DDR1, DDR2, DDR3: Navigating The RAM Maze
AnandTech: DDR3 vs. DDR2
As to your second question, I believe , its simply because the hardware interface for each DDR,DDR2 and DDR3 are different from one another.The
way each type of chip connects with your motherboard is different. Generally,
these interfaces are extensions of the older version. I mean DDR2 extends DDR and DDR3 extends DDR2. So the motherboard can support older versions but not newer.
Maybe some one more well versed with system RAM h/w can elucidate further.



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