User's Manual

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OVERVIEW OF NUMERIC PROCESSING
Automatic Exception Handling
As described in the previous section, when the 80287 NPX encounters an exception condition whose
corresponding mask bit in the NPX control word is set, the NPX automatically performs an internal
fix-up (masked-exception) response. The
80287 NPX has a default fix-up activity for every possible
exception condition it may encounter
.. These masked~exception responses are designed to be safe and
are generally acceptable for most numeric applications.
As an example of how even severe exceptions can be handled safely and automatically using the NPX's
default exception responses, consider a calculation of the parallel resistance of several values using
only the standard formula (figure I-II). If Rl becomes zero, the circuit resistance becomes zero. With
the divide-by-zero and precision exceptions masked, the
80287 NPX will produce the correct result.
By masking or unmasking specific numeric exceptions in the NPX control word, NPX programmers
can delegate responsibility for most exceptions to the NPX, reserving the most severe exceptions for
programmed exception handlers. Exception-handling software is often difficult to write, and the NPX's
masked responses have been tailored to deliver the most reasonable result for each condition. For the
majority of applications, programmers will find that masking all exceptions other than Invalid Opera-
tion will yield satisfaotory results with the least programming effort. An Invalid Operation exception
normally indicates
afatal error in a program that must b~ corrected; this exception should not normally
be.masked.
The exception flags in the
NPX status word provide a cumulative record of exceptions that have occurred
since these flags were last cleared.
Once set, these flags can be cleared only by executing the FCLEX
(clear exceptions) instruction, by reinitializing the NPX, or
by overwriting the flags with an FRSTOR
or· FLDENV instruction. This allows a programmer to mask all exceptions (except invalid operation),
run a calculation, and then inspect the status word to see
if any exceptions were detected at any point
in the calculation.
.
R3
EQUIVALENT RESISTANCE -
1
G30108
Figure 1-11. Arithmetic Example Using Infinity
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