As we said, the ideal scenario to get the best performance for you PC is to match the CPU external speed with the system's RAM speed. Having a RAM memory faster than the CPU external bus is not a problem, the problem is having a RAM memory slower than the CPU external bus – which happens a lot.

AMD CPUs use a technique called DDR or Double Data Rate. With this technique the CPU transfers two data per clock cycle, doubling the performance of the bus since usually just one data is transfered per clock tick. This technique is used since the very first Athlon processor and then with Duron, Sempron and Athlon 64 processors.

Because of that, the advertised clock rate isn't the real external CPU clock. For example, Athlon XP 3200+ is said to have a 400 MHz external clock, but in fact its clock rate is 200 MHz transfering two data per clock, making it a processor with a performance similar if the CPU used an external 400 MHz clock but transfering just one data per clock.

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.

Refer to the following table to know the maximum transfer rate of your AMD CPU. "External Clock" is the clock speed advertised by the manufacturer, while "Real Clock" is the real clock signal speed used by the CPU.

All Athlon 64 uses a 400 MHz external bus. To known the bus speed of your Athlon XP or Sempron processor, please read our tutorial on that subject. Duron processors up to 1.3 GHz use a 200 MHz bus, while models from 1.4 GHz on use a 266 MHz bus.

External Clock	Real Clock	Maximum Transfer Rate
200 MHz	100 MHz	1,600 MB/s
266 MHz	133 MHz	2,100 MB/s
333 MHz	166 MHz	2,700 MBs
400 MHz	200 MHz	3,200 MB/s

Another – and better – way of knowing your CPU external bus speed is just 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 1: Running CPU-Z on an Athlon XP system. This CPU uses a 333 MHz external bus.

As you can see on Figure 1, 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 333.8 MHz which makes it a 333 MHz CPU.

So, once you know the maximum external transfer rate of your CPU it is just a matter of matching the memory transfer rate with it. Keep reading.