When microprocessor manufacturers ran into increasing complications as they tried to make chips runs faster, they shifted gears, and started building more powerful processors using multiple cores. We're still at the early part of the multi-core revolution, but researchers at the University of Rochester claim that microprocessor manufacturers will eventually run into similar challenges with today's multi-core processor design because of the "limits of miniaturization." So what comes next?

The answer still lies with multi-core processors; but instead of just packing cores in next to each other on the same plane, the cores will be stacked on top of each other vertically. The concept of a three-dimensional chip is not a new one; in fact, IBM has been working on a "3-D chip stack" processor for a while. The 3-D chip designs of IBM and others essentially take modern chips and stacks them on top of each other--similar to today's multi-core design, but turned on its side. The Rochester team's design, however, differs from what IBM and other researchers are doing:

"Unlike past attempts at 3-D chips, the Rochester chip is not simply a number of regular processors stacked on top of one another. It was designed and built specifically to optimize all key processing functions vertically, through multiple layers of processors, the same way ordinary chips optimize functions horizontally. The design means tasks such as synchronicity, power distribution, and long-distance signaling are all fully functioning in three dimensions for the first time."

Credit: University of Rochester

One of the co-creators of this new processor design, Eby Friedman, Distinguished Professor of Electrical and Computer Engineering at Rochester, doesn't even call it a chip anymore--he calls it a "cube." Along with co-creator, Vasilis Pavlidis, a PhD candidate in engineering at Rochester, the pair has created a prototype design, which runs at 1.4GHz.

"What makes it all possible is the architecture Friedman and his students designed, which uses many of the tricks of regular processors, but also accounts for different impedances that might occur from chip to chip, different operating speeds, and different power requirements. The fabrication of the chip is unique as well. Manufactured at MIT, the chip must have millions of holes drilled into the insulation that separates the layers in order to allow for the myriad vertical connections between transistors in different layers."

Vasilis and Friedman's "how to" book on 3D integrated circuit design. (Credit: Elsevier)

Friedman envisions that each layer could perform an entirely different function, while "all the layers interact like a single system... He says the chips inside something like an iPod could be compacted to a tenth their current size with ten times the speed."

Friedman's vision isn't that different from what today's microprocessor manufacturers say we should expect from tomorrow's horizontally oriented multi-core processors: As chips start to get many more processors, we're likely to see the different cores take on different, dedicated tasks. Tilera's 64-core TILE64 processor takes a similar approach, as does Intel's 80-core Teraflop prototype processor. These designs show that it is possible to integrate many--albeit specialized--cores onto a single processor. There will come a point of diminishing returns, however, when multi-core processors will either need to start going vertical or an entirely new processor architecture will take microprocessors in a whole new direction.