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|
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.
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.
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|
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:DRAM 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.
There is one feature that is exclusive to AMD, and it is "Core Unlocker". Essentially, many of the dual and tri-core CPUs from AMD are actually just quad cores with some of the cores disabled. Sometimes this is because they are unstable, but often it is merely done so that they actually have some cheaper dual cores to sell. In either case, many motherboards allow you to unlock cores. If you do this, be sure to test the unlocked cores before getting on with any overclocking. Guide Part 3