Platform Trends: A Phenom in 2007?

AMD has been under intense pressure from Intel all year, with the CPU colossus landing blows both high and low. The launch of the Core 2 Quad left Intel alone in the quad-core desktop market, while a wave of low-cost Core 2 Duo processors has punished AMD at the entry level. AMD has tried everything from dropping prices to introducing new, low-power versions of the Athlon X2, but what the company really needs is to get its new K10 architecture out on the streets.

In mid-May of this year, AMD formally named its next line of dual- and quad-core desktop CPUs: Phenom. This is a rather bold choice, as the word denotes a singularly remarkable person or thing -- often either a fast-rising or legendary sports star, in the class of Michael Jordan or Wayne Gretzky. AMD can only hope the new line lives up to its lofty billing, and not only exhibits phenom-like qualities but makes short work of Intel's upcoming quad-core "Penryn" lineup. The key here is timing -- and although AMD has been averse to talking about firm release dates, rumors are emerging that the Phenom could arrive before the end of this year.

True Quad

As AMD will be only too happy to tell you, Intel may have been first out of the gate with a quad-core PC processor, but the Core 2 Quad Kentsfield is really just two Conroe processor cores slapped onto a single package. There are obvious benefits to this strategy, such as accelerated development times, but it also comes with inherent negatives.

The dual dual-cores are physically distinct, and communicate across the CPU bus rather than a high-speed internal bus. Kentsfield's 4MB + 4MB of Level 2 cache is not shared among all four cores, sacrificing many of the benefits from Intel's Advanced Smart Cache architecture. Using two Conroe cores also translates into a larger processor with architectural duplication and no shared resources. Of course, when you've got the only quad-core show in town, this matters little to most end users.

The AMD Phenom looks to change all that, and will be the first genuine quad-core processor built from the ground up. Each of the cores communicates using an internal CPU bus and offers ample access to system memory, helping performance scale well with additional processors. AMD continues to use an integrated DDR-2 memory controller (up to DDR-2/1066), as well as a HyperTransport platform base.

Other improvements -- shared with the equally eagerly awaited "Barcelona" quad-core Opteron server CPU -- include 128-bit floating-point units, enhanced DRAM controllers, and a shared Level 3 cache. AMD has even made the Phenom compatible with existing Socket AM2 platforms, with some concessions in terms of the HyperTransport bus. The Phenom chips prefer the new Socket AM2+ with HyperTransport 3.0, but will work with the older AM2 socket and HyperTransport 2.0.

The Phenom X2, X4, and FX

Apart from dropping the numeral 64 (everybody's CPUs are 64-bit capable nowadays), AMD will maintain its existing brand format, offering Phenom X2 (dual-core), X4 (quad-core), and FX (two quad-core CPU sockets on one motherboard) products. All of the Phenom desktop models will be 65-nanometer-process parts that will follow the existing market segmentation of their Athlon 64 X2 and FX predecessors.

The Phenom X2 dual-core and X4 quad-core processors will be the mainstream and performance sections of the line, with both supporting the Socket AM2/AM2+ package. The top-end Phenom FX models are similar to the current enthusiast-level Quad FX platform, and use a different Socket 1207+ platform. What is a bit strange is that AMD documentation currently has a Phenom FX listed under the AM2+ category, possibly muddying the waters the company clearly delineated in 2006.

There will also be Athlon X2 processors in the active line, which might be either today's existing models or Phenoms with the L3 cache absent (similar to what Intel has done with its new entry-level Pentium and Core 2 models). Either way, these Athlons will represent the new entry level for desktop processors, and probably condemn single-core CPUs to the scrap heap -- AMD still lists a Sempron processor on its roadmap, but maybe the company will come to its senses before then. Proposed thermal design power (TDP) ratings for the Phenom are looking extremely good, and compare well against existing Athlon 64 X2 and FX 90-nanometer-process parts.

The Phenom X2 will feature dual cores with 512K of L2 cache each, while the Phenom X4 steps up to four cores. As with today's Athlon 64 X2 and FX models, there are really no architectural differences between the Phenom X4 and FX, though the latter will be released at higher clock speeds.

Overall speed ranges are expected to be similar to the current Intel dual- and quad-core lineups; the Phenom X4 is whispered to debut at up to 2.4GHz, with the higher-end Phenom FX reaching 2.6GHz and the dual-core Phenom X2 design allowing an even higher 2.8GHz clock. Due to the number of cores, overall performance should still be relative to the product branding, but on the gaming side (where multithreaded software is still scarce) we might still have the scenario of a 2.8GHz Phenom X2 outperforming a 2.6GHz Phenom FX.

One of the more interesting facets of the Phenom is the presence of a shared L3 cache, a core feature we haven't seen on the desktop since the first-generation Pentium Extreme Editions. Level 3 cache is kind of a second line of defense between the L2 cache and slower system memory, but serves the same purpose: The more data you can keep on the processor instead of having to fetch from system memory, the better.

The speed, size, and latency of the L3 cache are sure to have a performance impact, as will the type of data involved, but it's certainly not going to hamper overall performance. Exactly how much it enhances throughput will be a topic of much discussion, and this type of architecture really needs to be put to a real-world test.

DDR-3: Just Say No

The announcement that the Phenom would continue to support DDR-2 system memory was a surprise to some, especially as Intel had already announced its highly publicized transition to DDR-3. But moving to a new technology just because it's available is not the way AMD does business, especially since the company doesn't expect DDR-3 to be affordable for the mainstream until sometime in 2009.

Instead, AMD is concentrating on matching processor bandwidth requirements while keeping an eye on end-user costs. By supporting dual-channel DDR-2/1066, AMD ensures that the necessary bandwidth is there now and for the future, as opposed to Intel's tactic of overloading its upcoming 1333MHz, 10.6GB/sec processors with DDR-3 capable of between 17GB/sec and 21GB/sec of bandwidth. AMD plans to stay away from that kind of overkill for now, as buyers have had to wait for the Phenom long enough without finding it saddled with a steep price.