4/4/97
It seems that the ghost of the FDIV bug lives on in Excel spreadsheets created using Pentiums affected by the problem.
I finally got rid of my Intel Pentium computer with the FDIV bug. Last night, while checking out my new Pentium Pro computer with Microsoft Excel 97 I decided to open up the spreadsheet which demonstrates the Pentium Floating Point divide bug. I was surprised to find that that the calculation of:
8391670 - (8391670 / 3145727) * 3145727
within the spreadsheet still showed the answer 512 instead of zero.
Click here to download a copy of the spreadsheet as calculated on a Pentium with the FDIV bug.
I pressed the recalculate key (F9) to no avail. So then, I retyped the formulas in neighboring cells. Where I had retyped the formulas the answer(s) were correct, but the same formulas that had been saved from when I had done the calculation on the defective Pentium still showed the wrong answer! In other words, here, on the same spreadsheet was the same problem with two different answers, one correct and one incorrect.
The only way I could find to "correct" the incorrect answers was to retype the formulas over the originals or to copy the cells and re-paste them into the spreadsheet.
It seems to me that this "ghost" could represent some risk since it is logical to assume that if you're no longer using a defective Pentium, you needn't be concerned about wrong answers on the spreadsheets you've moved to a new machine. This obviously is not so.
I've sent in a bug report to Microsoft as of 4/4/97.
As several people have pointed out, this behavior is explained by the fact that the spreadsheet program optimizes recalculations. If a cell hasn't changed, then the formulas that reference that cell shouldn't need to be recalculated. Obviously, this logic breaks down when an error appears as the result of a processor malfunction. What's particularly disturbing is that hitting the key that's supposed to force a recalculate still doesn't correct the errant result.
The bulk of this material first appeared in the RISKS digest.