC off (VDroop on) the voltage will naturally be a bit higher because there is a smaller load (2 cores aren't as power demanding as 4). However, when overclocking, it is common to disable Turbo Boost. In this case, enabling LLC can be beneficial to getting a stable overclock. It's not required unless going for close to or above 4ghz. There is a note of caution here, technically speaking when the Vcore fluctuates and the motherboard tries to compensate, it can for just a fraction of a second apply an overvoltage which may damage your CPU. This is extremely unlikely, but remember that the quality of your motherboard plays a huge factor in this.
CPU Ratio I have already hit on, but I want to mention a couple things. Most CPUs can go as low as 9x multiplier. The highest they can go will depend entirely on the CPU. My i5 750 tops out at 21x, the i5 760 at 22x. Both AMD and Intel have a series of CPUs that are "unlocked". From AMD, they are the Black Edition (BE) CPUs and from Intel they are the "K" series (2500k). What unlocked means, is that the multiplier is unlocked and can be raised to a very high number - like 60+. This makes overclocking significantly easier as you don't need to worry much about changing the base clock, which also means no worries about it interfering with your RAM and QPI.
DRAM Frequency is set as a multiplier of the BCLK. Generally on Intel, you only have 3 options: 10x, 8x, 6x. This means that to hit your RAM's rated speeds can require a specific BCLK. If you need to change the BCLK, keep this in mind. As you see in the image, my BCLK is at 200 which makes it really nice to hit my 1600mhz RAM speed with the 8x multiplier. This does limit my CPU OC options, though, to 200mhz increments. I have it at 20x (4.0ghz) and other options are 4.2ghz, 3.8ghz, 3.6ghz, etc. I'll delve into RAM OCing at another time, but for now it's just important to know how BCLK will affect RAM speed. With AMD CPUs, you have an FSB
RAM ratio which acts in much the same way.
The QPI frequency isn't something that needs to be worried about unless you hit an extremely high BCLK (over 200). Generally you want this fast because it will help speed up all data access on the motherboard. On AMD boards, I believe this is often the Hyper Transport speed and it also helps to overclock that.
Both AMD and Intel have features that can change the CPU multiplier automatically. The first feature is a turbo feature, which increases the CPU frequency automatically when less cores are being used in order to speed up single threaded apps. At least for Intel CPUs, it is possible to overclock the base clock and gain a higher turbo-enabled overclock - however there is a limit. I personally hit a wall at 177 base clock, which still delivered a very high turbo clock. Of course when all 4 cores are in use, the overall frequency is lower than doing a straight 4ghz for example.
The other multiplier changing feature they have is EIST/Speed Step from Intel and Cool'N'Quiet from AMD. In both cases, these features are used to reduce CPU frequency and voltage in order to save power at idle. Generally it is good to have these enabled, but when overclocking to a high frequency that starts to push the limits of your CPU, you may have to disable these functions in order to achieve a stable OC.
CPUs also have a feature called C States, which are related to EIST. The C States are what engage to turn off cores at idle. They are necessary for Turbo Boost and EIST to function properly. There are also different levels of C States, with the higher number (C6) being the most advanced stage of idle mode. Usually these can and should remain active, but you may wish to run C3 instead of C6.
Guide to Overclocking - Part 3
Testing
The first step to overclocking is to download appropriate apps to test your settings. There's a large variety of programs available, so I'm just going to list a few here.
First of all you're going to need system health monitoring programs. You can try:
HWMonitor
CPU-Z
Real Temp
Speed Fan
Secondly, you're going to need some stress tests. My favorite is IBT, but here's a few options:
Intel Burn Test
Prime95 32bit / Prime95 64bit
OCCT
Memtest86+ Bootable CD
Testing is actually fairly simple. I'd suggest running HWMonitor and Real Temp at the same time, to make sure you have accurate sensor readings. Leaving these programs open to monitor temperatures, run a stress test. Generally speaking, the more RAM you use, the more stability will depend on RAM. Specifically with IBT, you can set a low RAM usage (standard) with a very high amount of passes (20-30) to check almost exclusively CPU stability. If CPU passes, then it is recommended to do at least 10 passes on Maximum stress level to make sure everything is good.
Prime95 is much less sensitive to errors than IBT, but it is still a good test to run. However, it takes quite a bit longer. It would be a good idea to run it overnight, as it is typically recommended to get an 8 hour test without errors to be almost guaranteed stable. This, obviously, should be the last test you run as it is the most time consuming.
With any CPU, there is a thermal limit you shouldn't go too far over. Typical for Intel CPUs is 72C. AMD CPUs are a little different, but overall if you keep a CPU under 70C you should be ok. If stress tests are below 80C, it might be a little bit of a risk but you can still call that acceptable because under normal loads like in gaming and encoding videos, the temperature should be lower - just make sure you verify that.
Overclocking
Ok, so you should know understand most of the important options in your BIOS, how they are related, and how to test an overclock once you make changes. But what changes should you make? This will depend highly on your system, so I can only give some more general advice. Remember that OCing is more of an experience than anything. Take your time with it, don't jump too high very quickly, and always, always test.
If you have an unlocked CPU, this becomes rather simple. First and foremost, set your RAM to manufacturer specifications and test it - without overclocking the CPU. This is important to make sure your RAM is ok. Once you've done that (let's say 10 IBT passes at Max usage - or at least 2 passes of Memtest86+) you can start increasing your CPU multiplier. I would suggest not making more than 400mhz jumps above stock up to 4ghz, and not more than 100mhz after that. Test after each change.
If you have a locked CPU and need to start increasing the FSB/BCLK, overclocking gets much more complicated. The first thing to consider, which I mentioned previously, is that the speed of your RAM is affected by the BCLK. This of course poses a problem, and is one reason why overclockers like to buy fast RAM (1600+ MHz). For now, the RAM will have to be set to a lower than rated speed to ensure stability, while you overclock the CPU. Often, 200 BCLK or less is easily acheivable, so consider what values you can use to eventually get your RAM to 1200, 1400, 1600, or higher speed (if rated for that). We'll worry about actually hitting those values later. Much like for the unlocked CPUs, you don't want to take huge steps in speed increase.
Guide to Overclocking - Part 4
Knowing What to Change
This is probably the hardest part of overclocking. After you increase your speed and run a test, where do you make changes if it failed?
The first rule is to make sure you're changing as few variables at a time as possible. By this, as I've mentioned before, I mean overclock your CPU independently from your RAM or anything else. Focus on the CPU and then you'll know what to change. The first course of action is to increase the Vcore. I like to use the +/- keys for this, and generally give it "two increases". After that test again. If it fails, you can either do Vcore again or increase the VTT by a similar amount. If you are using an unlocked CPU, you mostly only need to focus on Vcore.
If you are at a fairly reasonably high overclock, like 4+ghz or 4.5ghz on a Sandy Bridge, you're probably going to hit a wall. What this means is that as you increase the CPU clock, it will lose efficiency. Eventually a small increase in speed will require a large increase in voltage. If you notice as you go that you start requiring more and more voltage increases to get stable, you are definitely nearing the wall and should consider whether you are happy with your current overclock. I've mentioned before that CPUs do have a voltage limit to them, particularly if you want it to last a while.
You also need to watch your temperature. Higher voltages will produce higher temperatures, and high temps are a detriment to CPU stability and longevity. If you are heating up at an abnormal rate, you might need to check that your CPU cooler is attached properly. If it is, and it's clean, and you have good case airflow, then you reached the limit of that cooler and will either need to just live with the clock you've reached, or buy a better one.
If you are pushing a very high overclock - perhaps for benchmarking - and are not terribly concerned by the voltage and/or temps, then you might find other factors than voltage are hurting your stability. You may need to disable EIST/Speed Step, C States, Cool N Quite (AMD), and other things. If there is an option you are unfamiliar with, it is a good idea to check your motherboard manual and to google for information on it.
Setting the RAM
Finally, I hope, you'll have reached a stable CPU overclock. I also hope that you are using a BCLK that will allow you to set your RAM to it's rated speeds. Ideally, you will just select the right multiplier to get the proper speed out of it. You also need to set the voltage to the appropriate amount - many RAM kits are 1.5V but for example I use RAM rated for 1.65V. Just verify your DRAM voltage is right.
If you have the speed and voltage right, there is still one more thing to adjust - the timings. If you see a RAM rating, it is usually something like "1600mhz 1.5V CL9". The CL9 means the CAS Latency = 9. The 9 refers to how many clock cycles it can take. There are, however, many more timings than just CL9. Usually, CL9 actually means 9-9-9-27 which are the 4 most important timings. The first 3 timings are almost always identical, like the former example. They could also be 8-8-8-24, for example. The 4th value is usually just the first 3 added together.
For a lot more detail on that, this is a pretty good article:
Understanding RAM Timings
Suffice it to say that as you SPEED goes up, your TIMINGS do as well. Essential, any given RAM will run at a particular speed for a particular timing. So if you have 1400mhz RAM at CL7, you might be able to run it at 1600mhz CL8 or 1800mhz CL9. This is not a guarantee, but it's very possible if it is good RAM. Also increasing the voltage can help the RAM run at higher speeds.
Ideally, you will not have to make the RAM run at too high of a speed, but if you CPU overclock gives a solid 1600mhz possibility for the RAM, you might wish to try it by either increasing or decreasing the timings based on what they are rated for at stock speeds. If you can't find your stock settings, check CPU-Z under the Memory tab. It will list JEDEC specs, which are universal, but if it has XMP specs then those are specific to your RAM.
XMP means "Extreme Memory Profile". Technically, anything over 1333mhz is "overclocked". XMP is fine in that it does set your RAM to it's rated speed, but the problem is that sometimes it will mess with your BCLK/FSB. You may want to use it to view the recommended settings, but once you found your CPU's OC settings, do not accidentally undo it all by setting the XMP (of course, you can choose to not save the changes if you want).
After any changes, again, make sure to test. You'll want to use a maximum memory usage test in Intel Burn Test, or even rum Memtest86+ or a long Blend test in Prime95.
I also would greatly appreciate any and all feedback. I'm not the best writer, so I think there's probably a lot of overlapping and confusing paragraphs. I'm also not very familiar with AMD OCing, so if I made some big mistakes with that, please let me know and I'll make the changes.
Also, I will keep the blog posts the most up to date, particularly when I add testing reports and such. There are also links and images that aren't incorporated in this post at this time.
Guide to Overclocking - Part 1
Preface
As I already wrote about, a stable PC is very important. However, today I'm going to try and teach the basics behind how to destabilize your PC with faster clocks - and then, hopefully, get it stable again with safe voltage and temperatures.
There are many reasons to overclock, but for the current generation of CPUs it's more a matter of "why not?" than "why should I?" I'm not here to convince anyone to overclock, but if it's something you're interested in then I hope I can help you understand it a little better and get you on your way. Overclocking can be tedious, but also fun once you get the hang of it and achieve a successful overclock.
There are some differences between how Intel and AMD CPUs are overclocked. A lot of it, though, is nomenclature. Most of my experience is with Intel chips, so that is what I'm going to focus on. However, there are some terms that are, for all intents and purposes, interchangeable with what you see on AMD chips. More on that coming up.
One more thing, for the absolute beginner. If you are totally unfamiliar with overclocking, this is the gist of it: You increase frequencies and/or multipliers or ratios in order to increase the speed at which your hardware runs above and beyond the stock speeds. Once you reach a certain speed, the default voltages are no longer enough to keep it running stable so you need to increase them. The question about how much and which ones are what I hope to help answer by the end of this guide.
Prerequisites
There's a few things that you absolutely need before you can consider overclocking. The first is a motherboard that supports overclocking. Unfortunately, if you own any "big name" PCs like Dell, Gateway, HP, etc., you will not have the option to overclock. Some PC makers like Alienware and CyberPower will use motherboards like Asus, Gigabyte, and MSI which generally do allow overclocking. Just go into your BIOS at startup (typically DEL key during the POST/boot) and see what options you have.
The second thing you need, and this is very important, is good cooling. The case itself needs to have a decent amount of air flow. Typically most "aftermarket" cases will be sufficient, but if you upgraded the motherboard using one of the smaller HP/Dell/etc towers, you may need to make some changes. I've seen many that only have 1 exhaust fan, on the bottom. Ideally you would have at least 1 intake and 1 exhaust. On top of good case cooling, you also will need good CPU cooling. Typically the stock cooler that comes with a CPU is not going to cut it for overclocking. You might get a very small performance bump out of it, but it will quickly run out of thermal headroom. Ideally you need an aftermarket tower cooler, even a cheaper one like the Hyper 212 Plus.
Guide to Overclocking - Part 2
Some Terminology
There are a few things you need to know first. This can be rather tedious and, for the beginner, daunting. I assure that no matter how complicated it looks now, once you enter the BIOS and have a look for yourself, and start to change some values, it should all start making a lot more sense.
Example of BIOS voltages (img on blog)
Voltages
The first thing I'll be addressing is Vcore. This is the CPU core voltage, and is one of the biggest influences on CPU stability. All CPUs have manufacturer specifications on what the highest Vcore the chip is rated to handle, so make sure to check the documentation online before you start. As a general rule for Intel CPUs, 1.4V is the max you want to go, but some high end chips are ok up to 1.5V.
Secondly, Intel has what is sometimes called IMC voltage (Integrated Memory Controller), other times called QPI (QuickPath Interconnect), and yet other times VTT. On AMD rigs, you have a Northbridge Voltage - the NB is pretty similar to the IMC (and in a way the QPI) in that they both handle a lot of information flowing into and out of the CPU and back and forth to other components on the motherboard. Increasing this voltage along with Vcore will help stability. On Intel rigs, I generally keep this around 0.1 to 0.2V below the Vcore value, but that is not a "hard and fast" rule, just a preference. There is one hard rule for IMC/VTT/QPI voltage, though: it must be within 0.5V of your DRAM voltage.
DRAM Voltage is directly related to your RAM. RAM should always be set to the recommended manufacturer speeds. For example, you might have bought a 2x2gb 1333mhz CL8 1.5V RAM set. The 1.5V designates the DRAM voltage needed for that speed (1333mhz) and timings (CL8). I'll explain more on this later in the nitty gritty of OCing.
There are a few other voltages you can probably tweak. In the example picture, that is my BIOS. PLL means "phase-locked loop", and PCH "platform controller hub". The PCH refers to the "south bridge" which is where many information sources have to go through - for example most PCIe 1x slots will go through there as well as USB and SATA hubs. The PLL voltage rarely has to be changed, maybe 0.1V increase at high base clocks. Same goes for the PCH - although in my case I have a PCIe 4x slot that runs through it, with a GT 240 installed to run PhysX. I found that by increasing my PCH voltage to 1.15 helpled stabilize the GPU.
Clocks
Base Clock (Intel) and Front Side Bus (AMD) are quite similar. The BCLK is what all other speeds are multiplied from to get their final speed. So your CPU might run at 22x multiplier - this means 22xBclk. RAM, too, is multiplied by the BCLK. Many Intel CPUs have a 133mhz base clock - so 22x133=2.9ghz. 10x133=1333mhz RAM. The QPI is also a multiple of the base clock. Front Side Bus, or FSB, works in much the same way although usually they use ratios instead of multipliers. This can be a little confusing, but when I say "change the multiplier" consider this to also mean the ratio if you have an AMD rig.
The PCIe clock is a bit special. I have a first gen "i" core - an i5 750 - in which the PCIe bus is unrelated to the base clock. However, as I understand it, this is not the case for the current Sandy Bridge intel cores like the i5 2500k. I have mentioned PCIe clock before in a previous blog post, but suffice it to say you shouldn't touch this right now. I will delve into this deeper at a later time (links will be provided at the end).
CPU Frequency is, of course, the speed your CPU runs and this is the value you are trying to increase. Only a few years ago, 4ghz was considered "the top" and going beyond that was "insane". On any current generation CPU, 4ghz is actually fairly easy to achieve. That isn't to say all CPUs will make that number, though - bear in mind that all hardware is slightly different and has different potential for overclocking. You need to be slow and cautious to get the most out of your hardware.
Feature Terms
There's a lot of features present in most BIOSes. Many of them you don't need to worry about, some you do. Some are completely optional or unrelated to overclocking. For example, the ASUS RoG series of motherboards (extremely top end) have all sorts of features you can access - like being able to overclock from a smart phone. I'm not going to touch on these. Instead I'll focus on what is primarily related to CPU stability when overclocking.
What not to do with your PCIe frequency (img on blog)
On the previous picture, you'll see one very important term - Load Line Calibration (or LLC). What this means, is that the motherboard will try to counter act the nature drop in Vcore once a load is applied to the CPU. This drop actually has a term - called VDroop - and it is in fact a specification made by Intel. I'm not sure if AMD has this or not. One important thing about VDroop is that when an Intel CPU is running with Turbo Boost, it can modify it's multiplier on the fly based on how many cores are needed. If only 2 cores out of 4 are required, the CPU will run at a higher frequency. This can also require a higher voltage, so with LLC off (VDroop on) the voltage will naturally be a bit higher because there is a smaller load (2 cores aren't as power demanding as 4). However, when overclocking, it is common to disable Turbo Boost. In this case, enabling LLC can be beneficial to getting a stable overclock. It's not required unless going for close to or above 4ghz. There is a note of caution here, technically speaking when the Vcore fluctuates and the motherboard tries to compensate, it can for just a fraction of a second apply an overvoltage which may damage your CPU. This is extremely unlikely, but remember that the quality of your motherboard plays a huge factor in this.
CPU Ratio I have already hit on, but I want to mention a couple things. Most CPUs can go as low as 9x multiplier. The highest they can go will depend entirely on the CPU. My i5 750 tops out at 21x, the i5 760 at 22x. Both AMD and Intel have a series of CPUs that are "unlocked". From AMD, they are the Black Edition (BE) CPUs and from Intel they are the "K" series (2500k). What unlocked means, is that the multiplier is unlocked and can be raised to a very high number - like 60+. This makes overclocking significantly easier as you don't need to worry much about changing the base clock, which also means no worries about it interfering with your RAM and QPI.
DRAM Frequency is set as a multiplier of the BCLK. Generally on Intel, you only have 3 options: 10x, 8x, 6x. This means that to hit your RAM's rated speeds can require a specific BCLK. If you need to change the BCLK, keep this in mind. As you see in the image, my BCLK is at 200 which makes it really nice to hit my 1600mhz RAM speed with the 8x multiplier. This does limit my CPU OC options, though, to 200mhz increments. I have it at 20x (4.0ghz) and other options are 4.2ghz, 3.8ghz, 3.6ghz, etc. I'll delve into RAM OCing at another time, but for now it's just important to know how BCLK will affect RAM speed. With AMD CPUs, you have an FSB
RAM ratio which acts in much the same way.The QPI frequency isn't something that needs to be worried about unless you hit an extremely high BCLK (over 200). Generally you want this fast because it will help speed up all data access on the motherboard. On AMD boards, I believe this is often the Hyper Transport speed and it also helps to overclock that.
Both AMD and Intel have features that can change the CPU multiplier automatically. The first feature is a turbo feature, which increases the CPU frequency automatically when less cores are being used in order to speed up single threaded apps. At least for Intel CPUs, it is possible to overclock the base clock and gain a higher turbo-enabled overclock - however there is a limit. I personally hit a wall at 177 base clock, which still delivered a very high turbo clock. Of course when all 4 cores are in use, the overall frequency is lower than doing a straight 4ghz for example.
The other multiplier changing feature they have is EIST/Speed Step from Intel and Cool'N'Quiet from AMD. In both cases, these features are used to reduce CPU frequency and voltage in order to save power at idle. Generally it is good to have these enabled, but when overclocking to a high frequency that starts to push the limits of your CPU, you may have to disable these functions in order to achieve a stable OC.
CPUs also have a feature called C States, which are related to EIST. The C States are what engage to turn off cores at idle. They are necessary for Turbo Boost and EIST to function properly. There are also different levels of C States, with the higher number (C6) being the most advanced stage of idle mode. Usually these can and should remain active, but you may wish to run C3 instead of C6.
Guide to Overclocking - Part 3
Testing
The first step to overclocking is to download appropriate apps to test your settings. There's a large variety of programs available, so I'm just going to list a few here.
First of all you're going to need system health monitoring programs. You can try:
HWMonitor
CPU-Z
Real Temp
Speed Fan
Secondly, you're going to need some stress tests. My favorite is IBT, but here's a few options:
Intel Burn Test
Prime95 32bit / Prime95 64bit
OCCT
Memtest86+ Bootable CD
Testing is actually fairly simple. I'd suggest running HWMonitor and Real Temp at the same time, to make sure you have accurate sensor readings. Leaving these programs open to monitor temperatures, run a stress test. Generally speaking, the more RAM you use, the more stability will depend on RAM. Specifically with IBT, you can set a low RAM usage (standard) with a very high amount of passes (20-30) to check almost exclusively CPU stability. If CPU passes, then it is recommended to do at least 10 passes on Maximum stress level to make sure everything is good.
Prime95 is much less sensitive to errors than IBT, but it is still a good test to run. However, it takes quite a bit longer. It would be a good idea to run it overnight, as it is typically recommended to get an 8 hour test without errors to be almost guaranteed stable. This, obviously, should be the last test you run as it is the most time consuming.
With any CPU, there is a thermal limit you shouldn't go too far over. Typical for Intel CPUs is 72C. AMD CPUs are a little different, but overall if you keep a CPU under 70C you should be ok. If stress tests are below 80C, it might be a little bit of a risk but you can still call that acceptable because under normal loads like in gaming and encoding videos, the temperature should be lower - just make sure you verify that.
Overclocking
Ok, so you should know understand most of the important options in your BIOS, how they are related, and how to test an overclock once you make changes. But what changes should you make? This will depend highly on your system, so I can only give some more general advice. Remember that OCing is more of an experience than anything. Take your time with it, don't jump too high very quickly, and always, always test.
If you have an unlocked CPU, this becomes rather simple. First and foremost, set your RAM to manufacturer specifications and test it - without overclocking the CPU. This is important to make sure your RAM is ok. Once you've done that (let's say 10 IBT passes at Max usage - or at least 2 passes of Memtest86+) you can start increasing your CPU multiplier. I would suggest not making more than 400mhz jumps above stock up to 4ghz, and not more than 100mhz after that. Test after each change.
If you have a locked CPU and need to start increasing the FSB/BCLK, overclocking gets much more complicated. The first thing to consider, which I mentioned previously, is that the speed of your RAM is affected by the BCLK. This of course poses a problem, and is one reason why overclockers like to buy fast RAM (1600+ MHz). For now, the RAM will have to be set to a lower than rated speed to ensure stability, while you overclock the CPU. Often, 200 BCLK or less is easily acheivable, so consider what values you can use to eventually get your RAM to 1200, 1400, 1600, or higher speed (if rated for that). We'll worry about actually hitting those values later. Much like for the unlocked CPUs, you don't want to take huge steps in speed increase.
Guide to Overclocking - Part 4
Knowing What to Change
This is probably the hardest part of overclocking. After you increase your speed and run a test, where do you make changes if it failed?
The first rule is to make sure you're changing as few variables at a time as possible. By this, as I've mentioned before, I mean overclock your CPU independently from your RAM or anything else. Focus on the CPU and then you'll know what to change. The first course of action is to increase the Vcore. I like to use the +/- keys for this, and generally give it "two increases". After that test again. If it fails, you can either do Vcore again or increase the VTT by a similar amount. If you are using an unlocked CPU, you mostly only need to focus on Vcore.
If you are at a fairly reasonably high overclock, like 4+ghz or 4.5ghz on a Sandy Bridge, you're probably going to hit a wall. What this means is that as you increase the CPU clock, it will lose efficiency. Eventually a small increase in speed will require a large increase in voltage. If you notice as you go that you start requiring more and more voltage increases to get stable, you are definitely nearing the wall and should consider whether you are happy with your current overclock. I've mentioned before that CPUs do have a voltage limit to them, particularly if you want it to last a while.
You also need to watch your temperature. Higher voltages will produce higher temperatures, and high temps are a detriment to CPU stability and longevity. If you are heating up at an abnormal rate, you might need to check that your CPU cooler is attached properly. If it is, and it's clean, and you have good case airflow, then you reached the limit of that cooler and will either need to just live with the clock you've reached, or buy a better one.
If you are pushing a very high overclock - perhaps for benchmarking - and are not terribly concerned by the voltage and/or temps, then you might find other factors than voltage are hurting your stability. You may need to disable EIST/Speed Step, C States, Cool N Quite (AMD), and other things. If there is an option you are unfamiliar with, it is a good idea to check your motherboard manual and to google for information on it.
Setting the RAM
Finally, I hope, you'll have reached a stable CPU overclock. I also hope that you are using a BCLK that will allow you to set your RAM to it's rated speeds. Ideally, you will just select the right multiplier to get the proper speed out of it. You also need to set the voltage to the appropriate amount - many RAM kits are 1.5V but for example I use RAM rated for 1.65V. Just verify your DRAM voltage is right.
If you have the speed and voltage right, there is still one more thing to adjust - the timings. If you see a RAM rating, it is usually something like "1600mhz 1.5V CL9". The CL9 means the CAS Latency = 9. The 9 refers to how many clock cycles it can take. There are, however, many more timings than just CL9. Usually, CL9 actually means 9-9-9-27 which are the 4 most important timings. The first 3 timings are almost always identical, like the former example. They could also be 8-8-8-24, for example. The 4th value is usually just the first 3 added together.
For a lot more detail on that, this is a pretty good article:
Understanding RAM Timings
Suffice it to say that as you SPEED goes up, your TIMINGS do as well. Essential, any given RAM will run at a particular speed for a particular timing. So if you have 1400mhz RAM at CL7, you might be able to run it at 1600mhz CL8 or 1800mhz CL9. This is not a guarantee, but it's very possible if it is good RAM. Also increasing the voltage can help the RAM run at higher speeds.
Ideally, you will not have to make the RAM run at too high of a speed, but if you CPU overclock gives a solid 1600mhz possibility for the RAM, you might wish to try it by either increasing or decreasing the timings based on what they are rated for at stock speeds. If you can't find your stock settings, check CPU-Z under the Memory tab. It will list JEDEC specs, which are universal, but if it has XMP specs then those are specific to your RAM.
XMP means "Extreme Memory Profile". Technically, anything over 1333mhz is "overclocked". XMP is fine in that it does set your RAM to it's rated speed, but the problem is that sometimes it will mess with your BCLK/FSB. You may want to use it to view the recommended settings, but once you found your CPU's OC settings, do not accidentally undo it all by setting the XMP (of course, you can choose to not save the changes if you want).
After any changes, again, make sure to test. You'll want to use a maximum memory usage test in Intel Burn Test, or even rum Memtest86+ or a long Blend test in Prime95.
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