As you are probably aware, there is quite a bit of hype surrounding AMD's current X2 and X3 processors. The idea is that many of them are perfectly functioning X4's that have had one or two cores disabled to meet X2 and X3 demand. Read this Anandtech post for more information on this and some better analysis.

Long story short, I purchased a Phenom II X2 555 processor and ASRock M3A785GXH/128M motherboard from Newegg a few weeks back in the hopes that I could also get an X4 on the cheap. While I am very satisfied with my upgrades for the price I paid, I unfortunately did not have any luck unlocking the extra 2 cores on the Phenom II X2. Read on for the details.

The ASRock M3A785GXH/128M board shipped with the latest 1.70 BIOS. After activating ACC, it would display Phenom X4 955 in the POST screen, but upon Windows starting to load it would crash. I tried increasing voltages but there was no change. I also tried lowering the CPU multiplier considerably but there was still no change. After reading some posts at computer forums I discovered that some people were also having problems with the 1.70 BIOS. One user in particular was able to successfully unlock with the 1.60 BIOS but upon upgrading to 1.70 ran into numerous errors. The user reverted back to 1.60 and was able to successfully unlock again.

With this in mind I loaded the 1.60 BIOS onto my motherboard and tried again. I was able to unlock via ACC the same as before, and it displayed X4 955 in the POST screen. This time Windows successfully loaded! Unfortunately the excitement was short-lived. A quick check in CPU-Z showed that the CPU was only running as an X2 555. Rebooting and back into the BIOS confirmed that the BIOS settings were reset and ACC was off. I continued to mess with settings, trying different voltages and multipliers, but was unable to get it to load into Windows as an X4 without crashing. So unfortunately I am stuck with what appears to be an X2 that has one or two genuinely defective cores.

Relevant Hardware/Software specs (no overclocking):
OS: Windows 7 Pro 64-Bit
CPU: AMD Phenom II X2 555 (AM3)
MB: ASRock M3A785GXH/128M
RAM: A-DATA Gaming Series 4GB (2 x 2GB) 240-Pin DDR3 1600
HDD: Seagate Barracuda 7200.12 500GB 7200 RPM 16MB (boot drive)
PSU: SeaSonic 430W S12
GPU: GeForce 8800GT