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AR# 479

How are Xilinx programs affected by the Pentium bug?

ソリューション




PROGRAM: PPR, APR, XDE, XDELAY

VERSION: All

PLATFORM: PC (spec. Pentiums)

LINK TO: PPR, APR, XDE, XDELAY, PC - Other

SHORT DESCRIPTION: Pentium FPU bug, how will it affect Xilinx?

LONG DESCRIPTION:

A recently discovered bug (c. Fall 1994) in the Intel Pentium's
floating-point unit can cause round-off errors on calculations with as
little as four digits of precision to the right of the decimal point.
Several Xilinx software products do perform a significant amount of
floating-point math. The most important of these is PPR, but APR, XDE
and XDelay also rely on floating-point math.

What is the potential impact of the Intel FPU flaw to the customer?

For the most part we would expect that PPR or APR might get a slightly
different placement and/or routing result when run with the same seed on
machines with and without the flawed Pentium. However, the differences
should not affect the quality of the results. They will instead reflect
very slight changes in the decision-making done by the programs in
question. In most cases it will matter little which way the decision
goes in terms of placement or routing quality. The problem simply is
that the user will get a different, but equivalent, result.

For XDE or XDelay the problem could be a very slight difference in delay
calculation when run on machines with and without the flawed Pentium.
Since the latest information is that the flaw seems only to affect
accuracy to the fourth decimal place at worst and since we report delays
only in one-tenth- nanosecond increments there should be no difference
between the XDE or XDelay reports for most designs. However, there
could be slight differences in some designs.

Again, Xilinx software in scattered instances uses but does not absolutely
rely upon the operations which are flawed in the Pentium. However, if
you are still bothered by this potential problem, you can at least run
the non-floating-point version of PPR in \XACT\EMUL\PPR.EXE. (Note that
\XACT\PPR.EXE is just a springboard to the FPU or non-FPU version of PPR.)
Instead of copying this version into the main XACT directory, it is
recommended that you put \XACT\EMUL in your executable path before \XACT.


AR# 479
作成日 08/31/2007
最終更新日 03/31/1997
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