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Essex Junction (VT) - A statement from a senior IBM engineer buried deep within a Q&A with veteran journalist Ed Sperling, published last week by Electronic News, casts a sharp ray of light on an otherwise undiscussed topic: defects in the course of processor production. Defects, IBM vice president of semiconductor and technology services Tom Reeves admitted, crop up in about one in ten processors - specifically, digital ASICs - that are fabricated, and weeding those defects out is part of the everyday work of producing chips. But with today's multicore chips, that defect number is compounded as core counts grow. As a result, Reeves told Sperling, as few as one Cell processor for every ten fabricated may be defect-free upon inspection.
With standard silicon germanium (SiGe) single-core processors, IBM can achieve yields of up to 95%, Reeves told Electronic News. But "with a chip like the Cell processor," he then remarked, "you're lucky to get 10 or 20 percent."
But Reeves went further, making a comment that is raising the eyebrows of many game console enthusiasts who had thought the sole purpose of multiple cores in the Cell processor for Sony's upcoming PlayStation 3 was to improve performance: He implied that because the Cell uses as many as eight identical synergistic processing elements (SPEs), but Sony only requires the use of seven, some production units could, in fact, get away with one core in eight being defective without any impact on the customer.
It gets better. Reeves stated outright that we're entering an era of redundant logic, which enables manufacturers to produce processing components that compensate for their own defects. With such systems in place, he said, yields could conceivably increase in a best-case scenario to 40% - still significantly lower than the 95% yields that IBM and others enjoyed during the single-core, "one-by-one" era. The picture that emerged from the Electronic News interview is one not of multiple, powerful processor units pounding out code in parallel, but instead a kind of "RAID array" for the CPU, where unit failure could be considered part of everyday life.
Is this guy serious? We asked Insight64 principal analyst Nathan Brookwood. "He is serious," he told TG Daily. "Yields always go down as chip size increases, so designers of large chips often use redundancy to increase yields. Memory chips have done this for years, as have the cache blocks on CPUs, but it's harder to design redundancy into logic circuits - unless you replicate the entire logic block, which is what Cell does. Sony needs to balance performance, cost, and availability, so it makes sense that they would sacrifice a core or two in order to get lower cost or more useful chips."
By far the biggest single application for the Cell processor, in terms of acquiring installed base, will be its introduction in Sony's PS3 this November. In its quarterly report last April, Sony told investors it intends to sell 6 million PS3s between November 2006 and March 2007. If this is indeed the case, borrowing Reeves' numbers, the IBM/Sony/Toshiba joint effort (STI) will need to fabricate at least 15 million Cell processors, and toss out 60% or more of those units after fabrication. But even then, it would appear to be a safe bet, based on Reeves' logic, that about half the number of processors that complete the full production cycle will have one SPE unit that's defective. Since PS3 will only use seven of the eight SPEs anyway, the user should not know the difference.
IBM's engineering division for Cell was contacted by TG Daily for comment yesterday, and has yet to return our inquiries.