In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly, operating voltage is also increased to maintain a component's operational stability at accelerated speeds. Semiconductor devices operated at higher frequencies and voltages increase power consumption and heat. An overclocked device may be unreliable or fail completely if the additional heat load is not removed or power delivery components cannot meet increased power demands. Many device warranties state that overclocking or over-specification voids any warranty, but some manufacturers allow overclocking as long as it is done (relatively) safely.
Overview
The purpose of overclocking is to increase the operating speed of a given component. Normally, on modern systems, the target of overclocking is increasing the performance of a major chip or subsystem, such as the main processor or graphics controller, but other components, such as system memory (RAM) or system buses (generally on the motherboard), are commonly involved. The trade-offs are an increase in power consumption (heat), fan noise (cooling), and shortened lifespan for the targeted components. Most components are designed with a margin of safety to deal with operating conditions outside of a manufacturer's control; examples are ambient temperature and fluctuations in operating voltage. Overclocking techniques in general aim to trade this safety margin by setting the device to run in the higher end of the margin, with the understanding that temperature and voltage must be more strictly monitored and controlled by the user. Examples are that operating temperature would need to be more strictly controlled with increased cooling, as the part will be less tolerant of increased temperatures at the higher speeds. Also base operating voltage may be increased to compensate for unexpected voltage drops and to strengthen signalling and timing signals, as low-voltage excursions are more likely to cause malfunctions at higher operating speeds.
While most modern devices are fairly tolerant of overclocking, all devices have finite limits. Generally for any given voltage most parts will have a maximum "stable" speed where they still operate correctly. Past this speed, the device starts giving incorrect results, which can cause malfunctions and sporadic behavior in any system depending on it. While in a PC context the usual result is a system crash, more subtle errors can go undetected, which over a long enough time can give unpleasant surprises such as data corruption (incorrectly calculated results, or worse writing to storage incorrectly) or the system failing only during certain specific tasks (general usage such as internet browsing and word processing appear fine, but any application wanting advanced graphics crashes the system).
At this point, an increase in operating voltage of a part may allow more headroom for further increases in clock speed, but the increased voltage can also significantly increase heat output, as well as shorten the lifespan further. At some point, there will be a limit imposed by the ability to supply the device with sufficient power, the user's ability to cool the part, and the device's own maximum voltage tolerance before it achieves destructive failure. Overzealous use of voltage or inadequate cooling can rapidly degrade a device's performance to the point of failure, or in extreme cases outright destroy it.
The speed gained by overclocking depends largely upon the applications and workloads being run on the system, and what components are being overclocked by the user; benchmarks for different purposes are published.
Underclocking
Conversely, the primary goal of underclocking is to reduce power consumption and the resultant heat generation of a device, with the trade-offs being lower clock speeds and reductions in performance. Reducing the cooling requirements needed to keep hardware at a given operational temperature has knock-on benefits such as lowering the number and speed of fans to allow quieter operation, and in mobile devices increase the length of battery life per charge. Some manufacturers underclock components of battery-powered equipment to improve battery life, or implement systems that detect when a device is operating under battery power and reduce clock frequency.