How to unlock the Phenom's fourth core

24th Jul 2009 | 12:00

How to unlock the Phenom's fourth core

Push AMD's triple core to the limit!


When AMD launched its first triple-core processor, we laughed at both them and the CPU itself. Oh, how we laughed.

Even the weakly clocked quad-core versions of the first Phenoms were farcical and their bastard offspring, with three-quarters the appendages, were at best pointless.

Things have changed now though – seriously so. The release of the Phenom II with its speedier clocks, more elegant architecture and massive overclocking headroom has given AMD a family of chips that, while not on a par with the lightning Core i7, at least gives it a run for its money.

The triple-core version of the Phenom II has come into its own too. No longer is it a comically irrelevant chip; the fact that we still lack many multi-core apps or games means the price and raw clockspeed of the processor keep it at the races.

The other neat thing, though nowhere near as certain as the overclocking potential, is the fact that certain chips carry the possibility of unlocking a dormant fourth core. The triple-core Phenom IIs are essentially quad-core CPUs with one of the cores turned off. Inevitably that core has been turned off for a reason, usually owing to a fault in the silicon, but not always.

Some batches are designated to be sold as triple-core chips despite being manufactured in the same process as quad-cores. It's all down to modern economics and business practices. AMD isn't going to be able to sell every single yieldable quad-core it manufactures, so some will be 'down-binned' to cover demand for cheaper processors.

If you're lucky enough to get hold of one of these then chances are you're in for a lovely free core.

How to unlock a dormant core

Not all X3s on the market have this usable fourth core, but you're in luck if you have one with a batch number (the middle string of numbers on the front of the chip) starting '0904'

1. You'll also need a 790FX motherboard with an AMD SB750 in order to access that dormant core and the BIOS needs to be able to give you access to the Advanced Clock Calibration section to make it possible.

Should you happen to have all of these things at your disposal, getting the most out of your bargain-basement quad-core beauty will be a mercifully simple endeavour. We chose to use a Phenom II X3 720 with a Gigabyte MA790FXT-UD5P motherboard, although Biostar and ASRock 790FX boards should be more than capable of reproducing the trick too.

Unfortunately though, with Gigabyte at least, the functionality has been patched out of the latest F3L BIOS release. After a bit of fruitless messing about we decided to flash back to the second F3B BIOS – the first to offer Phenom II X3 support – and this gave us access to that elusive fourth core.

From this point on it should be relatively plain sailing. Although flashing the BIOS on a Gigabyte board is just as fraught with mobo-bricking possibilities as it is with boards from other manufacturers, it's not a complicated procedure.

AMD phenom x3 ii

FREEBIES:If your chip's batch number starts '0904', there's a fourth core waiting to be unlocked

You don't have to go through the rigmarole of creating a bootable USB drive or waste time hunting down that otherwise obsolete floppy disk drive; you can just drop the BIOS and BAT files onto a USB key and get going with the BIOS-based QFlash app straight away.

Once the BIOS is flashed, remember to dip in and hit the Load Optimised Defaults setting to finalise the update. Then it's just a question of navigating to the CPU tweaking section of the BIOS and hitting the Advanced Clock Calibration (ACC) option.

2. Set this to Auto, then save the settings and reboot.


ALL THE CORES:Set Advanced Clock Calibration to Auto to finish the job

3. At the post screen you should now see that the CPU is being identified as a Phenom II X4.


BOOT IT:If successful, the post screen should identify your CPU as a quad-core

4. So long as the core is stable you should also find it showing up in Windows. If it isn't stable, you probably wont even get it to boot your OS in the first place, so if you've been lucky enough to hit the desktop you can be reasonably sure it's functioning okay.


THE WINDOWS TEST:You know your core is stable if Windows identifies it

5. We used X264 and Far Cry 2 to test the stability of our core and it passed with flying colours. X.264 in particular is a great test because it stresses all four cores to 100 per cent for a decent length of time. If you want to be fully certain, you can run X.264 a couple of times to make sure that it's getting similar results each time and doesn't fall over. Simple.


BENCH IT:X.264 is a good tool to check the stability of your cores

Performance results


OVERCLOCK:With a bit of tweaking in the BIOS, you can also OC the X3 to get a bit more grunt


First published in PC Format Issue 228

Liked this? Then check out The ultimate guide to overclocking

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