Hardware historian Ken Shirriff has made a groundbreaking discovery by identifying the exact transistors responsible for Intel's infamous "FDIV bug" that triggered a $475 million recall in 1994. Using microscopic analysis, Shirriff examined the original Pentium processor's programmable logic array (PLA) to pinpoint the root cause of this historic computing flaw.
The bug resided in Intel's first P5 architecture CPU, manufactured using an 800nm process with 3.1 million transistors. Through detailed microscope photography, Shirriff was able to analyze the processor's die and locate the specific transistor grid containing the faulty division table.
The error stemmed from the Pentium's advanced floating-point unit, which utilized the SRT division algorithm to perform calculations at twice the speed of previous processors. This algorithm required a 2,048-cell lookup table arranged in 112 rows, with values ranging from -2 to 2 encoded by the presence or absence of transistors.
The investigation revealed that five entries in this table were missing their required transistors, defaulting to 0 instead of the correct value of 2. Surprisingly, Shirriff discovered an additional 11 missing data points that hadn't caused errors, which he described as "due to luck."
Intel eventually resolved the issue in subsequent Pentium revisions by filling all unused table entries with 2's, a simple yet effective solution that preserved die space. The discovery provides physical confirmation of what mathematicians had theoretically determined in 1995 by studying error patterns.
Shirriff plans to publish a detailed analysis of his investigation, including the possibility of physically modifying affected Pentium chips to correct the three-decade-old bug.
The finding offers unprecedented insight into one of computing history's most costly errors, demonstrating how far processor technology has advanced from the microscopically visible 3.1 million transistors of 1994 to today's chips containing tens of billions of transistors.
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