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I hear almost everywhere that CPU clock speed is not the only factor that defines the true speed of a CPU. Well. What are the other factors then? In which case a processor clocked at 1.6 Ghz is faster than one clocked at 2.0 Ghz?

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Here are some of the things that can affect overall CPU speed:

The quoted CPU speed is the speed at which the internal clock runs. This is faster than the external clock (the FSB clock) which determines how fast the CPU can access the resources on the motherboard. A fast internal clock means the CPU can process faster, but a slower FSB means it takes longer to access memory and such.

This can be partially offset by the L1 cache (very fast memory inside the CPU itself) which can pre-fetch data from memory while the CPU is doing other things, so a larger L1 cache can yield a faster performance in certain circumstances.

The number of cores is important as well. A 1.6GHz dual core processor has approximately twice the processing speed (in optimal circumstances) of a single core 1.6GHz processor. This never realistically gets achieved in real life, but it is still much faster to have 2 cores than one. It's almost like having 2 processors.

The efficiency of the CPU itself has an effect too. Each instruction that the CPU performs takes a certain number of clock cycles to complete. A more efficient CPU will use less clock cycles to perform complex operations (floating point, for example) which makes the overall processing quicker.

The chipset on the motherboard can have an effect as well when it comes to the CPU accessing resources on the motherboard.

Pretty much everything in the computer has a bearing to a greater or lesser extent on the overall speed of the system.

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Wow, it's Matt Jenkins. I don't need to read anything else. Thank you. –  overmann Apr 4 '11 at 17:25
    
@questionBot, while @Matt is awesome, there are also other users who might have something to add! –  Ivo Flipse Apr 4 '11 at 17:35
    
But it's M-A-T-T J-E-N-K-I-N-Ssss. –  overmann Apr 4 '11 at 17:36

CPU's 'TRUE' speed is not only realised by it's clock speed. Clock speed is only the speed at which things(registers or other sequencial elements) change their value inside CPU. Clock speed would be smaller (eg:- 1.5 Ghz) for CPU's having long Combinatinal circuits. That means those CPU though having low Clock rate still could perform more instructions per cycle.

Secondly, CPU's throughput also depends on the type of instuctions it is given to process. For eg: a CPU having an excellent multiplier would outperform with a set of instructions having large number of multiplications.

Thirdly, there are certain combined factors such as : Cache size and levels, Miss rate, Hit rate that determine overall 'Speed' or 'Throughput' of a CPU.

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