There are some rumors circulating this morning that it is possible to enable the fourth hidden / disabled core in a triple-core Phenom II X3, by simply enabling Advanced Clock Calibration (ACC) in the system BIOS on motherboards outfitted with SB750 southbridges and AMI BIOS derivatives. Regardless of the ACC setting used in the BIOS, however, we could not enable the fourth core on our Phenom II X3 720 Black Edition. Sorry.
So far, a couple of stories have surfaced that claim to have unlocked the fourth core in a triple core processor using ASRock M3A790GXH/128M and Biostar TA790GX motherboards. And in their testing, the processors simply worked just like a quad-core.
Although we do not have either of the ASRock or Biostar motherboards used in the reports in house at the moment, we tried to replicate their success with an Asus M4A79T Deluxe motherboard, which also sports and AMI BIOS and an SB750 southbridge.
AMD Phenom II X3 720 Tri-Core Vital Signs
If these rumors do turn out to be true, and the fourth core in a tri-core Phenom II X3 can be enabled via a BIOS setting, we expect a rash of user modified BIOS files to hit the web post haste that enable the necessary functionality, even it motherboard manufacturers patch their existing BIOS files to prevent core unlocking (if it is in fact possible). But be aware, depending on AMD's binning process, enabling the fourth core may not be possible with all Phenom II processors. It's likely that some X3 processors will be perfectly functional X4s that simply have a core disabled, but it's also possible that the majority of Phenom II X3 processors have a defective or marginal fourth core that was disabled for a reason.
We hope to have more information as this story unfolds. We have reached out to AMD to see if they have an official statement on the matter but have yet to hear back. We'll update this post as necessary with new information as it comes in.
Regardless of the ACC setting used in the BIOS, however, we could not enable the fourth core on our Phenom II X3 720 Black Edition. Sorry.