Overclocking Basics (particular to Intel systems, though many of the concepts apply to AMD systems as well)
1) Limitations - Each hardware component has physical limitations that, at a minimum, meet the rated (stock) specifications. In the process of thoroughly overclocking a system, the actual limitations of various components will be found. In general, the CPU, RAM, and motherboard will control the overclock, but other components such as the PSU and cooling will have a major impact on overclocking abilities.
Depending on which of the three primary components (CPU, RAM, and mobo) is the first to reach its limits, different steps can be taken to squeeze more out of the other components. Video card overclocking is generally independent of overclocking the components previously listed.
2) Overclocking in the BIOS vs. overclocking software - Whenever the option exists, manipulating BIOS settings is the best way to accomplish overclocking. BIOSes on value/low end motherboards and on proprietary systems such as Dells and HPs generally have few to no options available for overclocking. On such systems, there is the potential to overclock through software, though there is not a single piece of software to overclock every board.
3) CPU FSB vs. external clock speed - Intel overclocking is achieved via the front side bus or system bus. Depending on your system, it can be noted as FSB, CPU frequency, CPU speed, clock speed, or something similar in the BIOS. Intel CPUs more recent than Pentium 3s are “quad pumped”. This means that the external clock speed (the value shown in the BIOS) is one fourth the FSB, i.e. external clock speed = FSB/4.
4) DDR frequency vs. external clock speed - Conversely, DDR RAM transmits data on both sides of a tactical signal, effectively performing two functions per single clock cycle (i.e. DDR frequency = 2 x external clock speed). That is why it’s referred to as Double Data Rate RAM. The discussion of RAM applies equally to DDR, DDR2, and DDR3. There are a variety of ways this is displayed in the BIOS; some display the DDR frequency and other show it as a ratio of the CPU:RAM, which will be discussed below.
5) Dividers - The ratio of CPU:RAM is known as a divider. On older Intel systems, best performance is achieved through highest possible stable operation in synchronous (1:1) CPU:RAM operation. On such systems, the higher the FSB, the better performance. Newer Intel systems can benefit from a divider that favors the RAM (e.g. 3:4 which means the RAM runs as 4/3 the external clock speed - the CPU always operates at the external clock speed). It is generally best to start with a 1:1 divider and then test other dividers for potentially greater performance.
6) Multipliers - The multiplier is the ratio of external clock speed to processor frequency, i.e. external clock speed x multiplier = processor frequency. Older Intel CPUs had a locked multiplier, most current Intel CPUs have a multiplier that can be adjusted downward, and most Extreme Edition CPUs have multipliers that can be both lowered and raised.
CPUs tend to have a maximum frequency, which can be achieved through whatever combinations of external clock speed and multiplier that are available (e.g. if a CPU can handle 3.6 GHz, it can do so equally at 400x9, 450x8, and 600x6). Manipulating the multiplier permits fine tuning of CPU settings in relation to the RAM and mobo settings.
7) RAM Timings - All RAM has a series of latencies, generally referred to as timings. Smaller numbers are faster or "tighter" while larger numbers are slower or "looser". As RAM is overclocked, it is necessary to apply looser timings, and conversely, RAM can often be run at tighter timings by either running it below stock speed or by increasing the voltage.
8) Voltages - Different components of the system receive different amount of voltage, and it is generally necessary to increase voltages as frequencies are increased beyond stock speeds. The three most commonly tweaked are vcore (CPU voltage), vdimm (RAM voltage), and vMCH (Northbridge/memory controller). Excessive voltage can shorten the life of component or cause failure.
9) Temperatures and cooling - Quality cooling is essential to achieving and maintaining a good overclock. The temperature of various components should be monitored to ensure that they are being sufficiently cooled. CPU cooling receives the most attention. The stock cooler that comes with most retail CPUs is generally not suitable for overclocking. There is a wide variety of aftermarket air coolers that provide a correspondingly wide degree of cooling.
Water cooling is a popular, though more expensive, way of cooling components (generally limited to CPUs and video cards, though there are water blocks available for many types of components). Extreme cooling options such as phase change are also available.
In general, the cooler the component, the further it will overclock. Installing an aftermarket cooling on the Northbridge is common for moderate to high overclocks.
There are also aftermarket coolers for Southbridges and RAM, though those components do not often require additional cooling in most systems or sufficient additional cooling can be provided by placing a fan to blow across the component.
10) Steppings, batches, weeks - Intel occasionally makes a large update to a processor line, and it shows as a new stepping. Processors can often be identified by batches or weeks as well. This information can often be used to give a general prediction of overclocking potential, though it is not a guarantee. There are good overclocking processors that come out of "bad" weeks/batches and poor overclockers that come out of "good" ones. The odds of getting a good overclocker from a "good" week/batch is simply greater than from a "bad" one.
11) CMOS Jumper - Unstable overclocking settings can cause a system to freeze and/or not boot. Should rebooting not reset the system, stock settings can be restored by manipulating the CMOS jumper. Some motherboard have a CMOS reset button, and some have BIOS features to automatically prevent lock ups due to unstable overclocking settings.
BIOS 101
The BIOS on most boards can be accessed by pressing the DEL key at system startup. It is safe to browse through the BIOS options, and it is important to be familiar with the various options. BIOS options and terminology will vary from motherboard to motherboard, though the same basic options are available on all boards that can be overclocked (along with a host of advanced options).
BIOS menus are navigated with a keyboard. The arrow and Enter keys are used to browse and select menus and options. The ESC key accesses higher level menus, and when hit from the main menu, it will exit the BIOS (first prompting if the user wants to abandon changes and exit). The F10 key generally prompts the user to save changes and exit.
Before tweaking settings that directly affect overclocking, there are some standard settings that affect stability that should be set. They may not appear exactly as listed, but it will be something similar.
Spread Spectrum = disabled
PCI/AGP/PCIe = fixed, locked, or 33/66/100 (It is essential to lock the PCI and AGP frequencies, though some systems may benefit from a slightly raised PCIe frequency)
Stop unused PCI clock = enabled
Legacy USB = disabled
Furthermore, ensure that the Initial Display Adapter is set accordingly (i.e. PCI, AGP, or PCIe, depending on the video card’s interface). It is also a good idea to disable any unused features (e.g. serial port, parallel port, onboard audio, etc.) as this will free up resources.
Finally, any option relating to CPU frequency, RAM frequency, RAM timings, or voltages should generally be set to manual.
Overclocking Process
As stated above, overclocking is an art. Juggling the various settings can seem overwhelming initially, and it's often difficult to fight the urge to raise an overclock quickly. It is very important to be patient and take baby steps while making adjustments.
In general, the overclocking procedure is -
Increase the external clock speed by a small amount.
Exit BIOS and boot to operating system.
Test for stability and monitor temperatures.
Return to BIOS, tweak settings, and repeat process.
In greater detail -
1) Baby steps - Increase the external clock speed in small increments. "Small" is relative to the stock speed of the system, though 3-5 MHz is common for Pentiums while 5-10 MHz is common for newer CPUs. These numbers can be responsibly tweaked for a variety of reasons including personal experience and knowledge that a particular CPU stepping/week/batch is a good/bad overclocker. The steps can also be larger early in the overclocking process and smaller as the system gets closer to its limits. The important thing is to not take too large of a step as too many other variables can change if large jumps are made.
2) Boot up - Be sure to save your settings before rebooting. Some motherboards offer overclocking profiles, which can save settings after a CMOS reset or even a BIOS flash. Unsuccessful boots are not uncommon. Either return to step 1 and lower the external clock speed or jump to step 4 for other tweaks.
3) Stability testing - There are a variety of stability testing programs available, and they should be employed frequently during the course of overclocking. The extent of stability testing is up to individual preference, and there are a wide variety of philosophies concerning testing. It is generally a good idea to do at least a brief test at every step with a more thorough test every few steps. Some quality testing programs are -
Super Pi - Good for quick tests and benchmarking. This program will not provide robust stability testing.
Prime95 and Orthos - These programs provide thorough testing, and some versions work automatically on multiple core processors.
OCCT - Another thorough stress testing program.
Memtest86 - An excellent RAM testing program. Great for ruling out or identifying the RAM settings as an issue.
A brief test with one of these programs might be for several minutes to an hour with a thorough test ranging from several hours to a full day. Be sure to monitor temperatures when stress testing.
4) Return to BIOS and tweak - If stability testing was successful, return to step 1 and further increase the external clock speed. If the system booted but did not test stable, there are several settings which may help.
They include -
Adjust vcore - Increase the vcore one notch and repeat the testing. If more than two notches are required, try adjusting another setting.
Adjust RAM timings and vdimm - If a bit of vcore doesn't do the trick or Memtest86 identified the RAM as the source of instability, tweak the RAM settings. Loosening RAM timings and/or increasing vdimm may address this issue. Be aware that excessive vdimm will void most manufacturers' warranties.
Adjust Northbridge voltage - Higher frequencies require additional voltage to the NB. In general, this setting only goes up a few notches from stock speed to extreme overclocks. Stock Northbridge coolers may not be able to handle additional voltage, so it may be necessary to invest in aftermarket cooling.
As with increasing the clock speed, it is important to change these settings in small steps, reboot, and test for stability.
Maximizing the Overclock on a System
One way to simplify overclocking is to initially take the RAM out of the equation. Select a divider such that the RAM does not exceed stock speeds; this permits attention to be focused on the CPU and motherboard. Once the maximum overclock of those two components is found, manipulate the divider to determine the optimal frequency for the RAM. Be sure to use Memtest86 to test RAM stability. A few complete passes with that software is generally a good indication of stability.
Manipulating the CPU multiplier can lead to better performance on systems that support that feature. First, find the maximum CPU frequency as described above with the stock multiplier. Then, determine other combinations of external clock speed and multiplier that equate to the same CPU frequency. Using the example from item number 6, above, that CPU could equally handle 400x9 and 450x8. If the RAM and motherboard could safely handle the higher frequencies, the lower multiplier would most likely produce the best performance. Trial and error plays into this equation as well, due to the complexities of modern systems. It is important to benchmark a system with appropriate applications (e.g. using gaming benchmarks for a gaming system, productivity benchmarks for an office system, etc.) to see which combination of settings provide the best performance. Remember that each set of components is unique, and that the goal of overclocking is performance not any specified settings.
