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AMD's 790FX chipset


The itsy bitsy chipset
— 11:00 PM on November 18, 2007

Processors and graphics chips are easily the rock stars of this industry, and for good reason. They're largely responsible for overall system performance, and that's even more appealing to enthusiasts than eyeliner and tight leather pants are to hair metal groupies. It's fitting, then, that the bulk of attention and excitement surrounding AMD's new Spider enthusiast platform will narrow on its new Phenom processor and Radeon HD 3800 series graphics cards.

If Spider were a rock band, Phenom would undoubtedly be the front man, with the Radeon HD 3870 and 3850 filling in at lead and rhythm guitars. On drums, you'd find AMD's new 790FX chipset, hidden away not behind a massive array of snares and cymbals, but under a heatsink amongst a sea of other chips on a crowded motherboard.

Like drummers, chipsets spend most of their time outside the limelight; even shirtless and sweaty, they don't pack much in the way of sex appeal. But they're the glue that holds everything together, and that counts for a lot if you value system stability. On occasion, chipsets can also bring important new features to the table that will shape system architectures for years to come. This is one such occasion, with the 790FX packing second-generation PCI Express, support for two-, three-, and four-way CrossFire configurations, and the first desktop implementation of HyperTransport 3. Join us as we explore how these features tie the Spider platform together, and how the 790FX's performance, power consumption, and initial motherboard implementations measure up.


The itsy bitsy 790FX
The heart of the 790FX chipset is a north bridge chip that shares the same name. This chip is built using 65nm process technology, allowing for a chipset thermal design power (TDP) of just 10 watts. AMD is quick to hype the 790FX's power efficiency, no doubt in part due to the notoriously high power consumption of its nForce rivals, particularly the 590 SLI. We'll see how actual motherboard power consumption shakes out a little later.

With all of the 790FX's new features packed into the north bridge, it's fitting that the name extends across the entire chipset. The most important of these new features is easily a HyperTransport 3 interconnect that allows the 790FX to make the most of AMD's latest Phenom processors. This interconnect retains HyperTransport's 16-bit data pathway, but doubles its clock speed to 2GHz, yielding a twofold increase in bandwidth between the processor and north bridge.

790FX nForce 590 SLI
Processor interface 16-bit/2GHz HyperTransport 16-bit/1GHz HyperTransport
PCI Express lanes 42* 18
Multi-GPU support CrossFire SLI
Chipset interconnect PCIe x4 16-bit/1GHz HyperTransport
Peak interconnect bandwidth 2GB/s 8GB/s

Doubling bandwidth is a bit of a theme for the 790FX, whose second-gen PCI Express lanes offer twice the bandwidth of their gen-one counterparts. As with the 790FX's HyperTransport 3 implementation, PCIe's second coming doesn't actually change the width of the data pathways. Instead, the increase in bandwidth comes courtesy of faster signaling rates that give each PCIe 2.0 lane 1GB/s of bandwidth—twice that of their predecessors.

In addition to offering substantially greater bandwidth, second generation PCI Express lanes are also capable of throttling lane speeds to conserve power at idle. PCIe 2.0 retains backwards compatibility with gen one hardware, as well, allowing the 790FX to play nicely with such outdated gen-one graphics cards as the, er, GeForce 8800 GTX.


The 790FX block diagram. Source: AMD
With a total of 42 lanes of PCIe 2.0 at its disposal, the 790FX has connectivity to spare. 32 of those lanes are reserved for graphics cards, with lanes split evenly for dual-card CrossFire configurations. The 790FX will also be compatible with three- and four-way CrossFire setups that are due to be enabled in future graphics driver releases. For these multi-card configs, graphics cards will receive eight lanes of bandwidth each—plenty considering the fact that PCIe 2.0 offers twice the bandwidth per lane of its predecessor.

Of the 790FX's 10 remaining PCIe lanes, six can be used for additional expansion slots or onboard peripherals. The final four are consumed by the interconnect that links the chipset's north and south bridge components. Four lanes at 1GB/s per lane should yield 4GB/s of bandwidth here; however, because the 790FX's south bridge component is limited to first generation PCI Express lanes, interconnect bandwidth tops out at only 2GB/s.

SB600 nForce 590 SLI MCP
PCI Express lanes 4* 28
Serial ATA ports 4 6
Peak SATA data rate 300MB/s 300MB/s
AHCI Y N
Native Command Queuing Y Y
RAID 0/1 Y Y
RAID 0+1/10 Y Y
RAID 5 N Y
ATA channels 2 1
Max audio channels 8 8
Audio standard AC'97/HDA HDA
Ethernet N 2 x 10/100/1000
USB ports 10 10

So why doesn't the 790FX's south bridge component have fancy pants PCIe 2.0? Because it's the same SB600 chip that ATI introduced way back in May 2006. AMD is working on a new SB700 south bridge that will apparently make its way into the second wave of 790FX boards due out early next year. However, the SB700 isn't ready yet, and AMD has elected to launch Spider without it.

With all its PCIe lanes and graphics cards hanging off the north bridge, there isn't an overwhelming need for additional interconnect bandwidth. Heck, even Intel's latest X38 Express chipset makes do with a 2GB/s chipset interconnect. The SB600 shows its age in the features department, however, particularly when compared with the nForce 590 SLI.

There was a time when four Serial ATA ports was more than enough for most folks, but that was back when SATA optical drives were few and far between. Serial ATA burners are much more common today, and for users looking to rid their systems of bulky IDE cables, four ports looks a little thin. The SB600's lack of RAID 5 support also stands out, although it will probably be less of a concern for most users.

Motherboard makers can always add auxiliary storage controllers to help bolster the SB600's I/O capabilities and RAID support. At the very least, they'll have to add their own Ethernet controller to make up for the SB600's lack of integrated networking. Ideally, board designers will opt for PCIe-based GigE controllers that offer full Gigabit throughput and exhibit low CPU utilization. That's not guaranteed, though; we've seen enough motherboards equipped with slow PCI-based Ethernet controllers to know mobo makers are prone to cutting costs wherever they can, even if it means sacrificing peripheral performance.