This has probably been discussed before, but...

I'm a little at a loss to figure out how DDR2800 CAS 5 is going to
perform better than DDR400 CAS 2.5.

Any explanations would be appreciated.

TIA

"Cessna 310" <jolene310 (AT) sbc (DOT) com> wrote

I was wondering that too. I have 2 gig of corsait XMS 3200 CL2 DDR.

On Tue, 01 May 2007 17:29:04 +0100, $ wrote:

One major mistake about that is that they say the bus clock is doubled. It
isn't.

While it's certainly twice the bandwidth, the bus speed is not any
more than standard ddr memory, which is no more than regular non DDR
emory. I realize that pretty much all the manufacturers has started using
bogus bus speed numbers to advertise their ram, but the bus speeds are
actually the same across the board. What's changed is the throughput
by sending double the data with DDR and 4 times the data with ddr2 per
clock, not the actual bus clock speed. IOW's PC133, PC2100, and PC2-4200
ram all have the same bus speed of 133MHz.

So anywhere in the article you see something about the bus clock being
doubled, it's just wrong. And anyone that says otherwise is jjust ignorant.

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Wes Newell wrote:
I finally found a reference to that just the other day (in an Intel Northbridge
datasheet). A DDR2-533 stick receives a 266MHz clock. In the same way, a
DDR400 stick receives a 200MHz clock. So that relationship does not vary
between the two memory types.

DDR2 has twice as many banks inside, and the banks are accessed at one half
the clock rate. Which allows the sticks to clock higher. The bus termination
scheme is also different, and that improves the signal integrity (wish I
could find some nice comparitive pictures of that). [Info from the tutorial
below.]

So the two technologies are comparable. You do the same things when working
out the bandwidth. DDR400 * 8 = PC3200 or 3200MB/sec. DDR2-533 * 8 = PC2-4300
So multiplying the bus transfer rate by 8 bytes (the width of the module) gives
the peak transfer rate.

There is a tutorial here:

http://www.lostcircuits.com/memory/ddrii/2.shtml

Paul