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Athlon and Phenom are the biggest names in AMD's stable. Athlon put AMD on the map with a line of processors capable of beating the best Intel had to offer around the turn of the millennium. Phenom became famous for somewhat more dubious reasons, as many consider it to have been a major failure for AMD, but it was never the less the first consumer processor designed from the ground up to be a quad core. Now AMD offers new versions of both processors, predictably called Athlon II and Phenom II. While Phenom II and Athlon II processors inhabit somewhat difference markets, they meet around the hundred dollar mark. Now that the Athlon II is available as a quad core the lines are further blurred. So what is the difference between Phenom and Athlon?
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The Name Game
Let's get down to brass tacks - the Athlon II is a Phenom II. Or to put it more accurately, an Athlon II shares the same architecture as the Phenom II, but with certain handicaps which prevent it from gaining the full performance of a Phenom II processor.
What are these handicaps? Well, there is actually only one, and that is the lack of any L3 cache. This is the main difference between the Athlon II and the Phenom II in terms of performance at comparable clock speeds, and even calling it a handicap may be a bit of an exaggeration. While the lack of an L3 cache does result in some minor degradation of performance at any given clock speed, L3 cache requires power to function, and its absence means that Athlon II processors suck a bit less power from your socket.
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The Athlon II and Phenom II are so similar, in fact, that most Athlon II processors on the market are the same as Phenom II except that they have lower clock speeds and have had their L3 cache disabled. By disabled, I mean it is still physically on the processor, but it has simply been disabled so that its functionality cannot be reached. The sames often goes for cores on the dual and tri-core Athlon II and Phenom II processors. On some processors these cores actually are on the physical processor, but are disabled. With the Athlon II this is in reference to tri-core designs, as Athlon II dual cores do not include any disabled cores. All Phenom II processors have four cores physically on the processor, so Phenom II dual cores differ from Athlon II dual cores in that the Phenom II variants have four cores physically on the processor.
Why would AMD want to do this? The reason can be found in the manufacturing process. Processor yields are often imperfect. Certain processors in a batch will have deficiencies which prevent them from reaching the maximum potential of the architecture the processor is based on. Rather than discarding these processors, their functionality is limited and they are sold as less expensive products. This is why you'll find some users posting on the internet about how they managed to effectively turn a dual-core Phenom II X2 550 into a Phenom II X4 955 by overclocking and unlocking disabled cores. These cases are rare, however - remember that AMD is selling the products in their stock form for a reason, and that those who manage to overclock and unlock cores without suffering stability issues are simply lucky.
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Unlike the Core i5 and Core i7, which have numerous difference and which in some cases run on different chipsets and even sockets, no such platform barriers exist with the Athlon II and Phenom II. What you do need to pay attention to is if your motherboard is titled as a socket AM2, AM2+, or AM3 processor. All Athlon II processors currently being produced are socket AM3 processors, but the first Phenom II processors were AM2+ socket processors, while newer Phenom II processors use AM3. There is currently a large degree of compatibility between these different sockets, and it is often quite possible to user older hardware with newer processors. I have a separate article which explains the details and differnces of the AM2, AM2+ and AM3 sockets.
Besides that, there are no obstacles to worry about. AMD has numerous chipsets that work with both Phenom II and Athlon II processors and there are no concrete barriers between them. The only possible barrier which exists is power draw. The highest-end Phenom II processors draw a lot of power, and there are some motherboards which are not made to support them. These motherboards will only work with lower powered processors like the Athlon II and low-end Phenom II processors. Also, remember that older motherboards often require a BIOS update. A motherboard built a year ago when Phenom II just came out might require a BIOS update in order to recognize a new Athlon II processor, as those processors were not available at the time the motherboard was sold. That can be a big problem if you are ordering parts for a new PC and you get a brand new chip with a motherboard that has been on the shelf for a while, and don't have an extra, older AMD CPU to boot the motherboard and flash the bios.
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It will be at least a year before AMD has a chance to debut a new architecture, Bulldozer. While Intel currently holds the crown when it comes to processor performance, AMD is doing a better job of making sure that users have a clear upgrade path. There is little difference between Athlon II and Phenom II processors, so consumers should simply purchase whatever fits their needs at the time. Purchasing a low end Athlon II processor now is not going to exclude you from buying a better Phenom II processor in a year if you feel an upgrade is required.