Why does my CPU have two cores disabled? What are the advantages/disadvantages?

Four cores gives you double the parallel processing power of two. Processor manufacturers test each processor for their maximum speeds, then set the clock speed accordingly. However, if they get a really good batch they may sell good processors for cheaper brackets and limit their speed to meet demand. Maybe the other two cores aren’t capable of the clock speed they wanted, so they sold it as a dual core and locked off two cores to maintain their reputation. Just guessing.

4 cores = more grunt in modern games.

4 cores = more threads available for multi-threaded applications (see the above point).

Your OS should do it automatically, but imagine assigning a program/process to a particular processor. With a single core, they all used to have to go through the same CPU sequentially. On multi-core CPUs, there are multiple processors to run multiple applications at the same time (kind of parallel processing as @FireMadeFlesh mentioned).

In a scenario where you are running a game, one core could handle the AI programming, another the physics of the environment, another the interface between processor and GPU, etc…. – games like Bad Company 2 or Crysis 2 take advantage of this potential; older games do not.

As @FireMadeFlesh said, CPU manufacturers check the CPUs they produce for the clock speed they are capable of running at the stock voltages of the chip. If they chips can’t achieve the speeds, they CPU is raped down and it is checked again until a stable clock speed is found for the voltage going to the CPU. This is called speed binning. This thread on Anandtech has some useful info and a video.

If, on a multi-core chip, a disparity between the maximum speeds of the cores becomes apparent, they will deactivate the under-performing cores and sell a 4x core as a 3x core, while giving the buyer an opportunity to unlcok the cores. Often, a slight increase in voltage above that which the chip was designed for will allow for workable clock speeds on all the cores that don’t cause blue-screens and the like.

There’s a wonderful saying when it comes to this kind of thing: “If you have any doubt as to what it’ll do, leave it alone.”
Your chip was binned as an X2 for a reason, just because the option for four cores is available does not mean it will run properly when they are enabled. For example most ATI HD 5830s and 5850s are actually HD 5870s which didn’t make the cut and were binned for their prospective cards due to being unable to run a the higher speeds with the desired memory bandwidth and optimal voltage settings. Some of these chips can be unlocked to perform with their full potential, but more often than not you will end up damaging the card. Same lesson goes for CPUs.

1) The majority of games out, and planned to be out, don’t make any use of extra cores in it’s programming. Any speed gains come from everything else being distributed.

2) If a certain amount of the yield doesn’t meet expectations then the whole batch is sold as something lesser. So it’s entirely possible that you could end up with the parts that are working fine. It is, however, a crap shoot.

I am with @ApolloX64 on this one. Unless there has been a major change to AMD’s naming conventions, the X2 is, has been, and always will be a dual-core CPU. There is the possibility that they had an X4 with a couple of failed cores and they decided to sell it as an X2, and/or that your BIOS is made to handle an X4 but has two of them grayed out since they aren’t actually present.

As for the advantages, a dual-core pulls less wattage and generates a little less heat during full-load operation, but the quad-core will do things faster if the software you are running is set up for multi-threading. Many programs on the market now cannot use a quad-core to it’s full potential, though most can handle dual-cores. That means that quad-cores have an advantage under certain circumstances, but it’s conditional.

Well basically I had chosen my mobo/cpu combo because they were cheap and other people used them together. They had also mentioned that the mobo can unlock the extra cores on the cpu. When loading up one time I wanted to get into the BIOS but accidently clicked the “4” button which unlocked the extra 2 cores. I haven’t bothered to find a way to relock them, but the computer is running fine. The only thing I notice different though is that the task manager has an extra two graphs for the extra cores :P

Well, if it’s stable then you’re golden!

You should run a stress test like Prim95 or SuperPi for a few hours to see how stable the system really is.

If you find instabilities at stock speeds (that fact that it’s a speed-binned 4-core acting as a 2-core suggest to me that you wiil), try upping the voltage a little.

If you can’t get it stable at stock voltages and up the voltages somewhat, you might find that you need to consider an after-market cooler to keep the CPU cores stable and error-free at a temperature that won’t cook your processor. If this is the case, the choice is yours, but I wouldn’t moan about it if the thing just decides to stop working.