Pentium FDIV bug

On October 30, 1994, Professor Thomas Nicely of 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.

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, if you could prove that you were affected, Intel would replace your processor. Finally, public outcry forced Intel to agree to replace all flawed Pentium processors, at huge cost to the company.

Note that this problem occured only on some models of the original Pentium processor. Any Pentium family processor with a clock speed greater than 100 MHz is new enough not to have this bug.

Parts of this article were summarized from information available at http://www.mathworks.com/company/pentium/index.shtml.