a clock multiplier to run its internal logic at a higher frequency than its external bus, but still remain synchronous with it. This improves the CPU performance by relying on internal cache memories or wide buses (often also capable of more than one transfer per clock cycle) to make up for the frequency difference. Variants Some CPUs, such as Athlon 64 and Opteron, handle main memory using a separate and dedicated low-level memory bus. These processors communicate with other devices in the system (including other CPUs) using one or more slightly higher-level HyperTransport links; like the data and address buses in other designs, these links employ the external clock for data transfer timing (typically 800 MHz or 1 GHz, as of 2007). BIOS settings Some systems allow owners to change the clock multiplier in the BIOS menu. Increasing the clock multiplier will increase the CPU clock speed without affecting the clock speed of other components. Increasing the external clock (and bus speed) will affect the CPU as well as RAM and other components. These adjustments provide the two common methods of overclocking and underclocking a computer, perhaps combined with some adjustment of CPU or memory voltages (changing oscillator crystals occurs only rarely); note that careless overclocking can cause damage to a CPU or other component due to overheating or even voltage breakdown. Newer CPUs often have a locked clock multiplier, meaning that the bus speed or the clock multiplier cannot be changed in the BIOS unless the user hacks the CPU to unlock the multiplier. High end CPUs, however, normally have an unlocked clock multiplier. Clock doubling The phrase clock doubling implies a clock multiplier of two. Examples of clock-doubled CPUs include: the Intel 80486DX2, which ran at 50 or 66 MHz on a 25 or 33 MHz bus the Weitek SPARC POWER µP, a clock-doubled 80 MHz version of the SPARC processor that one could drop into the otherwise 40 MHz SPARCStation 2 In both these cases the overall speed of the systems increased by about 75%. By the late 1990s almost all high-performance processors (excluding typical embedded systems) run at higher speeds than their external buses, so the term "clock doubling" has lost much of its impact. For CPU-bound applications, clock doubling will theoretically improve the overall performance of the machine substantially, provided the fetching of data from memory does not prove a bottleneck. In more modern processors