ASROCK X79 Extreme6/GB Installation Manual page 40

Step 2. Connect two Radeon graphics cards by installing CrossFire Bridge on
CrossFire Bridge Interconnects on the top of Radeon graphics cards.
(CrossFire Bridge is provided with the graphics card you purchase, not
bundled with this motherboard. Please refer to your graphics card vendor
for details.)
Step 3. Connect the DVI monitor cable to the DVI connector on the Radeon
graphics card on PCIE1 slot. (You may use the DVI to D-Sub adapter to
convert the DVI connector to D-Sub interface, and then connect the D-Sub
monitor cable to the DVI to D-Sub adapter.)
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ASRock X79 Extreme6/GB / X79 Extreme6 Motherboard
CrossFire Bridge
or