Users usually unlock CPUs to allow overclocking, but sometimes to allow for underclocking in order to maintain the front side bus speed (on older CPUs) compatibility with certain motherboards. Unlocking generally invalidates the manufacturer's warranty, and mistakes can cripple or destroy a CPU. Locking a chip's clock multiplier does not necessarily prevent users from overclocking, as the speed of the front-side bus or PCI multiplier (on newer CPUs) may 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 may require different procedures.