The Pentium chip wasn't verified

Sat, 19 Nov 1994 22:44:10 -0800

I'm passing on a message I saw today, illustrating the potential applications
of hardware verification.

From: (Cleve Moler)
Subject: MATLAB and the FDIV bug
Date: 15 Nov 1994 23:30:22 -0500
Organization: The MathWorks, Inc., Natick, MA 01760

I realize that many followers of this group are sick and
tired of reading about the Pentium floating point division
bug. But I've learned a great deal from the postings here
over the last few days. Thanks. I thought you might be
interested in the following summary I've just made for
MATLAB users and posted to comp.soft-sys.matlab.

-- Cleve Moler


Pentium Floating Point Division Bug

There has been a flurry of activity the last fews days on the
Internet news group,, that should interest MATLAB
users. A serious design flaw has been discovered in the floating
point unit on Intel's Pentium chip. Double precision divisions
involving operands with certain bit patterns can produce incorrect

The most dramatic example seen so far can be extracted from a
posting last night by Tim Coe of Vitesse Semiconductor. In MATLAB,
his example becomes

x = 4195835
y = 3145727
z = x - (x/y)*y

With exact computation, z would be zero. In fact, we get zero on
most machines, including those using Intel 286, 386 and 486 chips.
Even with roundoff error, z should not be much larger than eps*x,
which is about 9.3e-10. But, on the Pentium,

z = 256

The relative error, z/x, is about 2^(-14) or 6.1e-5. The computed
quotient, x/y, is accurate to only 14 bits.

An article in last week's edition of Electronic Engineering Times
credits Prof. Thomas Nicely, a mathematics professor at Lynchburg
College in Virginia, with the first public announcement of the
Pentium division bug. One of Nicely's examples involves

p = 824633702441

With exact computation

q = 1 - (1/p)*p

would be zero. With floating point computation, q should be on
the order of eps. On most machines, we find that

q = eps/2 = 2^(-53) ~= 1.11e-16

But on the Pentium

q = 2^(-28) ~= 3.72e-09

This is roughly single precision accuracy and is typical of the
most of the examples that had been posted before Coe's analysis.

The bit patterns of the operands involved in these examples
are very special. The denominator in Coe's example is

y = 3*2^20 - 1

Nicely's research involves a theorem about sums of reciprocals
of prime numbers. His example involves a prime of the form

p = 3*2^38 - 18391

We're not sure yet how many operands cause the Pentium's floating
point division to fail, or even what operands produce the largest
relative error. It is certainly true that failures are very rare.
But, as far as we are concerned, the real difficulty is having to
worry about this at all. There are so many other things than can
go wrong with computer hardware, and software, that, at least, we
ought to be able to rely on the basic arithmetic.

The bug is definitely in the Pentium chip. It occurs at all clock
rates. The bug does not affect other arithmetic operations, or the
built-in transcendental functions. Intel has recently made changes
to the on-chip Program Logic Array that fix the bug and is now
believed to be producing error free CPUs. It remains to be seen
how long it will take for these to reach users.

An unnamed Intel spokesman is quoted in the EE Times article as
saying "If customers are concerned, they can call and we'll replace
any of the parts that contain the bug." But, at the MathWorks,
we have our own friends and contacts at Intel and we're unable
to confirm this policy. We'll let you know when we hear anything
more definite. In the meantime, the phone number for Customer
Service at Intel is 800-628-8686.

-- Cleve Moler
Chairman and Chief Scientist
The MathWorks, Inc.


From: (Scott R. Eliason)
Subject: Consequences of Intel Pentium Chip Bug
Date: 18 Nov 1994 21:15:04 GMT
Organization: U. of Iowa - Weeg Computing Center

Just a point of information for any who may care. I do quite a bit
of numerical calculations and quantitative analyses as a social
scientist. I've written programs for statistical analysis
of categorical data and am a heavy user of other statistical
packages such as Gauss, SPSS, SAS, LIMDEP, and some others.
I currently have a 486 machine both at home and in the office,
as do some of my colleagues and students.

First, I won't go near a Pentium machine until this bug is
fixed and I hear it in the mass media from someone other than Intel.

Second, I'm going to advise those I know about this problem and
to steer clear of Pentium machines.

And finally, I won't believe the results from any analysis conducted
on a Pentium machine.

Scott R. Eliason (
Department of Sociology
The University of Iowa