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CPU Terms and Definitions

CPU Frequency

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:

  • Kilohertz, or kHz, equals to 1,000 Herts
  • Megahertz, or MHz, equals to 1,000,000 Herts or 1,000 kHz
  • Gigahertz, or GHz, equals to 1,000,000,000 Herts, or 1,000,000 kHz, or 1,000 MHz.
First microprocessors ran at frequencies close to 1 MHz. Modern microprocessors run at frequencies exceeding 3 GHz, or 3,000,000,000 Hertz.

Bus clock multiplier

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)

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:

  • Wider system bus allows the CPU to transfer more data during each bus cycle.
  • Faster system bus allows the CPU to transfer data faster from/to memory or peripheral devices.
Please note that buses of many modern microprocessors utilize dual-data and quad-data rate transfers, which effectively doubles and quadruples bus speed. For instance, the bus of first Pentium 4 microprocessors was clocked at 100 MHz. Because the data could be sent 4 times during each bus cycle, the bus effectively worked as 400 MHz bus.

Thermal Design Power (TDP)

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.

Core name

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:

  • Major micro-architecture overhaul, where all parts of the CPU are redesigned from scratch, often using new methods and techniques of increasing CPU performance. These changes are introduced as new CPU micro-architectures.
  • Adding new and/or changing existing parts of the CPU core with the purpose of improving processor performance. These modifications are presented as new core generations.
  • Minor tweaks to the core with the purpose of fixing existing errata, increasing microprocessor stability and/or reliability, or enhancing other CPU characteristics. These changes are classified as new core steppings.
Each new core generation adds new, or substantially changes one or more of existing CPU features, without changing the underlying micro-architecture the CPU is based upon. During development phase of each new core generation, the core is assigned a unique name, so called "codename". While this name is widely used in companies' roadmaps and other pre-launch documents, it is not mentioned in product documentation or advertisements aimed at general public. Codenames, or core names, are still used by hardware enthusiasts for historical reasons, and because they may be more descriptive than, say, family names. Some CPU families, like Celeron and Sempron, span not only multiple core generation, but also multiple micro-architectures. Using core name for processors from these and similar families gives much better understanding about CPU features and functionality than the family name