nForce 780a Features/HybridPower (cont’d)

PWShort

One feature first launched in the nForce 780i SLI chipset, and now carried over to the nForce 780a SLI and 750a SLI chipsets is Posted-Write Shortcut (PWShort). PWShort provides direct GPU-to-GPU communication without having to rely on the link between the PCIe and memory controller. In previous chipsets, data sent from one GPU to another had to go through this link, but the PCIe controller inside the nForce 780a chipset can send the data directly to its destination. This feature reduces the amount of latency for traffic among GPUs and helps to reduce the amount of congestion on the Memory-to-PCIe controller link.

Broadcast

This is another feature introduced with nForce 780i SLI. In systems with multiple GPUs, the CPU often sends the same data to all the GPUs. Instead of having to replicate this same data to each GPU individually, one broadcast is sent out from the CPU to the front-side bus to the chipset, which replicates it in parallel to all the GPUs.

This feature reduces the amount of data that has to go out across the front-side bus and reduces the amount of latency for CPU-to-GPU commands.

More On HybridPower

The nForce 780a SLI, nForce 730a, and 750a SLI chipsets will be the first NVIDIA platforms to hit the market with HybridPower support. Other chipsets that support the technology include the GeForce 8100/8200/8300 and the nForce 720a chipset. Besides the right motherboard/chipset, you will also need the right GPU in order to take advantage of the technology. Right now the GeForce 9800 GTX and 9800 GX2 are the only graphics processors with support for HybridPower. NVIDIA will also be providing a list of certified power supplies on slizone.com.

In order for HybridPower to work, the monitor must be connected to the motherboard, not the GPU. This allows the system to use the IGP when the discrete graphics cards are powered down. When multiple displays are attached to the IGP and the discrete graphics card, the motherboard picks the primary display based on settings you select inside BIOS.

HybridPower Modes

HybridPower runs in one of three different modes: Save Power, Boost Performance, and Additional Displays.

All three modes function exactly how they sound. In “Save Power” mode, the motherboard shuts down the discrete GPU(s), using the IGP to handle all graphics tasks. By shutting down the discrete GPU(s), this allows the system to consume less power, as well as run cooler and quieter. This feature really comes in handy as you add more GPUs. With previous chipsets these cards ran powered up at all times, but under HybridPower’s Save Power mode, these cards are completely shut off.


In “Boost Performance” mode, all GPUs, both integrated and discrete, are powered on to increase performance. In this mode, all graphics data is passed from the discrete GPU frame buffer through the PCIe 2.0 interface to system memory, where it is read by the motherboard GPU and sent to the display. This can result in a performance hit, although in the words of NVIDIA, the performance hit is small, less than 5%. The exact amount varies based on the screen resolution, the application being used, and the frame rate delivered.

Users who find this performance hit unacceptable will have to disconnect the display from the motherboard GPU, hooking it up directly to the discrete graphics card instead.

Hybrid SLI supports up to two displays. If the user wishes to run more displays than this, the system will have to run in the “Additional Displays” HybridPower mode in order to run the additional monitors (since the motherboard’s IGP is limited to one digital output, you will also have to run in the additional displays mode if you wish to connect two DVI displays to the system as well).

When the Additional Displays mode is selected, Save Power and Boost Performance modes are disabled. In this mode, all GPUs (including the motherboard and all discrete GPUs) are turned on and work independent of each other in order to drive all displays on both the motherboard and the discrete GPUs.