Understanding CPU clock signal processing

That's because there is a limit to the clock frequency but not to the pipeline length.

When you make a digital circuit the maximum clock frequency is determined by the so called "critical path": you have a clocked register, some combinational logic and another clocked register. If your clock period is shorter than the time required from the combinational logic to output a valid result then you have a problem. This requirement must be met for each possible input, so in the logic there usually is a longer path, the critical path, that is the most time consuming one. Have a look here:

^{simulate this circuit – Schematic created using CircuitLab}

Note that I omitted the clock signal for clarity (and laziness).

Assuming that each nand and each nor have the same propagation delay \$t_D\$, while the AND1 is built with two nand having a propagation delay of \$2t_D\$ you can see that the longest path is from REG1 (or REG2) to REG4 through AND1 and NOR2, the total delay being \$T_{D_{max}}=3t_D\$. Your clock can't be faster than this, so that $$f_{CK}\leq\frac{1}{T_{D_{max}}}$$

Now imagine a 64bit floating point multiplier. That can have a critical path that is way, way longer than this. What can we do about that?

1. Keep a low clock frequency, saving power and relaxing the constraints on some other, non critical transistors
2. Break the critical path with another D flip flop

The second option would make the longest path shorter, thus allowing higher clock frequencies.

You might say well then, why don't they just make shorter critical paths rising the clock frequency? That's because transistors work well till a certain frequency that can not be exceeded, after that you can't increas clock frequency anymore but you can add pipeline stages to make more computations per clock cycle. Moreover, distributing a faster clock is way more difficult than a somewhat lower one, and the compilers nowadays can make a very, very smart use of the pipeline to don't waste any clock cycle.

I'd add that a modern superscalar processor such as the one heating the enviroment in your (and my) pc is way, way more complicated than a "pipeline vs frequency" battle. I suggest this site, there are some white papers and quite a number of talks about a new architecture, so the guy makes a lot of comparisons with a modern CPU. And a plus: the speaker is Gandalf.

Actually, the clock cycle wont tell the speed at wich a single transistor can switch, but, how much time it does take for a signal to travel the slowest/longest path. A single cmos transistor on a modern cpu can switch at speeds much higher than the clock used on the cpu, but, the clock is not based only on transistor switching speed but on signal travel times. Thats the whole point of dividing the cpu into shorter stages (pipelining), to decrease the length that the signal must pass before finishing the execution of a certain task. That length versus speed of electron flow will determine the longest propagation delay and so will limit the max clock of the system.