Editing Central processing unit (section)

===Clock rate===
{{Main|Clock rate}}
Most CPUs are [[synchronous circuit]]s, which means they employ a [[clock signal]] to pace their sequential operations. The clock signal is produced by an external [[Electronic oscillator|oscillator circuit]] that generates a consistent number of pulses each second in the form of a periodic [[Square wave (waveform)|square wave]]. The frequency of the clock pulses determines the rate at which a CPU executes instructions and, consequently, the faster the clock, the more instructions the CPU will execute each second.

To ensure proper operation of the CPU, the clock period is longer than the maximum time needed for all signals to propagate (move) through the CPU. In setting the clock period to a value well above the worst-case [[propagation delay]], it is possible to design the entire CPU and the way it moves data around the "edges" of the rising and falling clock signal. This has the advantage of simplifying the CPU significantly, both from a design perspective and a component-count perspective. However, it also carries the disadvantage that the entire CPU must wait on its slowest elements, even though some portions of it are much faster. This limitation has largely been compensated for by various methods of increasing CPU parallelism (see below).

However, architectural improvements alone do not solve all of the drawbacks of globally synchronous CPUs. For example, a clock signal is subject to the delays of any other electrical signal. Higher clock rates in increasingly complex CPUs make it more difficult to keep the clock signal in phase (synchronized) throughout the entire unit. This has led many modern CPUs to require multiple identical clock signals to be provided to avoid delaying a single signal significantly enough to cause the CPU to malfunction. Another major issue, as clock rates increase dramatically, is the amount of heat that is [[CPU power dissipation|dissipated by the CPU]]. The constantly changing clock causes many components to switch regardless of whether they are being used at that time. In general, a component that is switching uses more energy than an element in a static state. Therefore, as clock rate increases, so does energy consumption, causing the CPU to require more [[heat dissipation]] in the form of [[CPU cooling]] solutions.

One method of dealing with the switching of unneeded components is called [[clock gating]], which involves turning off the clock signal to unneeded components (effectively disabling them). However, this is often regarded as difficult to implement and therefore does not see common usage outside of very low-power designs. One notable recent CPU design that uses extensive clock gating is the IBM [[PowerPC]]-based [[Xenon (processor)|Xenon]] used in the [[Xbox 360]]; this reduces the power requirements of the Xbox 360.<ref>{{cite web | last = Brown | first = Jeffery | title = Application-customized CPU design | publisher = IBM developerWorks | url = http://www-128.ibm.com/developerworks/power/library/pa-fpfxbox/?ca=dgr-lnxw07XBoxDesign | year = 2005 | access-date = 2005-12-17 | archive-url = https://web.archive.org/web/20060212002837/http://www-128.ibm.com/developerworks/power/library/pa-fpfxbox/?ca=dgr-lnxw07XBoxDesign | archive-date = 2006-02-12 | url-status = dead}}</ref>