How does raising the multiplier of a CPU increase its performance?

How can a CPU perform any faster than its FSB? How can raising the multiplier increase its performance if it communicates with the other components at a lower speed? Does raising the multiplier allow the CPU to communicate with the cache more?

Let me give an example in case I'm unclear:

If the FSB is 200mhz, and the multiplier is ten, the CPU speed is 2000mhz.
If it communicates with the other components at 200mhz, what's the extra 1800mhz doing?

mkay, i saw that you got 30 views and no replies, so i decided to help you out. you should really read a book i have, basics of computer hardware 101, but i'll give you a crash course.....

Think about FSB as being a little schoolbus that takes info to and from the processor to other components like memory. theoretically 200mhz fsb moves 3.2g a sec of information, this is the bandwidth. the clock speed of the chip is different. all your cpu basically does is math, it gets the speed by multplying the fsb frequency with a multiplier. when you change the multi you are changing the core speed of your chip. so, when you change the multi you are not changing the speed that the cpu communicates with the rest of your pc, but you are drastically changing the speed that the processor operates internally.

ogboot said:

mkay, i saw that you got 30 views and no replies, so i decided to help you out. you should really read a book i have, basics of computer hardware 101, but i'll give you a crash course.....

Think about FSB as being a little schoolbus that takes info to and from the processor to other components like memory. theoretically 200mhz fsb moves 3.2g a sec of information, this is the bandwidth. the clock speed of the chip is different. all your cpu basically does is math, it gets the speed by multplying the fsb frequency with a multiplier. when you change the multi you are changing the core speed of your chip. so, when you change the multi you are not changing the speed that the cpu communicates with the rest of your pc, but you are drastically changing the speed that the processor operates internally.

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So, does 200mhz FSB measure something different from 200mhz CPU clock speed? If so, what's the difference?

It measures 200MHz system bandwidth speed. Say some information is sent to the processor from the memory - it's some task to be done. The info is sent to the processor along that 200mhz bus. The processor gets the task, and works on it at whatever it's internal clock is. Say we have a multiplier of 10x - the CPU works on the task at 2000mhz. Then it goes wherever it needs to along that 200mhz bus again.

Multiplier changes internal CPU clock, FSB changes overall system clock, to put it simply. The FSB isn't doing any work, just the CPU.

johan851 said:

It measures 200MHz system bandwidth speed. Say some information is sent to the processor from the memory - it's some task to be done. The info is sent to the processor along that 200mhz bus. The processor gets the task, and works on it at whatever it's internal clock is. Say we have a multiplier of 10x - the CPU works on the task at 2000mhz. Then it goes wherever it needs to along that 200mhz bus again.

Multiplier changes internal CPU clock, FSB changes overall system clock, to put it simply. The FSB isn't doing any work, just the CPU.

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This is what I don't understand:

One hertz is one cycle per minute.

The frontside bus is 200mhz. When the CPU communicates with the memory, it accesses the memory at a rate of 209,715,200 cycles per minute.

If the CPU is running at 200mhz as well, it processes the data at a rate of 209,715,200 cycles per minute.

If the CPU multiplier is 10, it is running at 2000mhz, but it is recieving the data at a rate of only 209,715,200 cycles per minute, so it cannot process the data at a rate of 2000mhz--2,097,152,000 per second.

If this is right, the CPU multiplier has no effect in performing this action.

So, where am I wrong?

Lets say I were running a pc repair business.

The processor speed would be related to how fast I could fix a PC, and the FSB would be related to how fast I could drive to someone elses house to fix their pc.

Fixing a PC is a more complicated task than driving to someones house, and as a result should take more time to complete.

The speed I drive with only relates to how fast me and my tools arrive at the house.

Using the tools I brought with me I fix the computer which is the main task, and the more difficult task. What matters most is how fast I can fix the computer.

What you are saying is like asking why the human brain can make millions of calculations at a time when blood can only travel so fast through the body.



Hope this helped.

The unit "hertz" refers to 1/s (s being secconds). The speed of the CPU is geenerally much fater than the speed of the BUS. This is for several reasons. The first of which is that one operation may take several cpu clock cycles, even if it is on a small amount of data. This is the reasons for pipelines in the cpu, so that multiple instructions can be worked on at once, giving the appearence of executing the instructions faster. The second reason ius that some data is stored on the cpu in caches, and does not need to be fetched from main memory. I don't really have the time or expertise to go into a full explanation of how a microprocessor works and give you an exact answer to your question, so I'll just give you some good links:


arstechnica.com had an understanding the microprocessor series that delt with things like pipelining, superscalar execution, bandwidth and latentcy. I'll just post some links to the articles.
caching: http://arstechnica.com/paedia/c/caching/caching-1.html
multi-threading: http://arstechnica.com/paedia/h/hyperthreading/hyperthreading-1.html
bandwidth/latency: http://www.arstechnica.com/paedia/b/bandwidth-latency/bandwidth-latency-1.html
Understanding the microprocessor: http://arstechnica.com/paedia/c/cpu/part-1/cpu1-1.html
pipelining/superscalar execution: http://arstechnica.com/paedia/c/cpu/part-2/cpu2-1.html

and just for fun Moore's law: http://arstechnica.com/paedia/m/moore/moore-1.html

The ars article comparing the G4 and P4 talks about work per cycle I think, but I haven't read it in a long time:
http://arstechnica.com/cpu/01q2/p4andg4e/p4andg4e-1.html
http://arstechnica.com/cpu/01q4/p4andg4e2/p4andg4e2-1.html

Most of the other cpu theory and praxis articles are very good.

If you go to aceshardware.com and click on the cpu arch & tech link, you can find some very nice articles on the differences and performance of various cpus (some of the older articles from when the P4 first came out are very interesting).

http://www.chip-architect.com/ has some very in-depth exploration of the architectures of different chips.

It's really simple to explain

It takes a lot longer for the CPU to process the data then it takes to recieve/send it over the FSB, that's why every internal MHz helps.

Don't confuse him with all those details and examples
(even if they were good)

Finken said:

It's really simple to explain

It takes a lot longer for the CPU to process the data then it takes to recieve/send it over the FSB, that's why every internal MHz helps.

Don't confuse him with all those details and examples
(even if they were good)

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Actually, Gnufsh was the only one who didn't confuse me. Thanks, Gnufsh!

Haha how did I confuse you?!
Anyway, as long as you got the help you needed...

Btw I liked the links, very usefull!
Thanks Gnufsh!

Ok, I skipped everything and will just say something and hope that its what you were looking for.

FSB is, just like you said, the speed in which things communicate with eachother, that simple. Most commonly effected by this is the CPU and RAM.

But now the CPU needs a guideline as to how fast to do its calculations. Its guidelines are the multiplier, and the FSB. If the CPU is running at 2ghz, then its the processor that is calculating at 2ghz, and nothing else.

A Simple brief summary, hope it helped.

Stupid Boy

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I looked at this thread and it all came together when I saw your name. j/k

FSB and processor speed are TOTALLY UNRELATED. (at least in terms of how they work.) That is like wondering why the engine in my car goes 5000rpm when I'm only going 60 miles an hour.

Here is a good explanation. Lets say I am making milk shakes. I can only put the ingredients into the blender at a slow speed. The blender works a lot faster, but it does something totally different than me.

mkay, i saw that you got 30 views and no replies, so i decided to help you out. you should really read a book i have, basics of computer hardware 101, but i'll give you a crash course.....

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stupid boy

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sorry i just found it funny <3