Pentium FDIV bug

"Pentium bug" redirects here. For the 1997 bug affecting Pentium processors, see Pentium F00F bug.

66 MHz Intel Pentium (sSpec=SX837) with the FDIV bug

The Pentium FDIV bug is a hardware bug affecting the floating-point unit (FPU) of the early Intel Pentium processors. Because of the bug, the processor would return incorrect binary floating point results when dividing certain pairs of high-precision numbers. The bug was discovered in 1994 by Thomas R. Nicely, a professor of mathematics at Lynchburg College.[1] Missing values in a lookup table used by the FPU's floating-point division algorithm led to calculations acquiring small errors. While these errors would in most use-cases only occur rarely and result in small deviations from the correct output values, in certain circumstances the errors can occur frequently and lead to more significant deviations.[2]

The severity of the FDIV bug is debated. Though rarely encountered by most users (Byte magazine estimated that 1 in 9 billion floating point divides with random parameters would produce inaccurate results),[3] both the flaw and Intel's initial handling of the matter were heavily criticized by the tech community.

In December 1994, Intel recalled the defective processors in what was the first full recall of a computer chip.[4] In its 1994 annual report, Intel said it incurred "a $475 million pre-tax charge ... to recover replacement and write-off of these microprocessors."[5]

  1. ^ Edelman, Alan (January 1, 1997). "The Mathematics of the Pentium Division Bug" (PDF). SIAM Review. 39 (1): 54–67. Bibcode:1997SIAMR..39...54E. doi:10.1137/S0036144595293959. Archived (PDF) from the original on August 14, 2024. Retrieved April 11, 2021.
  2. ^ "'A Discussion of and Fix for the Pentium FDIV Bug' from the Notebook Archive (2002)". notebookarchive.org. Wolfram Research, Inc. Retrieved April 11, 2021.
  3. ^ Tom R. Halfhill (March 1995). "An error in a lookup table created the infamous bug in Intel's latest processor". BYTE. No. March 1995. Archived from the original on February 9, 2006. Retrieved December 19, 2006.
  4. ^ Carlton, Jim; Yoder, Stephen K. (December 21, 1994). "Computers: Humble Pie: Intel to Replace its Pentium Chips". The Wall Street Journal (Eastern ed.). p. B1.
  5. ^ "1994 - Annual Report". Intel. June 20, 2020. Archived from the original on February 26, 2017. Retrieved June 20, 2020.