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I've read a bunch of threas from here and around the web, best seems to be the 100 mhz below unless you do a bios flash and then run them on linux?

any development for windows 7 32 or 64 bit?

My MSI 6990 does just fine for underclocking it's memory, I run it's mem at 450, And it's core at 910
Using MSIafterburner to do the clocking

Windows: MSI Afterburner.

Is Afterburner capable of breaking the 125Mhz memory/gpu clock difference on a 6xxx card to lower the memory further ?

I haven't tried it for quite some time now (ever since switching from GUIminer to CGMiner 4+ months ago) but I have some 6xxx cards that I would love to get the memory lower on if it's possible and are all Windows7 x64 machines.

Could you share a bit of info ? does it require 'unofficial overclocking' to be enabled ?

MSIA will look like it does it but if you actually hook a power meter up you'll see nothing's really changing.

I couldn't get the memory down to 300MHz on my 6870 until I started using a combination of AB and clocktweak. Even now, I have to set the speed to 300MHz in AB, and then set the l1 memory speed to 300MHz in clocktweak. Just AB doesn't work, and setting all the levels to 300MHz in clocktweak doesn't work. Have to use both for me.

It work for me

Yes it require the unofficial overclocking thing.

Then you start Afterburner and put the frequency at the minimum. Apply, open again and you will be able to go even lower. That's how i am at 310mhz on my 6950 (300mhz would require another close-reopen but i'm too lazy)

do you have true confirmation that it actually lowered your memclock? like reduced energy consumption with a kill-a-watt or readout from your miner? Do you use a miner like cgminer which would verify the clock speed actually changed? I've seen tons of threads about msi afterburner, but the clock doesn't really change.

I've already tried afterburner and it doesn't do shet for me.

Having a surefire way to underclock memory on 6990's would Be huge for a lot of people. Tons of power and heat savings when you're talking 4gb of ram.

I'm talking windows 7 pro 64 bit here and i use cgminer, but cgminer isn't allowed to lower the memclock.

:O Cgminer cant change the memclock? Thats one hell of a surprise.
MSIafterburner works Flawlessly for me on my 6990, And yes, The memclock is acutally freaking changing. I dont lose 14° of temp, And gain 8mh/s with the fans running 20% slower, With a "Non effectivly changed" clock....

MSIa Also works Flawlessly on my XFX6870BE's I have two of them, I use them for gaming, I love the memclock drop, Saves me so much heat when mining.

MSIa Also works Flawlessly on my Shappire 5830 extreme (not lightning)

You absolutely must "enable unofficial overclocking"
Win7.64bit MSIa CGminer

I had to use Sapphire Trixx to keep the memory clocks to be persistent, a reboot and afterburner would not go below the 150 mhz difference unless you did the close and open trick a few times.

With Trixx, it works fine after a reboot automatically : https://www.sapphireselectclub.com/ssc/TriXX/TriXX.aspx

Whats wrong with the close/open trick? (Wich BTW you dont have to do) Just select a different gpu..

Just flash the bios and be done with it ...

The primary bios on a 6990 is typically write protected, but you can flash the "overvolt" bios in position #2. Don't mess with the other profiles only change the main profile with the 830 mhz and 1250 mhz mem speed. Leave the rest alone, and boom ... your set.

BTW: Dropping mem from core clock - 125 to 150 mhz will save you nearly 25 watts per gpu .. or a total of 50 watts per 6990 card.

Rebooting would pull up afterburner at the wrong clocks even through the profile was set for the correct speeds. Trixx always works without fail.

Agreed aside from i when i try trixx my systems just Haaaate trying to work with it... So i never use it

Reduced temperature. Without the  memory downclock GPU is much hotter

CGminer doesn't work for underclocking memory clock of 6900 serie. Or better, it seems only MSI Afterburner work on underclocking the memory clock.

Tried Trixx, nothing.
Tried CGMiner, nothing.
CC Catalyst thing is useless since it only allow you to go at 1250 -.-

Tried MSI Afterburner, it work. Memory clock is really underclocked.

Cgminer works fine for adjusting memory clocks on my 6970 as long as the initial clock rate is set by afterburner or trixx on boot, same with gpu clock rates, I'm pretty sure it's a driver bug.