It's barely two months since the mighty Core i7 tore up our CPU performance records, but already AMD is fighting back. This month sees the arrival of its most powerful desktop CPU yet, the Phenom II. It's launching in two flavours: the Phenom II X4 920, running at 2.8GHz, and the multiplier-unlocked X4 940, which has a stock speed of 3GHz. Slower parts, including triple-core models, are expected to follow in a few months. Could these be the chips that finally turn the tide for AMD?

The Phenom II's model numbers cheekily echo those of the Core i7, but its design is nothing like as revolutionary. The biggest material difference between the new CPUs and their quad-core predecessors is a die shrink, making these AMD's first chips to be fabricated with a 45nm process.

This explains the higher clock speeds (the old 65nm parts were never sold above 2.6GHz), but it's hardly ground-breaking stuff - more a case of better late than never. Intel has been enjoying the greater efficiency and lower costs of 45nm processors for more than a year now.

Beyond that, the basic architecture is unchanged. The chip retains the classic Phenom design of four cores on one die, each equipped with 128KB of L1 cache and 512KB of L2 - although the shared L3 cache has been boosted from 2MB up to 6MB.

For now it remains a DDR2-only platform as well, with a HyperTransport 3 bus running at 1.8GHz, just like the mid-range Phenom X3 and X4 processors. Although the 45nm process should allow better power efficiency, the maximum quoted power draw remains at 125W.

The upside of this conservative approach is that there's no need for a new motherboard design: many existing Socket AM2+ boards will need only a BIOS update to support a Phenom II. Future additions to the Phenom II line-up are expected to maintain compatibility with AM2+ while adding support for the forthcoming Socket AM3 architecture, which finally brings DDR3 support to AMD's platform.

But it's debatable whether any of this really amounts to a whole new generation of processors. As we booted up our test system, we were interested to see how AMD's refinements to the Phenom would affect performance.

RESULTS

We tested the Phenom II in an MSI motherboard based on the AMD 790GX chipset. We used our standard 2D benchmark suite on a fresh installation of Vista Home Premium with 2GB of RAM.

As you'd hope, the new chips proved faster than older Phenoms, but the gap wasn't huge. The Phenom II X4 920, running at 2.8GHz, achieved a benchmark score of 1.62 - only around 5% faster than the 1.55 achieved by the 2.5GHz Phenom X4 9850.

In fact, that's a slightly smaller gap than you'd expect from a simple clock-speed boost. This may be down to teething troubles with BIOS support for the new chips, but another factor is probably the 9850's faster HyperTransport, clocked at 2GHz rather than the Phenom II's 1.8GHz.

Stepping up to the 3GHz part put more convincing distance between new chips and old, boosting the overall score to a respectable 1.71. And the Phenom II didn't stop there: the high-end X4 940 is a multiplier-unlocked Black Edition, enabling you to ramp the CPU clock up beyond its advertised speed.

We found ours ran stably at 3.4GHz on standard voltages and with a stock cooler: at this speed its overall benchmark score rose to 1.93. Installing an enthusiast heatsink and increasing the motherboard's voltages to the maximum recommended settings let us press on up to 3.7GHz, raising the benchmark score to 2.05.

When we tried to push the operating frequency higher than this, the system became unstable, but as always with overclocking some chips will go higher than others. AMD has demonstrated Phenom II chips, with cooling comparable to ours, running at 4GHz.

HOW IT STACKS UP

It may seem odd to compare a quad-core chip with a dual-core processor, but as you'll see from the graph on this page, the Phenom II X4 920's overall performance is similar to that of a 3.16GHz Intel Core 2 Duo E8500. Naturally, though, the equivalence isn't perfect.

In the Office and Encoding sections of our tests, the Phenom II lagged behind Intel's chip, coming in almost 10% slower than the E8500; but in the Graphics and Multitasking exercises it turned the tables, placing first by a similar margin. Arguably, that makes it better suited to multi-threaded computing, but at current prices it's around $110 more than an E8500 - a big premium for such an equivocal benefit.

The Black Edition 940 is a far more persuasive choice. Even at stock speeds it's faster than any Core 2 Duo or Core 2 Quad processor: some high-end Core 2 Extreme chips rival it, but those come in at twice the price. Evidently its real competition is the Core i7, and while it can't match the incredible speeds of the high-end i7-965 Extreme, if you turn up the clock it will outpace a 2.93GHz i7-940 for $450 less.

As you'll see from the ‘bang per buck' graph on this page, that makes the X4 940 a far better deal than any but the Core i7 920 - but that's before you factor in the expensive motherboard needed for Intel's flagship processors.

It's a compromise candidate, of course. Those looking for ultimate performance still have nowhere to turn but Intel, while bargain-hunters have several better options - including, embarrassingly, the original Phenom X4 9850. Right now it's hard to justify paying the extra for a Phenom II X4 920 given the modest performance gap, but AMD has already flagged a price drop to $350 (920) and $419 (940).

But the Phenom II X4 940 has found a sweet spot in the upper echelons of computing power. For high-performance computing on a budget its attractions are obvious, especially for enthusiasts who don't mind tinkering with clock speeds.

Indeed, AMD's historic unwillingness to invest in promoting the Phenom brand suggests that enthusiasts will probably be its core market. But while the Phenom II is unlikely to prove a knockout blow to Intel, it's a smart trade-off that keeps AMD convincingly in the game

Benchmarks - click to enlarge