Overclocking is the practice of making a component run at a higher clock speed than the manufacturer's specification. The idea is to increase performance for free or to exceed current performance limits, but this may come at the cost of stability.
Think of the 3GHz on your new 3GHz Pentium 4 as a speed limit asking to be broken. This can be done to several components in your computer. This often takes advantage of the fact that many manufacturers mark higher end components as lower in order to meet demand for a lower end component. You will be able to get extra performance out of your components for free. It is possible to get performance that is not possible even when using the top of the line components. And you can buy cheaper parts, and then OC them to the clock speed of the higher end component.
WARNING: OVERCLOCKING WILL VOID THE WARRANTY ON THE PARTS BEING OVERCLOCKED. DOING SO MAY ALSO CAUSE SYSTEM INSTABILITY, AND MAY ALSO CAUSE DAMAGE TO COMPONENTS AND DATA. BE CAREFUL AND CAUTIOUS WHEN OVERCLOCKING.
Things that can't be overclocked
Although it is possible to overclock many of the components of a computer (such as the CPU, FSB frequency and video card), it is not possible to overclock some components. For example, it is not possible to increase the read/write speed or access time of a hard disk or CD-ROM drive. The only solution to improving the performance of these components is to use faster components in the first place, or in some special cases of hard disks, change the configuration to a RAID. Many OEM computers have the CPU frequency locked. (But you wouldn't be reading this guide if you're using an OEM computer, would you?)
The CPU's clock speed is the FSB clock speed (base, not effective speed) times the CPU's multiplier. On most newer CPUs, the multiplier is locked, so you will have to adjust the FSB clock speed (However, it might be possible to 'unlock' the chip's multiplier on some older chips. See CPU Locking.) The FSB is not adjustable on a few motherboards, and many OEM systems. The FSB and multiplier, if not locked, are adjustable from within the BIOS. Note that upping the FSB clock speed also increases the clock speed of many other components, including RAM. When increasing the FSB clock speed, only do so in small increments of a few MHz at a time. After you do this, boot up your computer to make sure it works. If your computer successfully boots, increase the FSB some more. If it won't boot, lower the FSB until your computer properly boots up. Repeat until you have the highest setting with which your computer will boot up. Next test your OS to make sure it is stable with a burn application, or any application that uses 100% CPU power. If a crash or reboot results, lower the FSB speed some more until it runs smoothly. On some motherboards you are also able to change the voltage of the CPU and other components in order to help stabilize the system. However, this increases the components' heat output and can harm or shorten the life of your system instead.
Notes: On AMD K8-based chips (Athlon 64, Opteron, Turion, and Socket 754 and 939 Semprons), there is no FSB- there is an integrated memory controller (IMC) and a HyperTransport bus (HTT). The IMC has a base clock speed like a FSB does and for overclocking, it would be adjusted just like a FSB would. The HTT can also be overclocked like the CPU core can be. Its base clock speed is the exact same as the IMC's and thus by default you will overclock the HTT bus as you overclock the CPU. Note that the HTT bus has a multiplier and it is adjustable. Many motherboards do not function well with the HTT bus frequency much over the stock frequency- that's where the adjustible HTT multiplier comes in. (It should be noted that overclocked systems are most stable when the HTT is at or below 1000Mhz)
Also, some newer desktop processors (AMD K8 series with Cool 'n Quiet, Intel Pentium 4 6xx series, Pentium D 830, 840, stepping C1 Pentium D 9xx series with Enhanced Intel Speed Step) and most mobile processors (exc. Celeron M) can vary their operating frequency while running by lowering the CPU multiplier. This results in the multipliers below the highest one being unlocked. This allows for a very high FSB/base clock speed with a lower multiplier to use very fast RAM to its full potential without overclocking the CPU so much that it is not stable.
How to choose the best CPU for overclocking
Assuming that you have selected quality components like an excellent motherboard, high-quality RAM , an excellent thermal solution and an excellent power supply; you may wonder why your processor won't exceed a certain speed limit. Lets assume that you have a memory chip that is capable of taking the maximum frequency the motherboard can throw at it and yet, when you exceed a certain speed limit you realize that your system becomes unstable.
A PCI bus generally runs on 33 Mhz. When you exceed 35-36 Mhz, the hard disk and other IDE devices become unstable, because the IDE controller is controlled through the PCI bus. Oftentimes, you may encounter texture corruption, when your AGP bus exceeds a certain speed limit. This was often observed on older motherboards that wouldn't allow you to lock the AGP and the PCI bus at stock speeds. The good news: Immaterial of the FSB speed, most motherboards nowadays automatically ensure that the frequency of the PCI, AGP and other buses always remain constant (in other words; their speeds are locked unless you deliberately change them). This implies that the other components connected to the motherboard don't undergo stress. You have ensured that you have the fastest memory chip and the obscenely fast speed ratings on your memory chip ensure that you can extract the most by bumping up your FSB to the limits. The main culprit that is plays the spoilsport is your CPU. I'll explain: So even if you have an exceptionally good thermal solution, your CPU won't exceed a certain limit. Example: I had experimented with a Pentium III 700E Mhz processor and a Pentium III 800E MHz processor on an Asus CUBX-E motherboard using Kingston PC-133 SD-RAM. The reason I chose these 2 processors for experimentation was because they both used a FSB speed of 100MHz. This motherboard was really flexible, I was able to increment the FSB to 150Mhz. I was able to extract 1050 MHz from the stock 700Mhz. This is because the multiplier is 7, which unfortunately cannot be changed. So I bumped up the FSB from 100Mhz to 150 Mhz; which gave me the resultant speed of:
(Multiplier) × 150
(Front Side Bus) = 1050 MHz
Simple arithmetic? Yes. Now, logically speaking, if I can extract 1050Mhz from a 700Mhz processor; I should be able to extract 1200Mhz from an 800 MHz processor. This is not true. I tried doing exactly the same with the 800 MHz processor and the Computer crashed. However, it was stable when I set the FSB speed on a 133 MHz. When I set the FSB at 133 MHz ; this was the result:
(Multiplier) × 133
(Front Side Bus) = 1064 MHz
This simple experiment simply shows that a CPU gets saturated after a certain clock speed. Typical symptoms of an erratic CPU include instability and at times, you may not be able to boot up at all.
This particular CPU die was manufactured using a 0.18u process. When Intel launched a similar CPU using a 0.13u process; they shipped those CPUs with the stock speed of up to 1.4Ghz. This CPU core was based on the P6 Architecture and it used a 10 stage pipeline. Presently, Intel manufactures the Pentium-M CPU which is based on the P-6 architecture; difference being that they manufacture it using a 0.09u process and they have increased the depth of the pipeline.
These terms may seem cryptic and this concept may be difficult for some to grasp. So why would I publish something so difficult?The answer is simple: It is not difficult at all. To be a successful overclocker; you need to purchase the best CPU possible; not necessarily the fastest. Always go for a processor that uses the latest manufacturing process. A CPU rated at 3Ghz which is manufactured by using a 0.13u process won't overclock as well as a CPU that is rated at 2.6Ghz using a 0.09u process.
Deeper pipelines ensure that the CPU has the capability to scale higher in terms of speeds. The disadvantage is that a CPU with a deeper pipeline is slower than a CPU that uses a smaller pipeline assuming that they are running at the same speed). AMD Athlon CPUs are famous for their relatively short pipelines. Thats why they perform better than the Pentium 4 CPUs at the same clock speed. Before purchasing the fastest processor, always keep this in mind. Choosing a processor smartly helps you extract maximum out of your machine. You don't need to know what a pipeline exactly does. Refer to the processor spec sheet, find out these basic details of the CPU core and its architecture and choose accordingly.
Two different parts of a video card may be overclocked, the GPU (Graphics Processing Unit) and the RAM. In addition, disabled pipelines on a video card may also be enabled through third-party drivers, third-party software, or direct hardware modifications depending on your video card type. Overclocking a video card is usually done through third-party or proprietary software.
Recent ATI proprietary Catalyst drivers feature an interface called Overdrive that allows for dynamic GPU frequency scaling based on its temperature and load. Increase the load, the clock rate increases for performance, but it's balanced against the increasing temperature. Sufficient for simple increases in overall performance, but doesn't allow for the best performance increase which requires overclocking the memory. For this you need third-party applications or drivers. An example is ATITool. This program has many options, including GPU and memory overclocking, temperature monitoring, and fan control allowing for a much more complete solution to overclocking ATI based video cards. As for example drivers, for ATI there are many, omegadrivers.net is one of them, also hosted there are nVidia drivers as well. Both of which include integrated overclocking and many unlocked features, even including enhanced image quality for nVidia-based cards. nVidia video cards can also be OCed through a hidden feature in the driver called coolbits. Coolbits is a feature that can be unlocked by downloading a set of regkeys, it is a good overclocking tool as it has a fairly conservative "optimal clock" once you have this increase the core clock (not the memory clock!!) then run a gpu intensive task like 3dmark, repeat until you have a sudden drop in the benchmark score. This is the thermal throttling kicking in, do not push it any harder as it will result in permanent damage to your gpu. Back off the clockspeed by about 20-30mhz.
The most important thing to remember about overclocking a video card is cooling. This can't be stressed enough. Just the same as a CPU can be damaged or have a shortened lifespan by overclocking or excessive and prolonged heat, so can a video card. In the past year many inexpensive and easy to install options have surfaced for cooling a video card, from adhesive ram heatsinks to attach to un-cooled ram chips, to rather expensive water-cooling solutions. A good midpoint (both in cost and effectiveness) solution is to purchase and install a direct exhaust, "sandwich" cooling solution. Direct exhaust means all air from the cooling fan is blown across the video card and directly out of the computer case, usually using the open PCI slot below the AGP (or PCIe) slot. This allows for substantially lower GPU temperatures. A sandwich cooler is two aluminum or copper heatsinks, shape formed for a particular video card, that "sandwiches" the video card in between the two and are usually connected by some kind of copper heat pipe which allows for the hotter side to convey heat to the cooler side for dissipation. The GPU should never surpass 60 degrees celsius for optimal performance and to avoid damaging the card. Most of the latest video cards are rated to go up to 90c, but this is NOT recommended by anyone. The optimal temperature for a video card is 40-55c for the card itself (the GPU's temperature differs depending on which you have,) but the lower you can get it, the better.
One important thing to note. Many think that the option which says "AGP voltage" in their BIOS can be used to "voltmod" a video card to get a bit more power out of it. In fact, it's used for something else, and raising the AGP voltage can and probably will cause damage to a video card.
Getting the few extra MHz out of a chip
When increasing the speed of any computer components you are making the components work harder and by doing so they output more heat. Heat can cause system instability so cooling is necessary to help keep your components stable at higher speeds. Without good cooling you could harm or shorten the life of your system. CPU temperature can usually be checked from within the BIOS. However, these are inaccurate as your CPU is under almost no load in the bios. SiSoftware Sandra may be used within Windows to check temperature. This should be done when your CPU has been under a heavy load for a while for optimum results.
There are three types of cooling that are generally accepted for overclocking: Air, water, and peltier.
With both air cooling and water cooling some type of transfer material is needed to move the energy away from the sensitive electronics. The device used for this purpose is a heatsink. The two most popular heatsink materials are Aluminium and Copper. The heatsink that is stock on factory computers by major manufacturers (Dell, Gateway, IBM) is usually made of aluminium, which has satisfactory heat transfer characteristics. However when overclocking more heat is being produced from the increase in power consumption and in order to obtain lower tempertures a material with better heat transfer properties is important. For this reason Copper is the material that offers the best ratio of price/performance.
Chips at higher speeds may need more power. Raising the vcore voltage on a CPU might enable it to go at slightly faster speeds but by doing so you add a lot more heat output from the CPU. The vcore of a processor is the voltage at which a chip is set to run at with the stock speed. This voltage may need to be changed when the multiplier is raised because otherwise the transistors in the chip wouldn't switch fast enough - transistors switch faster, the higher the supply voltage. If there is not enough voltage then the chip will begin to make mistakes and give bad data results. Good cooling is needed to keep the system stable at higher speeds. Raising the vcore too much may harm or shorten the life of your system. Raising the vcore can also greatly affect the stability of the system. This is where a high quality PSU will come into play. While many cheap, no-name brand PSU's will crash and die with more vcore, a good quality one will live to serve you for a long time.
Note: increasing the speed (multiplier or fsb) without changing the voltage will also increase heat output, but not as much as when also increasing voltage. Having said that, increasing the multiplier or fsb without adjusting the voltage may make your system unstable (undervolt).