Microprocessor frequency specifies the operating (internal) frequency of CPU's core. The higher the frequency is for a given CPU family, the faster the processor is. Processor frequency is not the only parameter that affects system performance. Another parameter than greatly affects the performance is CPU efficiency, that is how many Instructions Per Clock (IPC) the CPU can process. Knowing these two parameters it's easy to calculate total number of instructions per second that can be processed by CPU: Frequency * IPC. All modern AMD x86 microprocessors and all Intel microprocessors based P6, mobile and Core micro-architectures tried to improve their performance by improving the IPC, and, whenever possible, by increasing processor frequency. Intel Netburst micro architecture used quite different approach - it tried to increase processor frequency at the expense of IPC. This didn't work well for this micro-architecture.
Modern microprocessors do not always operate at the same frequency. To save power, all processors with PowerNow! or SpeedStep technology may temporarily reduce their operating frequency. Some mobile Intel Core 2 Duo processors may temporarily increase frequency of one of their cores when another core is idle.
The CPU frequency is measured in Hertz. The frequency can also be expressed in:
Internal frequency of microprocessors is usually based on Front Side Bus frequency. To calculate internal frequency the CPU multiplies bus frequency by certain number, which is called clock multiplier. It's important to note that for calculation the CPU uses actual bus frequency, and not effective bus frequency. To determine actual actual bus frequency for processors that use dual-data rate buses (AMD Athlon and Duron) and quad-data rate buses (all Intel microprocessors starting from Pentium 4) the effective bus speed should be divided by 2 for AMD or 4 for Intel.Clock multipliers on many modern processors are fixed - it is usually not possible to change them. "Extreme" versions of processors have clock multipliers unlocked, that is they can be "overclocked" by increasing clock multiplier in motherboard BIOS. Some CPU engineering samples may also have clock multiplier unlocked. Many Intel qualification samples have maximum clock multiplier locked - these CPUs may be underclocked (run at lower frequency), but they cannot be overclocked by increasing clock multiplier higher than intended by CPU design. While these qualification samples and majority of production microprocessors cannot be overclocked by increasing their clock multiplier, they still can be overcloked by using different technique - by increasing FSB frequency.
Front-Side Bus (FSB) is an interface between microprocessor core and other computer components, such as memory, peripheral devices and other microprocessors. FSB is often called a "system bus". Microprocessor performance highly depends on the speed, width and latency of the system bus:
The Thermal Design Power (TDP) is the average maximum power a processor can dissipate while running commercially available software. TDP is primarily used as a guideline for manufacturers of thermal solutions (heatsinks/fans, etc) which tells them how much heat their solution should dissipate. TDP is not the maximum power the CPU may generate - there may be periods of time when the CPU dissipates more power than designed, in which case either the CPU temperature will rise closer to the maximum, or special CPU circuitry will activate and add idle cycles or reduce CPU frequency with the intent of reducing the amount of generated power.TDP is usually 20% - 30% lower than the CPU maximum power dissipation.
Microprocessor manufacturers are constantly improving their products by re-designing and tweaking all or some CPU internal parts. These modifications fall into the following three categories: