If i cross fire a hd6850 and 6870 both BLACK editions from XFX, will one underclock to another automatically. AMD's chart says link em up and your good to go. I just want to know what will happen exactly.
I'm not sure if the 6870 "actually" downclocks or not. I suspect it will simply finish before the 6850 does and will then sit idle waiting for the 6850 to finish so it can work on the next frame. Doesn't matter, either way the same results is achieved.
You are pairing two cards together, one slower then the other. The options are the 6870 actually disables some shaders and downclocks itself which is what most will tell you, or it renders its frame before the 6850 is done and then waits to be told when to do the next one which is what I believe. Those are the options. It doesn't matter as the end result is the same, CF "6850s".