How a CPU Works

External Clock

So the CPU manufacturers started using a new concept, called clock multiplication, which started with 486DX2 processor. Under this scheme, which is used in all CPUs nowadays, the CPU has an external clock, which is used when transferring data to and from the RAM memory (using the north bridge chip), and a higher internal clock.
To give a real example, on a 3.4 GHz Pentium 4 this “3.4 GHz” refers to the CPU internal clock, which is obtained multiplying by 17 its 200 MHz external clock. We illustrated this example in Figure 4.

Figure 4: Internal and external clocks on a Pentium 4 3.4 GHz.

The huge difference between the internal clock and the external clock on modern CPUs is one major roadblock to overcome in order to increase the computer performance. Continuing the Pentium 4 3.4 GHz example, it has to reduce its speed by 17x when it has to read data from RAM memory! During this process, it works as if it were a 200 MHz CPU!
Several techniques are used to minimize the impact of this clock difference. One of them is the use of a memory cache inside the CPU. Another one is transferring more than one data chunk per clock cycle. Processors from both AMD and Intel use this feature, but while AMD CPUs transfer two data per clock cycle, Intel CPUs transfer four data per clock cycle.

Figure 5: Transferring more than one data per clock cycle.

Because of that, AMD CPUs are listed as having the double of their real external clocks. For example, an AMD CPU with a 200 MHz external clock is listed as 400 MHz. The same happens with Intel CPUs: an Intel CPU with a 200 MHz external clock is listed as having an 800 MHz external clock.
The technique of transferring two data per clock cycle is called DDR (Dual Data Rate), while the technique of transferring four data per clock cycle is called QDR (Quad Data Rate).