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NVIDIA nForce 790i Ultra SLI
by Marc Prieur
Published on April 8, 2008

After a lukewarm nForce 780i which actually was more like the nForce 690, three months later Nvidia is back in the foreground with its nForce 790i. Amongst other things, innovations include native PCI Express 2.0 support as well as the integration of a DDR3 memory controller – a first for NVIDIA.
A new chip
While the nForce 780i was in some ways just an nForce 680i with an additional chip, the nForce 790i integrates a brand new SPP. This SPP which is NVIDIA’s official name for the northbridge now has native PCI-Express 2.0 support.

On the nForce 780i SLI, you may recall that PCI-E 2.0 was managed by an additional chip, the nForce 200, which was relayed to the SPP via an ’’overclocked’’ PCI-E 1.0. In practice, performances were not equivalent to those offered by the X38’s PCI-E bus. Now there is native PCI-E 2.0 support which, we remind you, is the new specification enabling to attain data speeds of up to 500 MB /s on each lane versus 250 MB /s for the first version. In addition, there are two PCI Express 2.0 x16 ports.

NVIDIA also underlines its advances in PCI-Express support on the SPP that are useful in multi-GPU configurations. First of all, there is GPU-to-GPU Direct Link, which allows cards to directly communicate between each other without having to go through the memory controller. Second, there is Broadcast which means the CPU sends information only once which is then repeated to all GPUs. We will just have to see if these improvements have a notable impact in the case of Quad SLI, for example, or SLI with the recently released 9800 GTX.

PCI-E 2.0 is not the only innovation of the nForce 790i and actually it is the first non-Intel chipset to have DDR3 support. Introduced almost a year ago, this new type of memory differs from DDR2 by voltage that was reduced to 1.5V (versus 1.8V in DDR2 and 2.5V in DDR) as well as with its prefetch. The latter, which went from 2n to 4n bits in the change from DDR to DDR-2, now increases to 8n bits. The internal organization of memory cells has therefore been modified to obtain a doubled speed without increasing their frequency to the detriment of that of buffer inputs / outputs and the external memory bus.

DDR2 au dessus, DDR3 en dessous

Besides the gain in terms of energy consumption, this type of memory thus adds a noticeable gain in terms of bandwidth. It is officially available in versions ranging from DDR3-800 to DDR3-1600 or the double of DDR2 which varies from DDR2-400 to DDR2-800 and manufacturers now go up to DDR3-2133. The only problem is the very high price. DDR3 on the nForce 790i is matched with EPP 2.0 (2.0 only being related to DDR3 support). You may recall, EPP is a type of evolved SPD which allows memory manufacturers to transmit more information via their intermediaries to the motherboard so that there can be better use of memory depending on the type of EPP profiles they integrate. For example, this can be a profile with the best timings or another with the best frequency.

NVIDIA launches two versions of the nForce 790i, the SLI and Ultra SLI. While the first is officially ‘’limited’’ to DDR3-1333, the second goes further with NVIDIA announcing DDR3-2000 support. This is made possible without overclocking the FSB due to the fact that NVIDIA chipsets offer a flexibility which enables setting the memory and FSB asynchronously (without limits to FSB:DRAM coefficient ratios). The Ultra version was also announced as being more ‘’overclockable’’ although no precise figures were given.

The southbridge remains the same and thus has two Gigabit Ethernet ports, which are possible to combine via DualNet technology as well as six SATA ports configurable in RAID 0, 1, 0+1 or 5. It also manages the last PCI Express x16 port which is therefore a 1.0 type.

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