DDR Question

[Sethos]

Ok, this is probably an exceedingly stupid question, and indeed I am amazed at my own ignorance in the area, but having said that, here we go...

I am thinking of building a new system so I'm searching about the net on pricewatch and I find the processor I want and a mobo to support it. Now the mobo has 2 SDRAM slots and 2 DDR slots. In the past I've delt only with SDRAM of the PC 133 variety. When I got boards there it said something to the effect of this board requires PC 100 or PC 133 or what have you. I am now curious how does one determine the speed of DDR (ie the PC XXXX number) RAM that one needs. I looked all over the site (not pricewatch but the mobo site) and was unable to find anywhere anything like supports PC 2700, just supports DDR and SDRAM.

So any help here would be great, thanx,

~Sethos

[Tjalfe]

Forget about the SDRAM... get DDR. Get DDR which is at least as fast as your front side bus ( well this goes for athlon.. for P4, get the fastest type your motherboard supports )

[Gurm]

Heh.

Yeah, the numbers are a little confusing, no?

PC2100 is PC133, only DDR. Does that help?

I have spoken.

- Gurm

[Kooldino]

Crap, there's a mathematical formula somewhere that determines the DDR memory speed...it's a combination of Mhz, latency, and something else...

[Kooldino]

Maybe this is it? (although I remember a much more complicated formula some time ago)

Memory bandwidth (for all ram, not just DDR) = bus size (in bytes) x clock speed.

So DDR has an 8byte bus size.

So it looks like 8bytes x 200Mhz bus speed = PC1600
8x266=2128 (~2100?)
8x300=2400
8x333=2664 (~2700?)
8x400=3200

That kinda works. I could have sworn that latency was in the equation somewhere. Hmmm.

[Nowhere]

afaik it's just theoretical max speed in MB/s...

[rylan]

Yes the DDR ratings are max theoretical speed. PC2700 means 2700MB/sec max transfer rate. Speeds are shown above by Kooldino.

The older SDRAM markings were actually the memory clock speed, so PC133 was 133MHz and PC100 was 100MHz sdram.
Always bugged me that they went to the bandwidth type instead of staying with clock speed.

[Nowhere]

I guess then it just sounded better in comparision with rambus...

[Gurm]

Pretty much. I still don't know what Rambus's numbers mean.

"PC800"? Err... is that bandwidth too?

I have queried.

- Gurm

[az]

nope, 200 mhz quad-pumped AFAIK. But (and yes, I know you know Gurm ) it's really just marketing gibberish to make it sound much faster than SDR. So for DDR they had to change the nomenclature to again have higher numbers than Rambus...

AZ

[Sethos]

Ah, I begin to see through the fog now...so to clarify...with DDR it really doesn't matter what PC XXXX number I go with really, this is just the max data rate transfer possible? if that is the case then life is oh so sweetly better...Thanx a lot everyone for helping to educate me on this...

~Sethos

[az]

Not quite..

PC3200 = PC400 (or DDR400) = 200 MHz DDR
PC2700 = PC333 (or DDR333) = 166 MHz DDR
PC2100 = PC266 (or DDR266) = 133 MHz DDR
PC1600 = PC200 (or DDR200) = 100 MHz DDR

If they had stuck with the same nomenclature they used for SDR RAM, they would've called it PC200 etc, like in the second column. But since Rambus called their RDRAM PC800, they had to come up with even greater numbers, so they used the maximum transfer rate (first column). Last column is the real bus speed in MHz, which is then doubled (DOUBLE data rate) to get the "virtual clockspeed" - the times data is sent over the bus per second. With SDR this is once per clock cycle, with DDR it's twice.

AZ

[Taz]

Originally posted by Sethos
...with DDR it really doesn't matter what PC XXXX number I go with really

Er no, it relates to the speed of the memory bus. How fast that is depends on the CPU being used and whether your running the memory synchronously, (i.e. memory bus and FSB are equal), or asynchronously, (memory bus and FSB are different). For example if you've got a XP2100+ processor and your running the memory bus synchronously then you'd need PC2100 memory as that processor uses a 133Mhz FSB. For a Barton XP which uses a 166Mhz FSB you'd use PC2700 memory. If you plan to overclock your processor by increasing the FSB then you might want faster memory to be sure it will cope with it.

[Chrono_Wanderer]

okay let see...

DDR Memory...

DDR400 = 200MHz * 2 Data Rate --> 400MHz * (64bit/8bytes) = 3200Gbits/sec --> PC3200
DDR333 = 166MHz * 2 Data Rate --> 333MHz * (64bit/8bytes) = 2664Gbits/sec --> PC2700
DDR266 = 133MHz * 2 Data Rate --> 266MHz * (64bit/8bytes) = 2128Gbits/sec --> PC2100
DDR200 = 100MHz * 2 Data Rate --> 200MHz *(64bit/8bytes) = 1600Gbits/sec --> PC1600

RAMBUS Memory

Single Channel PC800 --> 800MHz * (16bits/8bytes) = 1600Gbits/sec
Dual Channel PC800 --> 800MHz * (16bits/8bytes) *2 = 3200Gbits/sec
Single Channel PC1066 --> 1066MHz * (16bits/8bytes) = 2132Gbits/sec
Dual Channel PC1066 --> 1066MHz * (16bits/8bytes) *2= 4264Gbits/sec

[az]

You should always run FSB and memory synchronously, except for a few select cases (VERY fast RAM, relatively low FSB), which shouldn't happen when buying a new, balanced system anyway

AZ