IBM PowerPC A2 (Power BQC)

Eighteen cores where only sixteen do user work: one runs the operating system and one is a spare, held in reserve so that a manufacturing defect in a single core does not scrap the chip. At Sequoia's scale, 98,304 of these, yield economics stop being the vendor's private problem and start determining whether the machine can be built at all. Each core ran four hardware threads to hide memory latency, and the network logic sat on the same die as the cores, so BlueGene/Q has no separate interconnect vendor anywhere in its bill of materials. That is why fabric-share charts that read only the interconnect field understate IBM's historical position.