At last, the long awaited support for DDR2 memory has arrived in the form of AMDs socket AM2, but there is a problem with the memory controller, I'll get to that, but first I will outline some background information.

In computer systems, the CPU clockspeed is determined by multiplying the system's Front Side Bus (FSB) in Intel systems, and the internal clock speed in AMD systems, by a multiplier that is hardwired into the CPU. In modern AMD systems using socket 939, and now socket AM2 as well, the internal clockspeed is 200mhz by default. For a 2.0ghz CPU, you would multiply 200 by a multiplier of 10 on the CPU to get 2000mhz (200*10=2000). Similarily, a 2.4 ghz CPU would have 200mhz multiplied by 12 to get 2400mhz (200*12=2400).

With socket 939, the memory clock was calculated by dividing the CPU clock by the CPU multiplier. So for a 2.0ghz CPU: 2000mhz/10=200mhz which at Double Data Rate (DDR) is 400mhz, thus we get the DDR-400 that 939 supports. With socket AM2, the maximum supported memory is DDR2-800, which means that the maximum memory clock is 400mhz. To determine the memory clock, AMD divides the CPU clockspeed by half the multiplier, so for a 2.0ghz CPU (multiplier of 10) and a 2.4ghz CPU (multiplier of 12), the calculations looks like this:

2000mhz/5=400mhz (800mhz DDR)
2400mhz/6=400mhz (800mhz DDR)

The problem is introduced when you have a CPU that has an odd multiplier, currently only 2 exist, 11, and 13. Dividing these in half will not give you a whole number. The way the memory controller deals with this is by rounding the number up. So for a CPU multiplier of 11, the number that the CPU clock would be divided by to get the memory clock is 6, and for a multiplier of 13, it would be 7.

2200mhz/6=366mhz (733mhz DDR)
2600mhz/7=371mhz (742mhz DDR)

As you can see, this means that you will not get the optimum memory speed. The only solution is to overclock the processor in question by 200mhz, which would result in higher power consumption, greater heat output, not to mention voiding your warranty.

So, which CPUs are affected? On AMDs current AM2 product list, the following CPUs will result in the less than ideal memory clock:

Athlon 64 3500 (2.2ghz, 512KB L2 cache)
Athlon X2 4200 (2.2ghz, 2x512KB L2 cache)
Athlon X2 4400 (2.2ghz, 2x1MB L2 cache)
Athlon X2 5000 (2.6ghz, 2x512KB L2 cache)

So, take this as a warning if you're planning on buying a new socket AM2 CPU. Do not buy a CPU with an odd multiplier.