I have never had a 6870, so I don't know if this works. To unlock Overdrive for the 6990M or 6970M, you open Regedit and navigate to HKEY_CURRENT_USER\Software\ATI\ACE\ Settings\Graphics\OverDrive2. Change the value of the key “Accessible” in the right panel from "Nobody" to "All" (right click on Accessible, select Modify and enter “All” in the Value Data box). Then drop down to Overdrive3 and Overdrive5 in the Graphics section and make the same change. For the 6990M, that's all it takes; restart and a new Performance tab will appear in CCC, which includes Overdrive. The problem is that the default o/c limits in Overdrive for the 6990M are 740/1000, so you need to edit the 6990M vbios using RBE to increase the limits by moving the sliders on the last screen to your desired levels. I use 900/1200, which has served me well. The 6970M requires more. Before Overdrive can be enabled (or the Performance tab appear in CCC), the 6970M vbios has to be modified to move the OD limit sliders referred to above from the default 0/0 to the desired levels. It is a tedious process since the settings can only double with each mod. So, when you first move both sliders to the extreme right, it increases the limits to 1Mhz/1Mhz. Save and reopen, repeat to move to 2Mhz/2Mhz, etc. Keep repeating. Since it doubles each time, it doesn't take as long as you might first think. When you reach the desired level (I use 900/1200), reflash your vbios with the new settings and Overdrive should be available. Ordinarily, it is easier to just post a pre-modified vbios for the 6970M. Unfortunately, I don't have one for a 6870M, so you'll have to go through the manual process. I don't know whether the 6870M supports OD. If it does, this process should unlock it.
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Problems? See this thread at archive.org.
