Memory Runs at Flex Memory Mode

Question

I had 4 GB RAM installed, running in dual channel mode.

RAM: Corsair TWIN2X4096-6400C5DHX (CM2X2048-6400C5DHX)
Motherboard: Gigabyte GA-EP45-UD3R

Channel 0: DIMM1, DIMM2
Channel 1: DIMM3, DIMM4

DIMM 1: 2 GB, CM2X2048-6400C5DHX
DIMM 2: --
DIMM 3: 2 GB, CM2X2048-6400C5DHX
DIMM 4: --

I added 2 GB more in DIMM 2. So now I have 6 GB in total.

So this is the current configuration:

DIMM 1: 2 GB, CM2X2048-6400C5DHX, channel 0
DIMM 2: 2 GB, CM2X2048-6400C5DHX, channel 0
DIMM 3: 2 GB, CM2X2048-6400C5DHX, channel 1
DIMM 4: --

But now when I boot up, BIOS reports as running in "Flex" mode.

Why is that? The module I added is of the same brand and part number as the other ones that were already installed. From what I have read on the web, the "flex" mode only appears if you mix and match different module sizes, or different brand and part numbers.

In fact, here is a quote from the motherboard manual about it:

When memory modules of different capacity and chips are installed, a message which says memory is operating in Flex Memory Mode will appear during the POST. Intel® Flex Memory Technology offers greater flexibility to upgrade by allowing different memory sizes to be populated and remain in Dual Channel mode/performance.

Here is a screenshot of a RAM report.

How do I tell if at least the original two RAM modules are running in dual channel? What do I look for?

I notice that the module in slot 2 has a week/year parameter. While the original two do not. I don't think the date is accurate. But could this indicate that they are not same modules, internally? Like different revisions maybe?

Why are there two different JEDEC frequencies? One is at 270 MHz and the other is at 400 MHz, why is that?

This is what the DIMM slots look like:

Similar color means disimilar channel, and disimilar color means similar channel (in lack of a better way of expressing this idea). So yellow and yellow means channel 0 and 1, which makes up a dual channel. Similarly, red and red means channel 0 and 1. Colors are not an indication of channel (same color does not mean same channel), but a way to help install memory in dual channel mode. (Then you add the third wheel to the mix, like I did, and you're in this weird "flex" mode.)

Solution 1

Your system is in flex mode because your channels are not of the same capacity. Since you have three chips but a dual-channel board/CPU flex is the best you can do. You must have an equal capacity on both channels to run full dual channel.

in your case, it it probably best that way. the additional ram is likely worth the small degradation in speed from the flex channel interleaving. When you get the opertunity, drop another stick in to even it out.

See more info about Ram channels and Flex mode here: https://web.archive.org/web/20081101200235/http://www.intel.com/support/motherboards/desktop/sb/cs-011965.htm

Solution 2

You are running in Flex Mode because you have a total of 4GB in channel 0 and only 2GB in channel 1. The first 2GB of memory in channel 0 is in Dual Mode with the 2GB in channel 1. The additional 2GB in channel 0 is in Flex Mode, that is the same as saying it is running in Single Channel Mode. So, you have 4GB in Dual Channel plus 2GB in Single Channel. It probably doesn't affect your performance at all but if you add one more 2GB module it will all be in Dual Channel Mode.

Comments

[Frank Thomas]

the jedec profiles are presets for how your ram could be run. when you underclock ram, other values like CL etc have to change as well, so the JEDEC profile defines what those values will be for each speed.

[Samir]

@Ramhound I have 4 GB on channel 0 and 2 GB on channel 1. So does that mean that DIMM 1 and 3 are in dual channel? And DIMM 2 is in single mode? Hence the "flex" mode?

[Samir]

@Ramhound I take it you're using a text mode web browser? ;) The motherboard model number is up in the screenshot. Alt="EP45-UD3R" So yes, it's an Intel P45 chipset and it's a Gigabyte board.

[Ramhound]

@sammyg - I see the screenshot. I had no idea that was the model number. Update the question to specifically list which motherboard your using. Because the Alt code didn't work in IE8.

[Ƭᴇcʜιᴇ007]

@sammyg yes, blue is one channel and black is the other. That's why they're coloured. :)

[Samir]

@techie007 I always get confused about this. I looked at it now, and I thought I had it there for a second. I even posted a picture and typed in a small note about this. But now you're confusing me again. Adjecent DIMM slots (or disimilar colors) means they are on the same channel. Say it ain't so?

[Samir]

@techie007 I don't know about Intel baords though. Maybe different manufacturers like to do things a bit differently. But I can assure you that on Gigabyte EP45-UD3R, GA-X38-DQ6, and Asus P5Q PRO, adjecent DIMM slots are on the same channel, and colors don't help identify channels, they help configure dual-channel mode. Even I RTFM... somtimes! ;) Usually not...

[Frank Thomas]

When I install a mobo, the ram configuration and the case pinouts are the only parts of the manual I read every time ;)

[Samir]

I still have this board, and it is still in use. It's operating in full dual channel mode by now. No flex! I installed another set of modules of the same type, so 4 Corsair modules are in now, for total of 8 GB.