We stated that most of the work was on the software side, but there is one new hardware feature required for Optimus, which NVIDIA calls the Optimus Copy Engine. In theory you could do everything Optimus does without the copy engine, but in that case the 3D Engine would be responsible for getting the data over the PCI-E bus and into the IGP frame buffer. The problem with that approach is that the 3D engine would have to delay work on graphics rendering while it copied the frame buffer, resulting in reduced performance (it would take hundreds of GPU cycles to copy a frame). To eliminate this problem, NVIDIA added a copy engine that works asynchronously from frame rendering, and to do that they had to separate the rendering effort from the rest of the graphics engine. With those hardware changes complete, the rest is relatively straightforward. Graphics rendering is already buffered, so the Copy Engine simply transfers a finished frame over the PCI-E bus while the 3D Engine continues working on the next frame.
If you're worried about bandwidth, consider this: In a worst-case situation where sixty 2560x1600 32-bit frames are sent at 60FPS (the typical LCD refresh rate), the copying only requires 983MB/s. An x16 PCI-E 2.0 link is capable of transferring 8GB/s, so there's still plenty of bandwidth left. A more realistic resolution of 1920x1080 (1080p) reduces the bandwidth requirement to 498MB/s. Remember that PCI-E is bidirectional as well, so there's still 8GB/s of bandwidth from the system to the GPU; the bandwidth from GPU to system isn't used nearly much. There may be a slight performance hit relative to native rendering, but it should be less than 5% and the cost and routing benefits far outweigh such concerns. NVIDIA states that the copying of a frame takes roughly 20% of the frame display time, adding around 3ms of latency.
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Problems? See this thread at archive.org.
