With its first desktop quad-core processor – arriving after delays of more than a year – AMD has finally abandoned the Athlon brand. The new Phenom range follows the initial launch of the Opteron-branded Barcelona server parts last month. The initial offering was supposed to have been for three Phenom CPUs designated the 9500, 9600 and 9700. All are based on AMD’s recently introduced 65nm fabrication process. The lower of the two models run at 2.2GHz and 2.3GHz respectively with a TDP (thermal design power) of 95W, and the 9700 at 2.4GHz and 125W.

From a performance point of view, the most interesting architectural innovation isn’t the native quad-core design but the cache. AMD has introduced a three-tier system, with the first two tiers as per the standard layout in Athlon processors: 128KB of L1 and 512KB L2 cache per core. But below that is 2MB of L3 cache shared between all four cores, making for an L2 and L3 total of 4MB. The idea behind the architecture is that the L2 cache gives multiple threads a consistent amount to avoid cache starvation, but the dynamically allocable third level gives single-threaded code a decent chunk when required.

Phenom now gains 128-bit floating-point registers and also introduces SSE4 streaming instruction extensions. The bad news is that it’s actually a different SSE4 – dubbed SSE4a – than the one Intel has introduced with Penryn, and the two aren’t compatible, so software will have to be optimised for both.

On the power management front, the best new trick is the Independent Dynamic Core Technology, allowing for fully independent frequency control of each core according to load. In addition, there’s now independent voltage control for the CPU core and memory controller.

The new processors continue AMD’s approach of integrating the memory controller into the CPU. Memory support remains with DDR2 rather than DDR3, though, with a maximum DDR2 speed of 1066MHz. DDR3 support won’t be available until 2009 with the AM3 platform. For now, the physical socket arrangement remains the same as the current Athlon 64 X2’s – the Socket AM2 – but for Phenom-specific motherboards, the design has been dubbed AM2+. It’s physically identical, but AM2+ platforms have HyperTransport 3 support to give a maximum I/O bandwidth to the rest of the system of up to 16GB/s; twice that of HyperTransport 2.

AM2+ boards can also deliver more power to the processor, and support the split voltage planes Cool ‘n’ Quiet needs for maximum power saving. This is all handled on the motherboard side by the new 790X and 790FX chipsets: the FX variant allows for quad graphics cards to form the platform AMD has dubbed Spider. Phenom should run in older AM2 motherboards, but there’s no universal guarantee – you can find a list of AMD-approved boards here.

For all the technical advances and the long wait for its release, Phenom’s performance is a disappointment, our test rig bringing in a good, but by no means stellar, application benchmark score of 1.33. The “native” quad-core approach has shown no perceptible benefits over the current two-by-two layout in Intel’s parts and, with all that cache onboard Core 2 processors, any limitations of the front-side bus architecture have been well masked.

Street pricing for the Phenom 9500 part is as close to equivalent to Intel’s Core 2 Quad Q6600 and, given that the basic, Q6600-equipped Acer Aspire M5630 (page 39) practically matches our Phenom test rig in our application benchmarks, the Phenom doesn’t look compelling.

The outlook becomes worse if you look a month or two into the future. Intel’s current quad-core generation is being replaced by 45nm parts with a lower TDP per clock, more total cache, and significantly faster performance.

We’ve yet to see the triple-core 8000-series Phenoms – which are, in fact, quad-core parts with one non-functioning core – and the traditional high-end FX series of parts (which will be dubbed the FX80 series). It’s possible AMD may pull off a coup in terms of price for the 8000-series Phenoms. But the FX series, which AMD has confirmed will be released in Q1 2008, will have to pull off a miracle in order to outstrip Intel’s upcoming QX9770 in terms of money-no-object performance for the enthusiast.