AMD has finally incorporated a DDR2 memory controller to AMD64 CPUs. The new DDR2-capable AMD64 CPUs use a new socket, AM2. In this review we benchmarked Athlon 64 X2 5000+, which is a dual-core Athlon 64 running at 2.6 GHz and supporting DDR2 memories. We are going to compare it to Athlon 64 X2 4600+ (which runs at 2.4 GHz) and Athlon FX-60 (a dual-core Athlon 64 FX running at 2.6 GHz). Check it out.
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 through 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 higher 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.
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|