Intel CPUs use a technique called QDR or Quad Data Rate, which transfers four data per clock cycle. With this technique the CPU achieves an external performance four times greater than if it was transfering just one data chunk. Because of that, the clock advertised by Intel is four times greater than the real clock used by the CPU, as you can see on the table below. It is still a mystery for us if the very high-end 1,066 MHz CPU uses a 400 MHz x 4 bus or a 200 MHz x 8 bus configuration. We believe on the latter, due to the physical difficulties involved in increasing the CPU external clock rate. So, a Pentium 4 processor with 533 MHz bus runs at 133 MHz but achieves a performance "as if" it was running at 533 MHz.

Since it is hard do compare clocks when you don't know how much data is transferred per time, it is better to know the maximum transfer rate, given in megabytes per second. The formula to calculate it is rather simple: real clock x number of data transferred per clock x 64 / 8. 64 is used because the CPU communicates with the memory 64 bits per time, and we have to divide by eight to have the result in bytes.

External Clock	Real Clock	Maximum Transfer Rate
400 MHz	100 MHz	3,200 MB/s
533 MHz	133 MHz	4,264 MB/s
800 MHz	200 MHz	6,400 MBs
1,066 MHz	200 MHz (our guess)	8,528 MB/s

So you need to known your CPU external clock rate to know which is its maximum transfer rate. The best way to check this is running CPU-Z software, available on our download section. Run it and check the external clock speed of your CPU on the "Bus Speed" field and check what is its maximum transfer rate on the table above.

Figure 2: Running CPU-Z on a Pentium 4 system. This CPU uses a 533 MHz external bus.

As you can see on Figure 2, the CPU clock speed may not be exactly the same as shown on our table, some slight differences may occur. For example, the external bus from our CPU is running at 535 MHz which makes it a 533 MHz CPU.

As we said before, once you know the maximum external transfer rate of your CPU it is just a matter of matching the memory transfer rate with it. As you can see by comparing AMD table with Intel table, Intel CPUs face more problem matching the memory speed, since Intel CPUs have a higher maximum transfer rate compared to AMD CPUs.

Let's talk now about memory speeds.