Adding a 3rd DIMM to Dual DDR

[EvelT]

I have a nForce2 board with the dual channel being utilised by a Corsair PC2700 2x256MB (Low Latency CAS2) matched pair. I want to add a 3rd DIMM.

From the article on Tom's I understand dual will still function at dual speed after I add a 3rd DIMM. However this means if I buy a 1GB DIMM that 512 Will run at twice the speed of the other 1GB. Is there any prioritisation so that data goes in the dual channel memory space so I get maximum performance?

Also what if the CAS timings are slower on the 3rd DIMM. Will it drag the dual channel to the same timings?

Any help would be appreciated...

 

[Prof133]

From the article on Tom's I understand dual will still function at dual speed after I add a 3rd DIMM. However this means if I buy a 1GB DIMM that 512 Will run at twice the speed of the other 1GB.

<b>Edit: Assuming the memory modules are installed to maximize the 128-bit access mode space, 1GB of the memory would be accessed in 128-Bit access mode. The rest would be accessed in 64-bit access mode.</b>

Is there any prioritisation so that data goes in the dual channel memory space so I get maximum performance?

The logical implementation would be to store data in the 128-bit access mode space first then the 64-bit access mode space second.

Also what if the CAS timings are slower on the 3rd DIMM. Will it drag the dual channel to the same timings?

As i understand it, by default the nForce2 Memory controllers will adjust to the weakest dimm. However, you can try to force the dimms to operate at certain memory timings and such by manually adjusting those parameters.


<P ID="edit"><FONT SIZE=-1><EM>Edited by Prof133 on 11/28/03 04:44 AM.</EM></FONT></P>

 

[MrBurns]

In this case, 1GB of the memory would be accessed in 128-Bit access mode. The rest would be accessed in 64-bit access mode.

He said, that he has 2x256MB and will add 1GB. So the 512MB will run in 128-Bit access mode and the rest would be accessed in 64-bit access mode.

As i understand it, by default the nForce2 Memory controllers will adjust to the weakest dimm. However, you can try to force the dimms to operate at certain memory timings and such by manually adjusting those parameters.

The problem is, that you cannot adjust the timings for each DIMM, only for all DIMMs. So if your new DIMM runs at the timings you used for the old (even if its not rated so high), you can still use the old timings. If the old timings are too low for your new DIMM you can still use them, but maybe your system wont boot or it will not be stable anymore (use prime95 to test stability). If you get a memroy error at post you will have to take the new module out or make a CMOS reset.

AMD Athlon XP 2700+
Epox 8rda+
Thermaltake Volcano 7+
2x256 MB PC2700
Creative GeForce 4 Ti 4800 (4600 Chip)
Chieftech Case
WD 80GB/8MB HDD
16x48 DVD-ROM
20x10x32 Burner

 

[Prof133]

He said, that he has 2x256MB and will add 1GB. So the 512MB will run in 128-Bit access mode and the rest would be accessed in 64-bit access mode.

Rest assured, it is as I described. There is fellow at the nForcersHQ.com forum named <b>Pulsar</b> that has some good explanations of Dual-channel for the nForce2 chipset. Search his posts. He has connections with a nVidia engineer. NForce2's DualDDR adjusts to the lowest common density that will facilitate 128-bit bandwidth. NForcersHQ has a wealth of information!

 

[MrBurns]

Maybe your version (1GB at dual channel, 512MB at single channel) will work, when he puts the 1GB module on Slot 1 and the both 256MB modules on Slot 2 und 3, but if he just adds the 1GB in the one free solt he has, it will be like I posted.

AMD Athlon XP 2700+
Epox 8rda+
Thermaltake Volcano 7+
2x256 MB PC2700
Creative GeForce 4 Ti 4800 (4600 Chip)
Chieftech Case
WD 80GB/8MB HDD
16x48 DVD-ROM
20x10x32 Burner

 

[Prof133]

Agreed. In order to maximize the amount in the 128-bit access mode space and minimize the amount in the 64-bit access mode space, the memory should be balanced across both channels as much as possible. For the 1GB+2x256MB case, the 1GB memory module should be in the memory slot that seems to stand alone and the two 256MB memory modules in the two remaining slots.

 

[Prof133]

In this case, 1GB of the memory would be accessed in 128-Bit access mode. The rest would be accessed in 64-bit access mode.

I should revise this as such:

Assuming the memory modules are installed to maximize the 128-bit access mode space, 1GB of the memory would be accessed in 128-Bit access mode. The rest would be accessed in 64-bit access mode.

 

[EvelT]

Ok, thanks for your help guys, I will have a look at that site also...

One thing you brought up is the prospect of running the 1GB DIMM in 128bit mode. I didn't think this was possible as you need to run pair in the dual channels (slots 1&2, i.e. 128bit address space).

How do I utilise 1GB in 128bit mode? When I have 2x256MB DIMMs and only intend to buy a single 1GB DIMM?

 

[EvelT]

Ok, ignore the question I got it all figured out. Thanks for your help again.

Pulsar's posts were very informative too...