Im quite confused about memory speed rating , especially when it comes to graphics card , for example the Radeon 8500 is said to be 275mhz for the memory clock , it has 3.3 ns memory which is okay because it calculated or 1000 is divided by 3.3 , this surely gives 275 , but its somehow multiplied by 2 to give a 550mhz rating , because its DDR or some reason , well what im wondering is why PC RAM even though its DDR doesnt corespond to multiplication by 2 even though its DDR ,as the mhz rating directly matches the ns latency time??

Strange for me!!!!

Instead of explaining, I found a good explanation of why DDR is what it is in terms of PC RAM:

With standard SDRAM, data is transferred from the memory to the processor when the clock signal bounces from LOW to HIGH. With DDR, data is transferred not only when the clock signal goes from LOW to HIGH, but also when the clock signal goes from HIGH to LOW. Voila! Twice as much data on each tick of the clock!

I've heard of PC1600 and PC2100. What does this mean, and what is the difference?

Well, it's kind of like PC100 and PC133. Since it is double datarate, you might expect that DDR would be PC200 and PC266. In fact, the RAM chips themselves are sold as PC200 and PC266. HOWEVER, us module guys did not want you to think that Rambus (at 800 MHz) is four times as fast as PC200 DDR. So, we went with a number that reflects the MODULE bandwidth. Since DDR DIMMs are eight bytes wide, the designation becomes PC200*8 = PC1600 and PC266*8 = PC2100. So, to repeat, PC1600 uses both edges of a 100MHz clock, and PC2100 uses both edges of a 133MHz clock. And now, you're only a little less confused than me!

Not sure if this really answers your question....I really don't remember it off the top of my head, but I can do a little research tomorrow and let you know if you're interested in more info.

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ActiveTuning - Partner & Director Of Sales & Marketing

I understand what you are saying , but DDR RAM memory like 266mhz has a corresponding ns rating i.e 1000 divued by 266 and its bandwidth is calculated as its data width is 64bits i.e 8 bits , thus pc2100 for 266*8 . but its never rated at 533mhz or something like that according to graphics card memory chips.

So why do graphics cards this?

I'm not really sure to tell you the truth. I'm hoping Paul, one of our editors who is a computer engineer major will be able to help out. I've IMed him a link to the thread, so hopefully we'll hear from him. If not, I'll be doing a little research to see what I can conjur up

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ActiveTuning - Partner & Director Of Sales & Marketing

Dave copy/pasted most of the right info about DDR.

With regards to nomenclature, the industry calls it "PC 1600" and "PC2100" because that's approximately how many Megabytes/second the memory can ("in theory") transfer per second. They are industry standards setup by an organization called JEDEC (you may have heard of them -- they're the ones that RAMBUS was a part of, and then left, and screwed them all by [secretly] submitting patents for SDRAM and DDRAM while getting them all to agree to them as standards, without telling them). PC2700 yeilds approximately 2.7Gigabytes of bandwidth per second ("in theory"). The memory industry put names to these because...well....because they needed some form of standard.

The graphics industry, on the other hand, changes the frequency of the memories they use all the time (generally going up). There is no "DDR standard" per se, in the same sense that there is in the consumer PC memory industry. They're not called "PC 7200" (or PC 10400) or whatever because they aren't industry standards. if this isn't enough let me know....but I gotta run to class now.

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paul@pleaseohpleasedontspamme.slcentral.com
A mathematician is a device for turning coffee into theorems -- P. Erdos