Fast Forward: AMD's Odd Phenom

Soon after AMD announced its new triple-core Phenom processor, the jokes began. Some people think a multicore processor with an odd number of cores is...well, odd. Others ridiculed AMD for making triple-core chips by disabling one core on a defective quad-core die. Intel CEO Paul Otellini cracked, “We see a distinct advantage in having all the cores on our dies work.”
Actually, there’s nothing odd about an odd number of processors. For 30 years we’ve had PCs with only one processor, and that’s an odd number.

But seriously, there’s no technical reason a multicore chip can’t have any number of processor cores. Sometimes it’s easier for chip designers to lay out an even number of cores because the die will be nearly a true square, which fits more efficiently on circular silicon wafers. However, different layouts are possible and often desirable. Designers can surround an odd number of cores with caches, buses, and other logic to achieve a square die. Indeed, such layouts are becoming more common as microprocessors integrate more components. Remember that AMD’s processors have on-chip memory controllers and soon will integrate graphics cores, too.

The slap that AMD is merely converting defective quad-core Phenoms into triple-core Phenoms hits closer to home. Rumors abound that AMD’s production yields—especially for the new 45nm fabrication process—are terrible. These rumors are hard to verify because semiconductor manufacturers closely guard their yields as trade secrets.

Although poor yields are definitely bad, there’s nothing wrong with salvaging chips by disabling the defective logic and relabeling the product. In the 1990s, Intel sold 486 processors either with (486DX) or without (486SX) an integrated FPU. Early 486SX chips were found to be 486DX chips with disabled FPUs.

More recently, some massively parallel processors have the ability to route around defective cores and keep working. One example is a 430-core chip from PicoChip Designs that’s found in cellular and wireless-network base stations. Should PicoChip scrap an otherwise functional die because it has “only” 429 working processor cores? As PC processors integrate large numbers of cores, inevitably some cores will be defective. It will become commonplace to salvage those chips by disabling the bad cores. And that’s fine, as long as the chips are honestly labeled.

Tom Halfhill was formerly a senior editor for Byte magazine and is now an analyst for Microprocessor Report.

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