Intel 80286, 80287 manual Arithmetic Example Using Infinity

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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. .

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EQUIVALENT RESISTANCE -

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Figure 1-11. Arithmetic Example Using Infinity

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