Cpu clock что это
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Cpu clock что это

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What are system clock and CPU clock; and what are their functions?

While reading a book, I came across a paragraph given below:

In order to synchronize all of a computer’s operations, a system clock—a small quartz crystal located on the motherboard—is used. The system clock sends out a signal on a regular basis to all other computer components.

And another paragraph:

Many personal computers today have system clocks that run at 200 MHz, and all devices (such as CPUs) that are synchronized with these system clocks run at either the system clock speed or at a multiple of or a fraction of the system clock speed.

Can anyone kindly tell:

  • What is the function of the system clock? And what is meant by “synchronize” in the first paragraph?
  • Is there any difference between “system clock” and “CPU clock”? If yes, then what is the function of the CPU clock?

Gilles 'SO- stop being evil''s user avatar

2 Answers 2

The system clock is needed to synchronize all components on the motherboard, which means they all do their work only if the clock is high; never when it’s low. And because the clock speed is set above the longest time any signal needs to propagate through any circuit on the board, this system is preventing signals from arriving before other signals are ready and thus keeps everything safe and synchronized. The CPU clock has the same purpose, but is only used on the chip itself. Because the CPU needs to perform more operations per time than the motherboard, the CPU clock is much higher. And because we don’t want to have another oscillator (e.g. because they also would need to be synchronized), the CPU just takes the system clock and multiplies it by a number, which is either fixed or unlocked (in that case the user can change the multiplier in order to over- or underclock the CPU).

The answer provided by @Benjoyo is not accurate. The CPU clock can be lower than the system clock, at least in microcontrollers.

As seen in this link, the PIC microcontrollers have the CPU clock, with a rate of $f_$ (also called instruction cycle frequency), and the system clock, with a rate of $f_$ (oscillator frequency).

Physically, what provides the clock «ticks» to the microcontroller is a crystal oscillator. The oscillator basically has a crystal ( X1 in the figure below) that when energized vibrates at a constant frequency.

enter image description here

The name «oscillator frequency», suggests that $f_$ is the input frequency of the microcontroller, that comes from the crystal oscillator. However, $f_$ usually refers to a scaled version of the crystal oscillator frequency, with PLL. The actual crystal oscillator frequency can be named as XTFREQ .

Fosc is the clock eventually with PLL, so with an 8MHz crystal and 4xPLL you would have an Fosc of 32MHz.

On the other hand, the CPU clock $f_$ is obtained by dividing $f_$ by 2 as seen in the image below (this depends on the microcontroller).

enter image description here

So a more precise statement would be that the «CPU clock can be much higher than the input crystal oscillator clock ( XTFREQ )». Also, depending on the system the CPU clock can actually be lower than the system clock (as mentioned above) or higher [ref].

Clock Speed Defined

cpu clock speed

You might be looking/hearing clock speed whenever a processor is discussed?

You might not know what it’s?

Or, you might be wondering what is over-clocking and under-clocking?

And, What is good for you?

Well, in this guide, I’ll be telling you all you need to know about CPU Clock Speed.

What’s Clock Speed?

Basic Definition: Clock rate or clock frequency at what CPU processor oscillates.

The processor in our CPUs has a built-in oscillator that completes an instruction cycle at a certain speed. This is known as clock speed and related to IPC (instructions per second). Processors are powerful nowadays, and their speed is usually told in MHz(MegaHertz)/GHz (GigaHertz).

1GHz processor can generate up to a billion pulses per second. On the technical side, a quartz crystal circuit determines the clock speed.

Why the term Clock Speed? Why not CPU speed?

Well, we can’t get its name changed now,

but, I can tell the reason behind it. As we measure most of the things against time and clock speed is measured in terms of seconds. So, it gets its name this way.

Secondly, clock speed doesn’t affect performance totally. It is one of the many measures that contribute to the overall speed. Doubling your clock speed might not have a great effect unless you change the rest of the hardware.

The clock speeds we are having commonly are enough as well, and the remaining hardware still barely copes up, as we don’t usually get an SSD with 4 billion writes per second, yet 4.0 GHz processors are becoming the new normal.

Other hardware tends to be a bottleneck in this case preventing our processor to be utilized fully when it needs tasks to be done by other hardware components.

Having said this, it stands true that you can get a performance upgrade if you overclock your CPUs, or you might get a better battery life by underclocking/undervolting.

Things are going to get interesting so keep reading.


It would be clear by its name that you would increase the base clock speed.


Now, if I can’t get my job done with the base speed and want to bring my processor to its maximum capability, increasing my processor’s clock speed would help me. Overclocking increases a CPU’s performance and you would be able to use that extra clock rate to play your favorite games.

You might be thinking why aren’t manufacturers providing processors with clock speeds less than what they are capable of?

This is due to the extra heat generated as it consumes extra resources, and your laptop would drain much sooner than you can expect. Additionally, it’s usually not beneficial for the hardware as it wears out and your processor might start to throw errors. When a processor is highly overclocked, it will behave in a reverse manner by underclocking itself to reduce heat generation.

You can reduce heat issues by undervolting your processor but that will not be of much help as you’d learn later in this post that it won’t be one of the best combinations to try with your processor.

Should I overclock my CPU?

If your PC has proper vents to maintain CPU temperature, and your workload isn’t being fulfilled at the base speed, then you might need to overclock.

It’s not recommended in cases where you’re doing it to just gain extra speed while you don’t know where to use it. An overclocked CPU’s warranty might not be claimed and possibly cause heat damage.

Under Clocking

Underclocking is clocking down the processor to a lower speed that remains usable.

CPUs use less power and generate minimum heat in an underclocked state. Using less power means your laptop would have longer battery life. You can improve battery life in a better way as undervolting seems to be a better choice that’s explained in the next section.

  1. cooler devices
  2. no fan noise

while you won’t get the optimal performance that may not be needed in your case.

Should I under-clock my CPU?

Underclocked processors are mainly used by server administrators. Some servers don’t require the maximum performance and they underclock to conserve power generating lesser heat.

If you have a similar situation where you don’t the maximum performance from your computer, then you are good to go.

What’s Undervolting? Is Undervolting & Underclocking the same?

Undervolting directly relates to your power usage and doesn’t directly affect your processor’s clock rate. An undervolt processor would consume less power, and you don’t need to have a lower clock speed for it.

Undervolting can be done along with underclocking. It’s not recommended as you are not giving enough resources to your processor, an result in a much lower CPU speed.

Undervolting conserves power while increasing your battery life.

Is clock speed the only way to measure CPU performance?

Clock speed is one of the main ways to understand how fast your processor is, but there are more factors as well.

As the processor is dependant on other components/factors, it’s clear that clock speed is not the only measure.

When you buy your CPU, different factors need to be kept in mind as well.

Firstly, clock speeds are for each core, so if you have a single 4.0 GHz or a Quadcore 3.0 GH. You’d likely go for the latter as 4 different cores are working at 3.0GHz.

Bus speed is the speed at which our processor and other peripheral communicate. A higher bus speed means robust speeds.

Other well-known components like your RAM and SSD play a very vital role, and these all often become a bottleneck.

Summing up

The clock speed cannot be taken at face value.

You need to look at options and all the metrics told when choosing your CPU. A multi-core CPU with reasonable clock speed, a higher bus speed, faster RAMs & SSDs will be far efficient than a single processor with high clock speed.

Clock Rate of CPU and its effects on Computer Performance

Are you familiar with the clock rate of your CPU? If you are a computer user, then you know that your computer’s CPU works like a brain on your computer. Your computer performance most of the time depends on the performance of your CPU. Performance of a CPU most of the time depends on the clock rate of CPU. So, if you want to understand the things that are responsible for the performance of a CPU, you should know what is clock rate of a CPU. Go through this article to know all about clock rate of a CPU.

Clock Rate of CPU

Table of Contents

What is Clock Cycle of CPU

You may know what is CPU in a computer. CPU is like a brain in a computer like a brain in a living kind. So, the speed of a computer is mostly depended on the speed of its CPU. Clock Cycle is referred to the speed of a CPU. The clock cycle is the amount of time between two Cycles. During a clock cycle, one or more instructions are processed. So, if a CPU can process a higher number of pulses per second, it will be able to process information at a high speed.

What is Clock Rate of CPU

You may have seen in your CPU description that your CPU clock speed is 1-3 GHz. Have you wondered what does that mean? Actually, it is your CPU’s clock rate. Clock rate means the number of pulses generated by CPU in one second. It is generally measured in MHz (Megahertz) or GHz (Gigahertz). Today’s computers generally run at a clock rate having more than one Gigahertz. Generally, clock speed or clock rate is determined by a quartz-crystal circuit.

History of Clock Speed in CPU

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If you look back to 1980, you will find that Intel 8088 processor was quite popular at that time. You will be surprised to know that this CPU has only a clock speed of 4.77 MHz. CPU clock speed then started doubling every year. Today, if you buy a cheap computer, it will have a CPU that has a clock speed of at least 1 GHz. Nowadays, a CPU having a clock speed of around 3 GHz is so common.

How Does Clock Speed Affect the CPU Performance

Think of your brain. How do you determine the speed of a person’s brain? The fast he can think and decide determine the speed of his brain. It is same for CPU of a computer. The fast a CPU can process instructions, the faster the CPU. CPU’s clock rate determines how fast a CPU can work. The speed of a CPU is the rate at which a CPU can complete a processing cycle. You know about the clock cycle.

Suppose, your CPU’s clock rate is 1 GHz. If your friend has a CPU which clock rate is 2 GHz, then your friend’s CPU’s clock speed is twice of your CPU’s. But it cannot be said that the 2 GHz CPU is two times faster than the 1 GHz CPU. It depends on many things including computer architecture. Suppose, the 2 GHz CPU can complete an instruction in 4 Cycles and the 1GHz CPU needs 6 Cycles to complete the same instruction then the 2 GHz CPU will perform the same operation more than two times faster than the 1 GHz CPU.
If the situation is opposite, that means, the 2 GHz CPU needs more cycles than the 1 GHz CPU to perform the same operation, then the 2 GHz CPU will work less than 2x fast than the 1 GHz CPU.

Other Factors That Determine the Performance of a Computer

  1. The type of microprocessor
  2. The bus architecture
  3. The nature of the instructions set
  4. The amount of Random Access Memory (RAM) etcetera

Some processors can execute only one instruction during a clock cycle. Most of the today’s CPUs can execute more than one instructions per clock cycle. So, the CPUs that can complete more instructions per cycle than others will work faster. In the same way, the computer with a 64-bit bus architecture will work faster than the computer work in a 32-bit bus architecture. So, you cannot find any simplistic or universal relation among clock speed, bus architecture.

Clock Rate Formula

CPU clock rate formula can be determined in many ways. Here we will talk about two formulas to determine the performance of a CPU. To understand that you have to know about instructions, CPI, Clock cycle time, Clock rate. You have known all these from previous sections of this article except CPI.

CPI: You have known about Clocks and instructions. CPI stands for Clocks per Instructions. That means, how many clocks are needed to complete an instruction.

CPU Time = I * T * CPI

I = Total number instructions in a program

CPI = Clocks needed to complete an instruction

T = Clock cycle time

CPU Time = I * CPI/R

R = Clock rate which can be measured by the inverse of Clock cycle time that means, R = 1/T

Note: T or R are generally measured for knowing the performance of a processor. I require special profiling software. CPI depends on many other factors such as RAM, computer architecture etcetera.

Fastest CPU Clock Speed

It is a difficult thing to say about the fastest clock speed. Because clock speed is depended on many things including cooling process, motherboard etcetera. Again, overclocking of a CPU can give higher clock speed. Vendors including AMD, Intel are manufacturing CPU having a clock speed of above 8 GHz. The clock speed of 8722.78 MHz was recorded as the world’s fastest clock speed until this article was written. The CPU is AMD FX-8370. The user of this CPU was The Stilt and liquid nitrogen cooling system was used.

Note: We haven’t discussed the clock speed of CPUs that are used in supercomputers.

Multi-Cores vs Multi-Processors

Before 2005, all processors on the market had a single core. So, clock speed was the main scale to measure the performance of a CPU. But nowadays, processors with multi-cores are common. Even a very cheap laptop nowadays comes with at least two or three cores. Even processors with multiple processors are becoming available. Each processor in the processor has multiple cores too.

Multi-core processors became popular because a processor having a single core cannot be given too much clock speed because of technological limitations. A core of a processor is a single processing unit. So, a multi-core processor has multiple processing units. A 2.0 GHz dual-core processor has two processing unit each having 2.0 GHz clock speed that comprising 4 GHz clock speed. So, a 5 core 2 GHz processor has a total clock speed of 10 GHz.

Clock Speed vs Cores

You now know the benefit of having a multi-core processor with a higher clock speed. But having too many cores with a higher clock speed is not possible most of the time because of the cost. So, it becomes difficult to choose a processor for buying. You can go for a processor having slower clock speed but many cores, or you can go for a processor having higher clock speed but fewer cores. To understand this matter, we are giving you pros and cons of these two possible choices.

Processor Having More Cores but Slower Clock Speed


  1. It will give more benefits to the applications that support multi-threading
  2. It is a cost-effective way of increasing performance
  3. This kind of processor can run more applications at once


  1. This kind of processors cannot give a higher single-threaded performance.

Processor Having Fewer Cores but Higher Clock Speed

  1. This kind of processors can give a higher single-threaded performance.
  2. Lower cost option
  1. It cannot give more benefits to the applications that support multi-threading
  2. Many cores are not available to split between applications

How to Check CPU Speed in Windows

There are many ways to check CPU speed/Clock rate on your computer. Among them, 4 best ways are given below:

From System Properties:

  1. Right-click on This PC or my computer (It can be named differently in your PC) on the desktop. An options menu will be seen.
  2. Click on the properties. System properties will be seen in a new window.

System Info of A PC

You will find CPU type and core numbers and CPU speed here as well as other system information. System Info

From Settings

  1. Press Windows key from the keyboard and type settings. Go to Settings.

In this window, you will find the CPU information including Click speed.

Processor Info of windows

Processor Info


  1. Press Windows key + R on the keyboard. The run application will be opened.


  1. Press Windows key + R on the keyboard. The run application will be opened.

1. Does clock speed determine the performance of a CPU?
Ans: No. It depends on cores too. See the section named “Other Factors that determine the performance of a computer.”

2. More cores, slower clock speed or fewer cores, higher clock speed, which one to choose
Ans: It depends on other things and choice. See the section named “Clock Speed vs Cores.”

Summing Up

Clock rate means the number of pulses generated by CPU in one second. CPU’s performance most of the time depends on the clock rate of a CPU. To understand all the things that are responsible for the performance of a computer, you must have a better idea about the clock rate of a CPU. What is clock rate and its effects on computer performance? If you have gone through the whole article, you surely know all about this.

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