# Correct explanation of CPU frequency, external frequency, frontend bus frequency, and memory frequency

Source: Internet
Author: User

Nowadays, there is no uniformity in the naming of the same thing on the Internet. It seems like there are many types of things, so it is necessary to clarify the clues first, find out the equivalence relationships between different names: 1. CPU external frequency = External frequency  2. CPU frequency = clock speed   3. Frontend bus frequency  4. System bus frequency   5. MultiplierRelationships:     The external frequency of the CPU is the baseline frequency of the CPU and even the entire computer system. The unit is MHz (Z ). In computer systems, most frequencies are based on external frequencies and multiplied by a certain number of multiples. The AGP bus frequency is usually fixed at 66 MHz. Secondly, the front-end bus is usually two to four times the external frequency of the CPU, and there is no fixed multiple, which is related to the CPU model and Motherboard chipset. Besides, the system bus should be a general concept. It can represent all bus in the system (Front-End bus, AGP bus, PCI bus, etc.), and sometimes guide bus between North bridges, there is no fixed frequency or algorithm.CPU frequency = External frequency X Multiplier (Or frequency = External frequency X frequency doubling)Intelcpu Front-End bus = External frequency * 4 (MHz)
Amdcpu Front-End bus = External frequency * 2 (MHz)
CPU data bandwidth = frontend bus x 8 (MB/s)
Memory bandwidth = memory equivalent operating frequency * 8 (MB/s) (note that the unit is byte. The unit of 8 here is byte B, not 8 bits, the result is 64bit/8 = 8B, and 64bit is the number of data digits transmitted each time by the computer)       A bus is a set of transmission lines that transmit information from one or more source components to one or more target components. Generally speaking, it is a public connection between multiple parts, which is used to transmit information between parts. People often describe the bus frequency at a speed expressed in MHz. There are many bus types. The English name of the front-end bus is the front side bus, which is usually expressed as FSB. It is the bus that connects the CPU to the North Bridge Chip.      The CPU is connected to the beiqiao chip through the front-end bus (FSB), and then the data is exchanged with the memory and video card through the beiqiao chip. The front-end bus is the main channel for data exchange between the CPU and the outside world. Therefore, the data transmission capability of the front-end bus has a great effect on the overall performance of the computer. If the front-end bus is not fast enough, A strong CPU does not significantly increase the overall speed of the computer. The maximum bandwidth of data transmission depends on the width and transmission frequency of all data transmitted simultaneously, that is, the data bandwidth = (bus frequency × data Bit Width) ÷ 8. Currently, the frontend bus frequency on a PC can be 266 MHz, 333 MHz, 400 MHz, 533 MHz, or 1066 MHz, up to MHz. The higher the front-end bus frequency, it indicates that the larger the data transmission capability between the CPU and the North Bridge Chip, the more powerful the CPU function is. The current CPU technology is developing rapidly, and the computing speed is increasing rapidly. The large enough front-end bus can ensure that enough data is provided to the CPU. The low front-end bus cannot supply enough data to the CPU, this limits the CPU performance and becomes a system bottleneck.              The concept of external frequency is based on the fluctuation speed of Digital pulse signals. That is to say, a 10 thousand MHz external frequency refers to a digital pulse signal that oscillates thousand times per second. the external frequency refers to the bus speed between the CPU and the chipset. The external frequency is the synchronous speed between the CPU and the motherboard, and the external frequency in most computer systems is also the synchronous speed between the memory and the motherboard. In this way, it can be understood that the external frequency of the CPU is directly connected to the memory to realize the synchronous running status between the two.     Before 486, the CPU clock speed is still in a low stage, and the CPU clock speed is generally equal to the external frequency. However, after the advent of 486, due to the continuous increase in the CPU operating frequency, some other devices (such as plug-in cards and hard disks) on the PC are restricted by the technology and cannot withstand a higher frequency, this limits the further improvement of the CPU frequency. Therefore, the frequency doubling technology is introduced. This technology can change the internal frequency of the CPU to a multiple of the external frequency, so as to increase the frequency by increasing the frequency doubling. The multiplier technology enables external devices to work at a lower frequency (the lower frequency is the outer frequency) without limiting the operating frequency (clock speed) of the CPU ), because the CPU clock speed is a multiple of the external frequency.

Furthermore, the concepts of the front-end bus and the outer frequency are confusing, mainly because, for a long time ago (mainly before the emergence of Pentium 4 and when the emergence of Pentium 4 ), the frequency of the front-end bus is the same as that of the outer frequency. Therefore, it is often called the front-end bus as the outer frequency, which leads to such misunderstanding. With the development of computer technology, it is found that the frequency of the front-end bus is higher than that of the outer frequency, so the QDR (quad
Date rate) technology (4 times of concurrency), or other similar technical implementation. The principles of these technologies are similar to the 2x or 4x of AGP. They make the front-end bus frequency twice, 4x, or even higher than the outer frequency, since then, the difference between the front-end bus and the external frequency has been paid attention. When the external frequency is still 133 MHz, the speed of the front-end bus is increased by 4 times to 533x4 = 200 MHz. When the external frequency is increased to 800 MHz, the front-end bus is changed to MHz, so you will see p4 of the 533 Front-End bus and P4 Of the 800 Front-End bus.
Generally, both the clock speed (CPU frequency) and the Front-End bus frequency are based on the outer frequency. The former is multiplied by the multiplier, and the latter by the 2/4/8, however, the coefficient of the latter multiplication cannot be called a multiplier because it is implemented by QDR (quad date rate) technology or other similar technologies. These three were concepts in the early stages.
As for the memory, there are usually three frequencies to describe it-core frequency/clock frequency/data transmission rate. The data transmission rate is the frequency marked on the memory stick, such as ddr333 and ddr400. In general, the memory frequency is the index data transmission rate by default. The core frequency of the memory is like the CPU frequency, which is the inherent frequency, and the clock frequency is what we call the external frequency. For DDR, the ratio of the three is. For DDR2, the ratio of the three is.      In the past P3, the core frequency of memory was 133, the data transmission rate was 133, and the front-end bus of CPU was 133. At present, the CPU of P4 reaches 133 MHz (× 4) with an external frequency of 533, and the memory frequency is 266 (ddr266 ). When the problem arises, the front-end bus is a bridge between the CPU and the memory. At this time, the front-end bus reaches 533, while the memory is only 266 faster, and the memory is half slower than the front-end bus of the CPU, theoretically, the CPU has to wait for the memory to transmit data before processing the data. This means that the memory is dragging the CPU down. This is indeed the case. This is true for 845 and 848 boards. Therefore, the concept of a dual-channel memory is proposed. The two memory uses two channels to work together to provide data, which doubles the speed again. The two ddr266 have a speed of 266x2 = 533, exactly p4
The front-end bus speed of the CPU is not slow. When the external frequency is increased to 200, the CPU Front-End bus is changed to 800. Two ddr400 memories constitute a dual channel, and the memory transmission speed is also 800. Therefore, to make good use of P4, you must use dual-channel memory. More than 865 of the motherboard provides this function. However, the 845 and 848 boards do not have a dual-channel memory function.
Related Keywords:

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: info-contact@alibabacloud.com and provide relevant evidence. A staff member will contact you within 5 working days.

## A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

• #### Sales Support

1 on 1 presale consultation

• #### After-Sales Support

24/7 Technical Support 6 Free Tickets per Quarter Faster Response

• Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.