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The AMD 790FX Chipset Explored

The AMD 790FX Chipset Explored

As you may know, AMD has just launched their brand new PC platform solution, codenamed 'Spider'. Spider is really an ecosystem consisting of the AMD Phenom processor, AMD 790FX chipset and the ATI Radeon HD 3800 series graphics. AMD is banking on these new components to revitalize its position in the consumer PC market. However, it is quite unfortunate that the one component with the biggest hype and the most promise - the AMD Phenom processor - is the only product we're missing to complete AMD's trifecta. While we aren't really one to speculate, AMD's inability to provide seeding units is troubling indeed. So while we have graphics cards and motherboards stacking up, we're left twiddling our thumbs without so much as a whiff of a CPU to test on.

Still, moving on, let's just have a look at what the new AMD 790FX chipset has to offer. The main features of the 790FX chipset is really to upgrade the backbone support of AMD's platform in order to fully take advantage of the AMD Phenom processor and ATI Radeon HD 3800 series GPUs. These features come in the form of HyperTransport 3.0, split power planes and PCI Express 2.0. The AMD 790FX also officially supports DDR2-1066, but since the memory controller is embedded in the processor, this is more of a CPU upgrade than chipset.

With HyperTransport 3.0, the 790FX will effectively double the bandwidth available between the Northbridge and CPU from 1GHz of the previous generation chipsets running with HyperTranport 2.0 to 2GHz today. Split power planes is also a new feature supported by AMD Phenom (and above) processors where the there will be two power planes feeding the CPU, one for the CPU cores and another for the memory controller. This is a highly anticipated feature since AMD features a built-in memory controller. With separate power planes, users can better tweak their systems. This should theoretically help power consumption and efficiency as well. Because of the upgrade to HyperTransport 3.0 and split power planes, the 790FX chipset will support what AMD calls Socket AM2+.


Basically, Socket AM2+ is fully pin-compatible with Socket AM2 and vice versa, so you can technically run a Phenom on any current Socket AM2 motherboard or run a current Athlon 64 X2 on the 790FX. However, running a Phenom on an AM2 will mean running in compatibility mode, without the benefits of HyperTransport 3.0 or split power planes (which consequently affects the memory controller's performance potential). Similarly, an older Athlon 64 X2 processor will not benefit from Socket AM2+ as it does not support the new features.

As expected, AMD has also moved towards PCI Express 2.0 and the 790FX chipset features a total of 42 PCIe 2.0 lanes. 32 lanes are dedicated for graphics with two full speed PCIe x16, each of which can split again evenly into four PCIe x8. This will facilitate the new CrossFireX design, which can support 2, 3 or 4-way CrossFire setups. For the remaining ten lanes, six are dedicated PCIe x1 lanes for peripheral connection while the last four seem to be reserved to form the interconnect between the North and Southbridge. In this light, AMD's solution trumps Intel's 3-series chipset because AMD is boasting PCIe 2.0 for almost the entire system, top to bottom, whereas Intel only offers PCIe 2.0 on their high-end X38 chipset, limited again to supplying the GPU lanes and nothing else.

However, there is a weak link in the AMD 790FX chipset, and that happens to be the Southbridge. It would seem that AMD has not been able to roll out their next generation SB700 Southbridge yet, which means that the current batches of 790FX motherboards will still be paired with the older SB600 Southbridge, which doesn't support PCIe 2.0. Compared to chipsets today, the SB600 is also slightly lacking in features with only four SATA 3.0Gbps ports and no built-in Ethernet MAC.

Lastly, the AMD 790FX is supposed to feature great power efficiency. Built on a new 65nm process technology, AMD claims that the 790FX runs with a nominal 8W and a maximum power draw of 10-12W at full load. Because of this, the chipset can even be passively cooled with a simple heatsink. Though from what we've seen, motherboard manufacturers are still sticking to their heat-pipe designs for enthusiast boards.

So in a nutshell, the AMD 790FX and Socket AM2+ isn't all that different from previous generation AM2 motherboards. It serves as an interim before AMD can roll out AM3. Current AM2 users have no real reason to upgrade to the new chipset unless you are investing in the complete Spider platform - motherboard, CPU and GPU. It is only then, will the Spider platform perform at its peak. However, having seen the workstation platform equivalent from AMD with their Barcelona processors, so far there doesn't seem to be much in the way of expectations in either performance or power savings with all the new features on the processor end. So as we've pointed out, this is just a platform renewal for AMD, but don't expect anything exceptional. With that said, we share with you a couple of new motherboards bracing the new AMD 790FX platform on the following pages.