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Dual Channel DDR question

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unwrittenLaw

Member
Joined
Oct 12, 2002
Location
San Diego
I'm a bit confused about this spec...Does a mobo with Dual DDR support mean you can use regular DDR and get double the bandwidth, or do you have to buy a new type of ddr to go along with these new motherboards?

Thanks
 

Tipycol

Member
Joined
Aug 20, 2002
DDR-II will require new ram (one reason is that they will run at different voltages). Dual Channel DDR will run with any DDR sticks out today, the only thing required is that you have two of them :)
 

Tipycol

Member
Joined
Aug 20, 2002
Well there's a lot of disputes about this (just one THREAD out of the many) but here is what PCphreak had to say about it:

Originally posted by PCphreak
It's surprising to see how many people do not understand how memory bandwidth and a data bus correlate. I explained this in his other thread, while not thoroughly- because of the complexity of the issue; I did explain enough to show the benefits of nForce2. Of course the benefits only come when nForce2 is not running @ spec. I'll quote my post below:

To get a full grasp on bandwidth and memory throughput, you need to understand it.

Right now DIMMs, whether SDR or DDR, are 64 bit's wide. This is important to note, because the CPU's external bus interface is 64-bits wide also. This is also why we are able to add memory a single DIMM at a time, whereas older SIMMS (32-bits wide) had to be added in pairs to match the CPU's external 64-bit bus interface.

The FSB and memory bus are two different busses, but must cooperatively operate together. They operate together more efficiently when their cycle time is lower and synchronous. As clock speed increases cycle time decreases. Example: a 2 GHz CPU cycles every .5 ns, while a DDR 400 bus cycles every 2.5 ns (ns = nanoseconds). ( 1 / MHz * 1000 = ns).
So what happens when a 2 GHz processor trys to directly access a DDR 400 bus? About 4.5 wait states (blank cycle CPU executes) till data is ready for the CPU on the DDR 400 bus's next event cycle. These wait states effectively slow down the CPU from 2 GHz to the 400 MHz of the bus. Same principle goes for any bus working together. Now you can see why it's important to run synchronous rather than async which introduces bottle necks.

I'm not going to get into the latency of memory, which is the time involved in a setting up a transfer. So I'll move on to bandwidth.

Memory bandwidth is figured by this:
(Bus Width) multiplied by (Clock Speed), and this product multiplied by (Data Cycles Per Clock).
Example: The Athlon's FSB is 64-bits wide and runs at clock speed 133 MHz and manages 2 data cycles per clock. This works out to be
( 8 bytes(64-bits) * 133 MHz * 2 = 2,128 MB/s ), or approx. 2.1 GB/s. To take full advantage of the FSB's bandwidth, you'd need to pair it with memory at the same bandwidth. DDR 2100 does this exactly. ( 8 * 133 MHz * 2 = 2,128 MB/s ). Or ( 8 * 266 MHz = 2,128 MB/s ); however you want to figure it.

Now you have it. Since Athlon is based on a DDR bus, it's better paired with DDR memory to directly match it. Remember, the higher the clock speed of the FSB the closer it's cycle time is to the CPU and less wait states occur in the event of a L2 cache miss.
As far as the P4 goes, it has a 128-byte line size and runs on a quad based bus (high bandwidth; 8 (64-bits) x 133 MHz x 4 = 4,256 MB/s ). So naturally it will benefit from a memory bus able to match. Right now, stock, Dual DDR 400 is right up it's ally.
I hope this helps a bit, as I spent some time typing this:).

*note*
1) A CPU will only access memory directly if the cache controller misses, and data needed isn't present in L2 cache.
2)The 'memory bandwidth' example for the AMD is illustrated @ stock specs. Naturally bandwidth is a direct function of clock speed- the higher the clock the greater the bandwidth.


-PC
 
Last edited:

Placid

Senior Member
Joined
Mar 13, 2001
Location
Connecticut
unwrittenLaw said:
I'm a bit confused about this spec...Does a mobo with Dual DDR support mean you can use regular DDR and get double the bandwidth, or do you have to buy a new type of ddr to go along with these new motherboards?

Thanks

You just need two sticks of regular ddr for the intel GB or Nforce 2.
 
OP
unwrittenLaw

unwrittenLaw

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Oct 12, 2002
Location
San Diego
Ok, I understand a little better, thanks for the info guys...So my next question is, would it be a good idea to upgrade to these mobos now or is there some better technologies on the horizon...
 

Tipycol

Member
Joined
Aug 20, 2002
Well that depends on you. What kind of computer are you using? Is it doing everything you need it to do without any bottlenecking problems? If you have a good computer, maybe you can wait a year, cause that's when the clawhammers and DDR-II "should" be available. If you have something really slow and are looking for the best upgrade out now, maybe you should get a nForce 2 mobo. It all depends on you and if you think you need the upgrade.
 
OP
unwrittenLaw

unwrittenLaw

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Oct 12, 2002
Location
San Diego
Well i have an xp1600 that i hear can oc pretty well, but my ecs mobo isnt helping much, so i thought if i got a really good mobo, it would help increase overall performance...is this true or will i just be gettin a mobo that will be obsolete next year?
 

Pooptacular

Registered
Joined
Oct 13, 2002
In your situation I'd get a Nforce2 mobo which should be out soon to oc the xp1600 if you have the cash. If not you can get a temp mobo for around 80, then wait for ddr-2 and clawhammers to come out and upgrade to that then. My suggestion for the temp mobo is the same mobo as me MSi kt3 ultra 2. Although there might be better boards out for the price, ask around or wait untill someone replies to this.
 

jstutman

Member
Joined
Oct 13, 2001
okay so now my question. with this dual ddr. i would get better performace with 2 256 mb pc3200 than say 1 512 mb pc3200?

so now i have 2 chips so i can use the dual ddr. corrrect>?
because im purcasing a nforce 2 board.
 

Tipycol

Member
Joined
Aug 20, 2002
You would get better performance due to using two sticks versus one (since two are needed for Dual DDR), but if you meant 2x512 XMS 3200, then the performance might be the same but you just have a gig of memory instead of 512MB (though the size might slow down the performance a little.) And I think if you mix a 512MB stick and a 256MB stick (both at say XMS 3200), and run Dual DDR, you would only be able to use 256MB of the 512MB, so you'd have a total of 512MB of ram instead of 768MB.
 

jstutman

Member
Joined
Oct 13, 2001
no i meant 2x 256 pc3200 per say 1x 512.
using 2 chips in place of one, would using two chips be a bottleneck or overclocking?

in retard, would the extra proformance from dual ddr be enough to not be worried about the few mhz that 2 chips would hold me back from.
 

Tipycol

Member
Joined
Aug 20, 2002
Well, considering it doubles your bandwidth, I'd say yes. Getting double the bandwidth due to a little less MHz is definitely worth it. And if I got an nForce 2 board, I'd just put the lock the AGP:pCI buses, run memory at a stable speed (since memory has its own clock generator) put the fsb at max, and lower the multi till it's stable, and even if the multi causes the proc to go to a stock speed (XP 2400 @ 333x6 = 2GHz) the high fsb is definitely worth it.