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A CPU lock or CPU locking is the process of locking down a CPU's clock multiplier, either permanently or until the lock is removed. The main purpose of this is to prevent users from overclocking the CPUs, making them operate in conditions that they were not designed for, and then possibly damaging them. This is also a common method of differentiating CPU models so that manufacturers can sell low-, mid- and high-end tier CPUs by locking down cores and multipliers to create a slower performance, making lower-tier models.
A CPU lock means that some function of the CPU is being inhibited, usually a core or a clock multiplier. This means that the CPU is locked to its current performance state and cannot be overclocked to increase performance or underclocked to reduce power draw. However, some CPU models are purposely unlocked to fill a specific segment of the market catering to enthusiasts and gamers to allow them to overclock the CPU. For Intel these are the models with a "K" designation, such as the Core i7-3770K, while AMD for a while used the "Black" designation before moving to the "K" designation as well.
CPU locking became a practice because during the earlier years of CPU manufacturing technology, as not all produced CPU die were the same; most were imperfect and some had areas that did not work. So instead of throwing away these imperfect yet working CPUs, they were sold as lower-end models. For example, for multiple-core CPUs that were supposed to have four cores, but due to manufacturing inconsistencies only had two or three cores active, the dead cores had to be locked so that they would not be used and cause problems, and then they were sold as lower-end models.
Further innovations in manufacturing ensured that most of the die that resulted from manufacturing were more or less perfect, so they no longer used CPU locks to lock down non-functioning cores; they instead used them to lock down functioning ones in order to continue to serve the lower-end segments.