CPU locking - Wikipedia

The locking of a central processing unit (CPU) means it has a fixed, preset clock multiplier. CPUs generally are either sold unlocked, locked (but unlockable with some difficulity), or unlockable to the point where not a single individual has ever been known to unlock one before.

AMD's CPU locking practices usually entail the release of unlocked CPUs for early sales of a new model, then the locking of all CPUs of that model thereafter. AMD CPUs are generally unlockable. However, CPUs from Intel for example are released locked, and are completely unlockable. This has the benefit of preventing fraud in that a CPU can not be passed off as being a faster model than it actually is, but it places limits on what speed a consumer chooses to run their CPU at.

The practice of unlocking CPUs requires some skill. It is done for one of three reasons:

to allow a CPU to be underclocked
to allow a CPU to be overclocked
to allow the CPU to run at its correct MHz speed with a slower or faster front side bus than its preset clock-multiplier was set with in mind.
For example, some individuals have unlocked Athlon XP CPUs for the purposes of manually setting a higher multiplier so the CPU will work at the correct speed with an older (and therefore slower) motherboard.

Unlocking a CPU has its pitfalls: it will invalidate the manufacturor's warrenty, and a mistake in the unlocking process can potentially destory the CPU's ability to function properly. Usually an unlocked state is realised by connecting jumper-like points (referred to as bridges) on the CPU's surface together with a conductive material, such as pencil lead or special paint. Different CPU models requite different procedures to be unlocked; usually the serial number marked on a unit belies what model it is and thus (with some research) what is required to perform the procedure.