Pentium FDIV bug
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[edit] Initial bug report
On October 30, 1994, Professor Thomas Nicely who was then at Lynchburg College reported a bug in the Pentium floating point unit. He reported that certain division operations returned a value which was wrong by a very small amount.
This result was quickly verified by other people around the Internet, and became known as the Pentium FDIV bug (FDIV is the x86 assembly language instruction for floating point division). Other people found division problems where the result returned by the Pentium was off by up to 61 parts per million.
[edit] Response by Intel
This report stirred up a huge controversy. Intel at first denied that the problem existed. Later, Intel claimed that it was not serious and would not affect most users; however, people who could prove that they were affected would get their processor replaced by Intel. However, although most independent estimates found the bug to be of little importance and have negligible effect on most users, it has caused a great public outcry. Companies like IBM (whose 5x86C microprocessor competed at that time with the Intel Pentium line) joined the condemnation.
[edit] Recall
Finally, Intel was forced to offer to replace all flawed Pentium processors, at huge potential cost to the company (however, it turned out that only a small fraction of Pentium owners actually bothered to get their chips replaced). As is often the case when a large company gets involved with a public-relations nightmare, Intel's stock price actually rose the day they finally acknowledged what just about everybody else had already realized: that they had to offer a no-strings-attached recall.
[edit] Models
This problem occurred only on some models of the original Pentium processor. Any Pentium family processor with a clock speed of at least 120 MHz is new enough not to have this bug. Some of the defective chips were later turned into keyrings by Intel.
[edit] Humour
Pentium's FDIV bug is famously laughed at in the computer folklore, see Pentium bug jokes.
[edit] References
- Personal website of Dr. Nicely, who discovered the bug
- A page with precise information, also about the cause
- Ivars Peterson's Mathland on the bug
- A Tale of Two Numbers, by Cleve Moler of The MathWorks
- ZIP-file containing more details (See ZIP file format for details on the file)
- Intel's official site
