What is a CPU's actual speed?

Sailor · Nov 17, 2006

Well this is an incredibly dum question but I need to know.
I have an Athlon64 3500+. Now is this "3500" the clock's speed? Because all the monitoring utilities give me a cpu clock of 2.21 GH. Is "3500" just the name of the processor series?

Mada_Milty · Nov 17, 2006

There's no dumb questions! Except the ones that don't get asked!

You're right; the Athlon 64 3500+ (I have the same CPU at home) does not run at 3.5GHz, but it does a number of operations equivalent to an Intel processor running at 3.5GHz.

I had a good article explaining how AMD achieves this, but I've since lost it.

candle_86 · Nov 17, 2006

its 2.2ghz and the way its measured is it runs equvilant to an Athlon Thunderbird at 3.5ghz if there ever was one. It manages to beat Intel Pentium 4 line because it can do more work per clock than the Pentium.

Mada_Milty · Nov 17, 2006

Sorry, that's not quite right. INTEL processors do more per clock cycle! I remembered where I found my info! DUH ME! It was right here on Majorgeeks!

From the Overclocking Guide:

Ace 256 said:

When you buy a processor, or CPU, you will see it's operating speed. For instance, a Pentium 4 3.2GHz CPU runs at 3.2GHz, or 3200 MHz. This is a measurement of how many clock cycles the processor goes through in one second. A clock cycle is a period of time in which a processor can carry out a given amount of instructions. So, logically, the more clock cycles a processor can execute in one second, the faster it can process information and the faster your system will run. One MHz is one million clock cycles per second, so a 3.2GHz processor can go through 3,200,000,000, or 3 billion two hundred million clock cycles in every second. Pretty amazing, right?

The goal of overclocking is to raise the GHz rating of your processor so that it can go through more clock cycles every second. The formula for the speed of your processor if this:

FSB (in MHz) x Multiplier=Speed in MHz.

Now to explain what the FSB and Multiplier are:

The FSB (or, for AMD processors, the HTT*), or Front Side Bus, is the channel through which your entire system communicates with your CPU. So, obviously, the faster your FSB can run, the faster your entire system can run.

CPU manufacturers have found ways to increase the effective speed of the FSB of a CPU. They simply send more instructions in every clock cycle. So instead of sending one instruction every one clock cycle, CPU manufacturers have found ways to send two instructions per clock cycle (AMD CPUs) or even four instructions per clock cycle (Intel CPUs). So, when you look at a CPU and see it's FSB speed, you must realize that it is not really running at that speed. Intel CPUs are "quad pumped", meaning they send 4 instructions per clock cycle. This means that if you see an FSB of 800MHz, the underlying FSB speed is really only 200MHz, but it is sending 4 instructions per clock cycle so it achieves an effective speed of 800MHz. The same logic can be applied to AMD CPUs, but they are only "double pumped", meaning they only send 2 instructions per clock cycle. So an FSB of 400MHz on an AMD CPU is comprised of an underlying 200MHz FSB sending 2 instructions per clock cycle.

This is important because when you are overclocking, you will be dealing with the real FSB speed of the CPU, not the effective CPU speed.

The multiplier portion of the speed equation is nothing more than a number that, when multiplied by the FSB speed, will give you the total speed of the processor. For instance, if you have a CPU that has a 200MHz FSB (real FSB speed, before it is double or quad pumped) and has a multiplier of 10, then the equation becomes:

(FSB) 200MHz x (Multiplier) 10= 2000MHz CPU speed, or 2.0GHz.

On some CPUs, such as the Intel processors since 1998, the multiplier is locked and cannot be changed. On others, such as the AMD Athlon 64 processors, the multiplier is "top locked", which means that you can change the multiplier to a lower number but cannot raise it higher than it was originally. On other CPUs, the multiplier is completely unlocked, meaning you can change it to any number that you wish. This type of CPU is an overclockers dream, since you can overclock the CPU simply by raising the multiplier, but is very uncommon nowadays.

It is much easier to raise or lower the multiplier on a CPU than the FSB. This is because, unlike the FSB, the multiplier only effects the CPU speed. When you change the FSB, you are really changing the speed at which every single component of your computer communicates with your CPU. This, in effect, is overclocking all of the other components of your system. This can bring about all sorts of problems when other components that you don't intend to overclock are pushed too far and fail to work. Once you understand how overclocking works, though, you will know how to prevent these issues.

*On AMD Athlon 64 CPUs, the term FSB is really a misnomer. There is no FSB, per se. The FSB is integrated into the chip. This allows the FSB to communicate with the CPU much faster than Intel's standard FSB method. It also can cause some confusion, since the FSB on an Athlon 64 can sometimes be referred to as the HTT. If you see somebody talking about raising the HTT on an Athlon 64 CPU and is talking about speeds that you recognize as common FSB speeds, then just think of the HTT as the FSB. For the most part, they function in the same way and can be treated the same and thinking of the HTT as the FSB can eliminate some possible confusion.
Click to expand...

Sailor · Nov 18, 2006

Thanks a lot. By the way, Hyper-Threading is disabled in my Athlon. How can I enable it and what exactly will the results be? I've heard that it is a common tweak for high performance.

candle_86 · Nov 18, 2006

Mada_Milty said:

Sorry, that's not quite right. INTEL processors do more per clock cycle! I remembered where I found my info! DUH ME! It was right here on Majorgeeks!

From the Overclocking Guide:
Click to expand...

not true at all, let me put this in a simpler way thats easy to understand.

The Pentium 4 has a 16 or 18 stage pipline, the Willimate and the Northwood are 16 stage and the Presscott is 18 stage. Now all Athlon 64's CPU's are 10 stage, so they can put the work out faster than a Pentium per clock. This is also the reason why Early Pentium 4's couldnt beat the Pentium III and why a Conroe is faster than PEntium and Athlon, having the old 8stage pipeline agian

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What is a CPU's actual speed?

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