The Phenom II X2 555 Black Edition is part of a recent update to AMD's range of dual, triple and quad core processors. The five new models look identical to each other as they all use Socket AM3, which means they have an integrated DDR2/DDR3 memory controller.

These processors are fabricated using the latest Global Foundries 45nm process and as the new models are speed bumps of existing CPUs we can speculate that the 45nm process has been improved since its introduction.

The five CPUs have a range of clock speeds and TDP (Thermal Design Power) ratings, with the low power Phenom II X4 910e standing out from the crowd, as this quad core processor manages to keep the TDP down to 65W thanks to a clock speed of a mere 2.6GHz. Unfortunately the price of the X4 910e is north of £140 which puts it head-to-head with Intel's Core i5.

The triple core Athlon II X3 440 with its clock speed of 3.0GHz and the Athlon II X4 635 that runs at 2.9GHz both have a TDP of 95W so it's a simple matter of deciding whether you want to pay £75 for three cores or whether you prefer four cores for £99.

Although there are obvious similarities between the Athlon II X2 and Phenom II X2 CPUs there are also a surprising number of differences lurking just below the surface. Athlon II X2 255 has a TDP of 65W with a clock speed of 3.1GHz and a price around £60, while our AMD Phenom II X2 555 is clocked at 3.2GHz with a price of £85 and a TDP of 80W.

The Athlon II X2 Regor core is designed as a dual core processor and has 234 million transistors in a core that is 117.5 square millimetres. By contrast the Callisto Phenom II X2 is massive as it has 758 million transistors in a core that is 258 square millimetres. In part this is because Phenom II is a quad core CPU which, in this case, has had two cores disabled.

We were hoping to use the Advanced Clock Calibration trick (Google for ACC if you need an explanation) to enable the extra cores but we failed with the Asus Crosshair III Formula we used for testing. Based on our experience you shouldn't expect to get a quad core CPU for the price of a dual core.

The other reason for the difference in die sizes is that Phenom II X2 255 has 1MB of L2 cache and 6MB of L3 cache whereas Athlon II X2 has 2MB of L2 and no L3 cache. 5MB of cache memory requires a huge number of transistors.

Phenom II supports DDR2-1066 and DDR3-1333 with its integrated memory controller but in theory Athlon II X2 only supports DDR2-800 and DDR3-1066. Shop around for a decent motherboard and you shouldn't have any trouble running your RAM at higher speeds than AMD intends. Of course that only works if your DDR3 memory is rated to run at 1333MHz or 1600MHz. While we were testing the Phenom II we had to increase RAM voltage to 1.8V in the motherboard BIOS settings before we could even achieve the default RAM speed of DDR3-1333.

Testing the Phenom II back-to-back with the Athlon II showed that there is little to choose between the two dual core CPUs in terms of performance. The clock speeds of 3.1GHz and 3.2GHz, along with the number of cores, are the key here and details such as the amount of L2 and L3 cache make very little difference.

Under load in Cinebench the Athlon II X2 system drew 125W so we were surprised to see that the Phenom II drew 155W. That's an extra 30W just from a change in the CPU although we noted that Cool 'n Quiet was disabled in the BIOS so there is scope to reduce the figure.

When it came to overclocking we raised the core voltage of each CPU to 1.4V, however the method of overclocking was different in each case. For the Athlon II X2 we raised the base clock from 200MHz to 238MHz for a clock speed of 3.69GHz. Things were simpler with Phenom II X2 as this Black Edition processor has an unlocked multiplier so we were able to leave the base clock at 200MHz and raise the multiplier from 16x to 19x, which gave a speed of 3.8GHz.

When both CPUs were overclocked the power draws of the two systems were 145W and 140W for Phenom II and Athlon II respectively.

Verdict
Phenom II X2 555 Black Edition is simple to overclock to 3.8GHz and we suspect that 4.0GHz is within reach with a bit of work. The performance of Phenom II is slightly better than Athlon II, however the price is rather steep for a dual core processor. In many respects this makes Phenom II a premium dual core CPU and by contrast the 2.9GHz quad core Phenom II X4 635 looks very tempting at £99.

Company: AMD

Contact: 01276 803100

Tags: advanced micro devices phenom ii central processing unit multi-core processor socket am3 athlon ii thermal design power amd