As many as 12 per cent of computer memory chips have to be thrown away after manufacture because they are defective. But researchers at Hewlett-Packard Laboratories and the University of California at Los Angeles have now found a use for these otherwise useless chips - a new defect-tolerant computer called the Teramac. The Teramac works by applying a software 'patch' around the defective chips (Science 280 1717).
Researchers are investigating new ways to make computer chips in an effort to increase processing speed. It is likely that these new methods, such as chemical fabrication and assembly, will produce large numbers of defective chips. Rather than throw these chips away, James Heath of UCLA and colleagues at Hewlett-Packard are investigating how to build a computer that will work even if some of the chips inside are faulty. To test their ideas they built the Teramac using faulty silicon chips produced by existing fabrication techniques.
Instead of a traditional design with the whole computer on a single chip, Teramac has all the components for computation laid out in a lattice formation. Each component also has high speed connections with its nearest neighbours. The computer is programmed so that if one route or component is defective, another route or component is used. Calculations by the team indicate that 10 per cent of the 864 processor chips inside the Teramac – plus 3 per cent of its 220, 000 memory chips and 10 per cent of the signals – were defective. However, by building the computer first, and then detecting the faults and programming the system to bypass problems, the Teramac was significantly faster than a high-end workstation.
