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CPU locking is the process of permanently setting a CPU's clock multiplier. AMD CPUs are unlocked in early editions of a model and locked in later editions, but nearly all Intel CPUs are locked and recent models are very resistant to unlocking to prevent overclocking by users. AMD ships unlocked CPUs with their Opteron, FX and Black Series line-up, while Intel uses the moniker of "Extreme Edition." Intel usually has one or two Extreme Edition CPUs on the market, leading their range in clock speed and starting at $999-1499 USD as well as K series CPUs synonymous with AMD's black edition. AMD has the majority of their desktop range in a Black Edition, with very little price differences from the standard editions.
Users unlock CPUs to allow underclocking, overclocking, and front side bus speed compatibility with certain motherboards, but unlocking invalidates the manufacturer's warranty and mistakes can cripple a CPU. However, locking a chip's clock multiplier does not necessarily prevent users from overclocking, as the speed of the front-side bus can still be changed to provide a performance increase. AMD Athlon and Athlon XP CPUs are generally unlocked by connecting bridges (jumper-like points) on the top of the CPU with conductive paint or pencil lead. Other CPU models (determinable by serial number) require different procedures to be unlocked.
One of the easiest ways to unlock an older AMD Athlon XP CPU was the pin mod method, because it was possible to unlock the CPU without a permanent modification of any bridges. Just putting one wire (or some more for a new multiplier/Vcore) into the socket would unlock the CPU.
There are unlocked intel versions available today that do not violate the manufacturer's warranty such as "4th Gen Intel CPUs".
(also referred to as "Haswell")
An example would be the Intel Core i7-4770K Processor: Intel Core i7-4770K Processor - Quad Core, 8MB L3 Cache, 3.5GHz, 84W, Fan, Unlocked Multiplier, 1250 MHz Graphics Core Speed.