Clock multiplier

The clock multiplier (or CPU multiplier or bus/core ratio) is the ratio of the internal CPU clock rate to the externally supplied clock. A CPU with a 10x multiplier will thus see 10 internal cycles (produced by PLL-based circuitry) for every external clock cycle. For example, a system with an external clock of 133 MHz and a 10x clock multiplier, will have an internal CPU clock of 1.33 GHz. The external address and data buses of the CPU (often collectively termed front side bus in PC contexts) are also using the external clock as a fundamental timing base, however, they might as well employ a (small) multiple of this base frequency (typically two or four) in order to transfer data faster.

Topology of a typical x86 computer. Notice the FSB interconnecting the CPU and northbridge.

Basic system structure

Modern computers have several interconnected devices (CPU, RAM, peripherals, etc - see diagram) that typically run at different speeds. Therefore internal buffers and caches are used when communicating with each other via the shared buses in the system. In PCs, the CPU's external address and data buses connect the CPU to the rest of the system via the "northbridge". Nearly every desktop CPU produced since the 486DX2 has employed a clock multiplier to run its internal logic at a higher frequency than its external bus, but still synchronous with it. This improves the CPU performance by relying on internal cache memories and/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 processors, 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

Most systems allow the clock multiplier to be changed 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 are the two common methods of overclocking and underclocking a computer, perhaps combined vith some adjustment of CPU or memory voltages (changing oscillator crystals is not commonly done); note that careless overclocking can cause damage to a CPU or other component due to overheating or even voltage break down. (Newer CPUs often have a fixed bus/CPU clock ratio, leaving the external clock as the only remaining means by which the internal CPU clock could be changed.)

Clock doubling

Clock doubling refers to a clock multiplier of two.

The most famous example of a clock-doubled CPU is the Intel 80486DX2, which ran at 50 or 66 MHz on a 25 or 33 MHz bus. Another example was the Weitek SPARC POWER µP, a clock-doubled 40 MHz version of the SPARC processor that could be dropped into the otherwise 20 MHz SPARCStation 2. In both cases the overall speed of the systems increased by about 75%.^{[citation needed]}

Today almost all high-performance processors (i.e. excluding typical embedded systems) run at higher speeds than its external buses, so the term clock doubling is redundant.

For CPU-bound applications, clock doubling will theoretically improve the overall performance of the machine substantially, where fetching of data from memory is not the limiting factor. In more modern processors where the multiplier is much higher than two, the bandwidth and latency of specific memory ICs (and/or the bus or memory controller) typically becomes a limiting factor.

See also

• Overclocking
• Memory divider