The dual core processor revolution is already in gear, and both Intel and AMD will be heavily pushing this technology through 2005. Intel has already unleashed a dual-pronged attack of the Pentium Extreme Edition 840 for the enthusiast, and the Pentium D line for the mainstream buyer, but while multi-threading speed is excellent, the relatively low 2.8 GHz to 3.2 GHz clock speeds hold them back from gaming superiority. That's where AMD and their Athlon 64 X2 line come in, and with these new processors sporting clock speeds from 2.2 to 2.4 GHz, expect high-end multi-threading and media encoding, along with cutting-edge game performance.
The Athlon 64 X2 line is comprised of two distinct architectures, in much the same format as the current Athlon 64 processors. The Athlon 64 X2 4800+ is clocked at 2.4 GHz, and features a full 1MB of L2 cache per core, for a total of 2MB per processor. The Athlon 64 X2 4800+ is essentially dual Athlon 64 4000+ (or FX-53) cores on the same processor die. The Athlon 64 X2 4400+ shares these same basic characteristics, but is clocked at 2.2 GHz. Each of the Athlon 64 X2 cores also features its own 64K of L1 instruction and 64K of L1 data cache, or 256KB of total L1 per processor.
The second level of the line includes the Athlon 64 X2 4600+ (2.4 GHz) and 4200+ (2.2 GHz), with the difference being the L2 cache being cut in half to 512K per core, or 1MB total per processor. These follow the standard Athlon 64 design, with the Athlon 64 X2 4600+ corresponding to a dual Athlon 64 3800+ and the 4200+ to a dual Athlon 64 3500+. This is the more mainstream portion of the AMD Athlon 64 X2 line, and is being targeted and priced for this market, especially the Athlon 64 4200+. Naturally, all Athlon 64 X2 processors are AMD64 models and can handle both 32- and 64-bit code.
The Athlon 64 X2 processors represent the first true dual core processors for the desktop, and offer a more streamlined format compared to the Intel Pentium D/Pentium EE design strategy. The Athlon 64 X2 architecture incorporates two cores onto the same die, but unlike Intel, it provides a System Request Queue and Crossbar Switch onboard the processor itself. That means that the two Athlon 64 X2 cores can communicate directly across a high-speed internal bus, rather than using the system bus like the Intel Smithfield dual core models. This can obviously speed up data and cache coherency transfers, as well as not relying on the platform or chipset to determine overall performance levels or processor compatibility. This design is also far superior to Intel Hyper-Threading, where a single core emulates two logical processors.
The integrated memory controller of the Athlon 64 X2 4800+ is much the same as a standard AMD64 processor, and can handle PC1600 to PC3200 memory in a dual-channel format, but also has a new wrinkle to it. There may be dual cores on the processor die, but there is only one HyperTransport link and only a single path to the system memory. This means the integrated memory controller is shared between the two cores, and memory bandwidth remains equivalent to current a single core Athlon 64 processor. The Athlon 64 X2 processors also receive all of the benefits of the 90nm Venice/San Diego revisions, including SSE3, the use of mismatched DIMMs on a memory channel, to fill all four sockets at DDR400 speeds, and improved memory mapping and lower latencies.
The Athlon 64 X2 4800+ shares a similar design to the Athlon 64-based processors, but there are a few differences. The Athlon 64 X2 4800+ is a 90nm SOI part, with a die size of 199mm2 and approximately 233 million transistors. The transistor count has jumped significantly, but it is interesting to note that while the actual die size has doubled compared to current 90nm parts, it has not increased very much compared to 130nm Athlon 64 FX models. Core voltages have not changed compared to 90nm Athlon 64 processors, and are still in the 1.35V-1.4V range. The Athlon 64 X2 4800+ thermal power rating is set at 110W, and actually compares quite well to the 89W rating of the 90nm Athlon 64 4000+ and the 105W thermal power of the 130nm Athlon 64 FX-55.
The AMD dual and single core models share the Socket 939 platform, so don't look for any radical departures in that area. The Athlon 64 X2 4800+ still employs a single HyperTransport link running at 2 GHz, and supports standard features such as Cool'n'Quiet. Due to the similarities between the 90nm Athlon 64 X2 4800+ and 130nm Athlon 64 FX-55, at least in terms of die size, thermals and power, there is no need for upgraded platforms or cooling hardware, assuming these are high-end enough to pass the current AMD requirements.
In fact, we didn't give a thought to the thermal or power requirements of the Athlon 64 X2 4800+ while testing, and unlike our experience with the Pentium EE 840/i955X, there was no problems with power draw on the 650VA UPS. Throughout our testing, the Athlon 64 X2 4800+ was no different from any high-end AMD processor, and it was as if AMD had developed a dual core processor that emulated the outward appearance of a single core model.
In terms of basic compatibility with Athlon 64 X2 processors, HyperTransport and the processor's onboard memory controller take care of the majority of issues, but there are still some potential challenges. A BIOS update will be required to properly identify the new Athlon 64 X2 models, and we've already seen many vendors releasing X2-compliant BIOS files for download. Assuming that the motherboard and chipset are up to specs, and can handle the power and thermal requirements, that is really all you need to worry about.
Of course, nothing should be taken for granted, as VIA recently announced that the K8T890 is incompatible with the Athlon 64 X2, while the K8T800 versions are not. The NVIDIA Socket 939 chipsets are compatible, and so far VIA is the only major chipset vendor to come out with X2 incompatibility news. But even BIOS updates are by no means a certainty these days. There are also vendors who have not even updated their nForce3 and nForce4 motherboards for the AMD 90nm Venice/San Diego cores yet, so we are not exactly holding our breath for an X2 update.