ddr number vs pc number, how are th

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March 5, 2009 at 18:12:29
Specs: Windows XP
someone told me that the two ways memory's bandwidth is labeled, for instance ddr2 1066 vs pc2 8500, are related to one another by doubling then quadrupling. i think i understand the quadrupling because intel's fsb's quadruple the speed before it dumps into the cpu, but how is it doubled?

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March 5, 2009 at 19:07:03
There are a few different numbers you need to understand.

1. frequency - that's the true bus speed that the RAM (or CPU) runs at.

2. data rate - for DDR (double data rate) , the "effective speed" is twice the frequency because two bits of data are sent per clock cycle. For QDR (quad data rate as used by Intel CPUs), the "effective speed" is four times the frequency because four bits of data are sent per clock cycle.

3. bandwidth - these numbers are rounded so it can be a little confusing, but basically it's the data rate x 8. The "8" represents bits per byte.

If you're given any one of the above numbers, you *should* be able to determine the other two. For example, if you find a stick of RAM & all it says on the label is PC2-4300, you can divide 4300 by 8 to get 537.5, then divide 537.5 by 2 to get 268.75. Hopefully you're familar enough wth RAM ratings to know that it's DDR2-533 at 266MHz frequency. The slight differences in the numbers are because the bandwidth rating was rounded to 4300. The true bandwidth is 4266.66MB/s (266.666 x 2 x 8). Some RAM manufacturers round down & call it PC2-4200.

If you run RAM in dual channel mode, the bandwidth is doubled, so using the above example, DDR2-533 in dual channel mode would be 4266.66MB/sec x 2 = 8533MB/sec.

If you have an Intel CPU that runs at 1066MHz FSB (QDR), the bandwidth would be 1066.66MHz x 8 = 8533MB/sec.

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March 5, 2009 at 19:30:47
thanks jam u r always so helpful

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