Athlon 64 FX-62 is right now the most high-end CPU from AMD, incorporating dual-core technology, socket AM2 (i.e. DDR2 memory support), 2.8 GHz internal clock and 1 MB L2 memory cache for each core. We had the chance of reviewing this beast and compare it to other dual-core CPUs from AMD: Athlon 64 X2 5000+ (socket AM2, 2.6 GHz, 512 KB L2 memory cache), Athlon 64 X2 4600+ (socket 939, 2.4 GHz, 512 KB L2 memory cache) and Athlon FX-60 (socket 939, 2.6 GHz, 1 MB L2 memory cache). Let’s see how this new CPU performs.

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Figure 1: Athlon 64 FX-62.

Since it is based on a new pinout, you will need to replace your motherboard by a socket AM2 one if you want to use DDR2 memories with Athlon 64.

The memory controller integrated on AM2 CPUs can support DDR2-533, DDR2-667 and DDR2-800 memories. The problem, however, is how the memory bus clock is achieved. Instead of being generated thru the CPU base clock (HTT clock, which is of 200 MHz), it divides the CPU internal clock. The value of this divider is half the value of the CPU multiplier.

For example, an AMD64 CPU with a clock multiplier of 12x will have a memory bus divider of 6. So this CPU will work at 2.4 GHz (200 MHz x 12) and its memories will work at 400 MHz (DDR2-800, 2,400 MHz / 6). Keep in mind that DDR and DDR2 memories are rated with double their real clock rate.

The problem is when the CPU clock multiplier is an odd number. For an AM2 CPU with a clock multiplier of 13x, theoretically its memory bus divider would be of 6.5. Since the AMD64 memory bus doesn’t work with “broken” dividers, it is rounded up to the next rounded number, seven in this case. So while this CPU will work at 2.6 GHz (200 MHz x 13), its memory bus will work at 371 MHz (742 MHz DDR) and not at 400 MHz (800 MHz DDR), making the CPU to not achieve the maximum bandwidth the DDR2 memory can provide.

This is the case of Athlon 64 X2 5000+. As it works internally at 2.6 GHz multiplying its HTT clock by 13x, it can access memory only up to 742 MHz and not 800 MHz. Athlon 64 FX-62, on the other hand, works internally at 2.8 GHz multiplying its HTT clock by 14x, allowing it to access the memory truly at 800 MHz. We will talk more about this later, as we thought it was important to take a closer look on this issue.
 
Here are some examples:

CPU Internal Clock	CPU Multiplier	Memory Divider	Memory Bus
2.8 GHz	14x	7	800 MHz
2.6 GHz	13x	7	742 MHz
2.4 GHz	12x	6	800 MHz
2.2 GHz	11x	6	733 MHz
2 GHz	10x	5	800 MHz
1.8 GHz	9x	5	720 MHz
1.6 GHz	8x	4	800 MHz