The AMD Phenom II and Athlon II can have between 2 and 4 cores (avoiding going into detail). A core is basically a CPU. Each version is sold at a different pricing point. Recent Athlon II are Phenoms with 2 cores locked out.
2- and 3-core chips have 4 physical cores on them. If everything tests OK, it's a 4-core CPU. If one or two cores fails either totally or performance-wise, it's locked out and the CPU is sold as a 2- or 3-core CPU.
HOWEVER, it's possible that market demand for 2- and 3-core CPUs can't be met from failed 4-core units, so in that case AMD may be locking perfectly usable cores on 4-core CPUs to create additional 2- and 3-core CPUs.
Therefore, if you buy a 3-core CPU, its 4th core may be locked because it's defective, because it misses a performance target somehow, or because it's functional but disabled to meet market demand for a 3-core CPU.
So some motherboards can unlock the "defective" core on a CPU, on the off-chance that you have what would otherwise be a fully-functional 4-core unit.
People report generally good results unlocking Phenom X3s. AMD may or may not be turning a blind eye to this, as it gives AMD a price- and performance-competitive CPU against the Intel i3 series. As a user, you end up with a machine which works somewhere between won't boot and perfectly.
You have a 4-core unit with all cores enabled (Phenom II X4: the "X4" tells you).
Incidentally, the same testing system works for speed, too: in general a CPU at a certain speed may have been manufactured for that speed, or it might be a higher-speed unit that failed testing at its design speed and was re-tested at the lower speed, or it might be a higher-speed unit marked down to fill a market demand.
Last edited by Bushfiva; 26th September 2010 at 02:18.
Reason: Added the "incidentally".