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Nvidia's nForce 790i SLI Ultra chipset


The high end gets even more exotic
— 8:00 AM on March 18, 2008

Barely three months have passed since Nvidia launched its high-end nForce 780i SLI chipset, and already the green team has another round in the chamber. To be fair, the 780i wasn't so much a new design as a second-coming of the nForce 680i decked out with PCI Express 2.0 support via a custom SLI bridge chip. The 780i wasn't the most elegant answer to the 32 second-generation PCI Express lanes lurking in Intel's X38 Express chipset, but it was the best Nvidia could do at the time to keep up with the Joneses. Today, however, Nvidia's real next-generation core logic arrives in the nForce 790i SLI and its spoiler-equipped Ultra variant.

Unlike its predecessor, the nForce 790i SLI has an all-new north bridge chip bristling with 32 lanes of bandwidth-rich PCIe 2.0 goodness. Within it also lies Nvidia's first DDR3 memory controller, which has been carefully tweaked with Penryn in mind. The 790i has native support for 1600MHz front-side bus speeds, too. And, of course, everything comes neatly packaged on a fresh motherboard design tuned for overclocking and packed with ESA functionality.

This is Nvidia's answer to the X48 Express—the very flagship of Intel's chipset aristocracy—leading us to a heavyweight bout for high-priced bragging rights and technical superiority. Keep reading to see if Nvidia has managed to unseat the incumbent.


Will the real 7-series nForce please stand up
Nvidia has made a habit of recycling older chipset components, so the nForce 780i SLI's 680i underpinnings and bodged PCI Express 2.0 functionality weren't much of a surprise. But the 790i is different. It's fresh silicon, albeit fabbed on a 90nm process that's starting to look a little antiquated next to the 55 and 65nm tech used to fab AMD's latest chipsets. Gone is the 780i's LinkBoosted PCI Express 1.1 connection to an auxiliary PCIe 2.0 bridge chip, and in its place, 32 lanes of second-generation PCI Express in all its 5.0GT/s glory.

This native PCI Express implementation inherits a couple of SLI-specific optimizations from the nForce 200 bridge chip, as well. Among those functions is a posted-write shortcut that allows the PCI Express controller to pass messages directly between graphics cards without having to hit main memory. A broadcast function also allows the PCIe controller to take a single command from the CPU and distribute it across multiple graphics cards, saving the CPU from having to generate multiple commands for multiple cards while reducing congestion on the front-side bus.


Such SLI optimizations are a welcome addition to the 790i's PCIe controller, but they'll only affect multi-GPU configurations whose graphics cards are connected directly to the north bridge. That's fine if you're only going to be running a two-way setup. However, more exotic SLI threesomes will be forced to hang a third graphics card off the 790i's south bridge component. The 790i south bridge's first-gen PCI Express controller is decidedly more pedestrian, not only losing out on posted-write shortcut and broadcast functionality, but also offering signaling rates of only 2.5GT/s per lane. Mind you, with the GeForce 9800 GX2 effectively enabling four-way SLI using only two graphics cards slots, there may not be much demand for three-way setups.


To keep up with Intel's upcoming Core 2 Extreme QX9770 processor, the 790i SLI chipset also features native support for a 1600MHz front-side bus. This isn't the most exciting development given that the 780i and even 680i easily overclocked to similar speeds, but it's a must-have for any new high-end chipset's resume.

Speaking of other high-end chipsets, the 790i SLI actually comes in standard and Ultra flavors. Both are based on the same north bridge silicon, with Nvidia binning particularly speedy samples for use as Ultras. This cherry-picking will probably ensure that Ultra chips offer the best overclocking potential, although the only official difference between the two variants is in their support for higher memory bus speeds. The standard 790i SLI can handle DDR3 memory up to 1333MHz, while the Ultra boasts support for memory bus speeds up to a whopping 2000MHz.

X48 Express X38 Express P35 Express nForce 790i SLI Ultra SPP nForce 780i SLI SPP
Front-side bus 1600/1333/1066MHz 1333/1066MHz 1333/1066MHz 1600/1333/1066MHz 1333/1066MHz
Memory controller DDR2-800
DDR3-1600
DDR2-800
DDR3-1333
DDR2-800
DDR3-1333
DDR3-2000* DDR2-800
PCI Express 1.1 lanes 0 0 16 2 0
PCI Express 2.0 lanes 32 32 0 32 32
Multi-GPU support CrossFire CrossFire CrossFire* SLI SLI
Chipset interconnect DMI DMI DMI HyperTransport HyperTransport
Peak interconnect bandwidth 2GB/s 2GB/s 2GB/s 8GB/s 8GB/s

DDR3-2000 support sounds impressive, and it is, but we should qualify it with a couple of limitations. First, the 790i SLI only supports memory speeds beyond 1800MHz with one module per channel, and then only in a motherboard's second and fourth DIMM slots. This "0101" memory configuration is necessary at high speeds because of how Nvidia's reference motherboard design for the Ultra configures slot addressing; it may not be a problem for custom board designs from other manufacturers.

The nForce 790i SLI marks Nvidia's first attempt at a DDR3 memory controller, so hitting 2000MHz is impressive regardless of the associated limitations. Nvidia boasts that the 790i can handle 2000MHz with memory voltages as low as 1.9V—only a modest boost over DDR3's default voltage of 1.5V. Even at more sedate speeds, the 790i's memory controller is tuned for performance. New prefetch algorithms within the memory controller's Dynamic Adaptive Speculative Preprocessor (DASP) are specifically optimized for the larger L2 caches found in Intel's new Penryn processors. Nvidia also claims the 790i's memory controller has the lowest latency data path for memory reads and writes.

As one might expect, Nvidia has brought its open Enhanced Performance Profile (EPP) spec to DDR3 memory. EPP 2.0 offers the same extensions to standard SPD settings as the original Enhanced Performance Profile spec, this time for DDR3 memory instead of DDR2.

Like so many Nvidia chipsets before it, the 790i SLI uses a HyperTransport interconnect to link its north and south bridge components. Although it's been used for years, this link offers a healthy 8GB/s of interconnect bandwidth—four times what's available with Intel's desktop chipsets. The extra bandwidth is really only necessary for three-way SLI implementations, so for most systems, it simply provides a massive pipe down to the south bridge.


It's at the south bridge that we find a familiar face in the form of the nForce 570 SLI MCP. Nvidia may call this an nForce 790i SLI Ultra MCP in its block diagrams, but the chip's markings clearly reveal its true identity. The 570 SLI first made an appearance back in 2006 as the south bridge component of the nForce 590 SLI chipset for Socket AM2. In the years since it's been passed around like a bong at a Grateful Dead concert, pulling duty in the nForce 570, 590, 590i, 680i, and 780i.

ICH9R nForce 790i SLI MCP nForce 780i SLI MCP
PCI Express 1.1 lanes 6 28 28
Serial ATA ports 6 6 6
Peak SATA data rate 300MB/s 300MB/s 300MB/s
AHCI Y N N
Native Command Queuing Y Y Y
RAID 0/1 Y Y Y
RAID 0+1/10 Y Y Y
RAID 5 Y Y Y
Matrix RAID Y N N
ATA channels 0 1 1
Max audio channels 8 8 8
Audio standard HDA HDA HDA
Ethernet 10/100/1000 2 x 10/100/1000 2 x 10/100/1000
USB ports 12 10 10

Even at almost two years after its first release, the nForce 790i 570 SLI MCP's feature set stands up reasonably well against the best of Intel's south bridge offerings. Nvidia's MCP chips have always packed more functionality than rival designs from Intel, and with 28 PCI Express 1.1 lanes and dual Gigabit Ethernet controllers with packet prioritization and teaming capabilities, the 570 SLI is no exception. Things are more even on the storage front, where the 570's six Serial ATA RAID ports neatly match those in the ICH9R.

The 570 begins to show its age in the USB department, offering a mere 10 ports to the ICH9R's 12. However, the Nvidia MCP's age ends up paying dividends in the form of an ATA controller that the ICH9R lacks. IDE ribbons may be rare these days, but they're not quite so uncommon that motherboards have banished "parallel" ATA altogether. With the nForce 790i SLI, mobo makers won't have to resort to third-party storage controllers for a fix of old-school ATA lovin'.