Subject: Re: CCL:DEFECTIVE PENTIUM FPU'S

From: David Heisterberg <[email-masked]>
Date: Mon, 21 Nov 1994 16:48:41 -0500

>NOW WHAT? COMPUTE STATISTICAL PROBABILITIES OF ERRORS IN COMPUTED RESULTS?

Tim Coe at Vitesse Semiconductor has done just that.  He's posted a
very detailed description of the problem in the comp.arch.arithmetic
news group.  For the likelyhood of running into this bug he says

  that needs clearing is that this is an extended precision problem.  This
  bug hits between 50 and 2000 single precision dividend divisor pairs (out
  of a total of 64 trillion.)

His analysis shows that the probability of getting a relative error
greater than 1e-5 is 3e-12, of less than 1e-5, 6e-12.  As to when this
error occurs

  Examination of the above divide failures reveals that both the dividend
  and divisor are integers minus small deltas.  Also notable is the induced
  error is roughly delta^(2/3).  The integers in the divisors are actually

>INTEL HAS JUST TOLD ME ON THE PHONE THEY DO NOT CONSIDER THIS A PROBLEM. THEY
>SAY THEY HAVE KNOWN ABOUT IT SINCE LAST JUNE.

It's supposed to have been fixed, but there are no markings on the
chips to distinguish the good ones.  The test John McKelvey posted
will do the trick, though.
--
David J. Heisterberg ([email-masked])      Gee, it's so beautiful, I gotta
The Ohio Supercomputer Center           give somebody a sock in the jaw.
Columbus, Ohio                          -- Little Skippy (Percy Crosby)