Silicon Scissors


Snippers can often be very useful, especially when you have a lot of loose ends hanging around. The photomicrograph above illustrates a set of silicon scissors placed on an Evans & Sutherland graphics chip by designer David Naegle and his coworkers Paul Israelsen and Steve Wald. Originally designed to compete with Silicon Graphics' geometry engine in the early 1980s, the integrated circuit was dubbed "The Clipper Chip".

We are told by designer David Naegle that the chip contains about 15,000 transistors and was intended to be used in groups of four to implement a "clipping FIFO" (First-In First-Out). At the time, most computer graphics CAD systems displayed only wireframe images. It was the job of the Clipper chip to clip portions of the wireframe geometry that extended beyond the boundaries of the screen so they would not "wrap around" to the opposite edge of the screen.

During layout of the chip, the designers tried to minimize the length of the carry chains in the various adder-accumulators around the chip in order to speed up the adders. This left a great deal of "open" area on the die that was used by the designers to place the scissors graphic that is illustrated above.

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