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Intel's $475M error: the silicon behind the Pentium division bug


In 1993, Intel released the high-performance Pentium processor, the start of the long-running Pentium line. The Pentium had many improvement...

In the article, Intel explained that the bug was in a component of the chip called a PLA (Programmable Logic Array) that acted as a lookup table for the division operation. The trick is that we only need 7 bits of the partial remainder for the table index, so we can use a different type of adder—a carry-lookahead adder—that calculates each carry in parallel using brute force logic. Colwell claims that the Pentium design originally used the same lookup table as the 486, but shortly before release, the engineers were pressured by management to shrink the circuitry to save die space.

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