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Libraries and Floating Point Support Guide

Preface The ARM C and C++ Libraries The ARM C Micro-library Floating-point Support About floating-point support The software floating-point library, fplib Calling fplib routines fplib arithmetic on numbers in a particular format fplib conversions between floats, long longs, doub fplib comparisons between floats and doubles fplib C99 functions Controlling the ARM floating-point environment Floating-point functions for compatibility with Mi C99-compatible functions for controlling the ARM f C99 rounding mode and floating-point exception mac Exception flag handling Functions for handling rounding modes Functions for saving and restoring the whole float Functions for temporarily disabling exceptions ARM floating-point compiler extensions to the C99 Writing a custom exception trap handler Example of a custom exception handler Exception trap handling by signals mathlib double and single-precision floating-point IEEE 754 arithmetic Basic data types for IEEE 754 arithmetic Single precision data type for IEEE 754 arithmetic Double precision data type for IEEE 754 arithmetic Sample single precision floating-point values for Sample double precision floating-point values for IEEE 754 arithmetic and rounding Exceptions arising from IEEE 754 floating-point ar Exception types recognized by the ARM floating-poi Using the Vector Floating-Point (VFP) support libr The C and C++ Library Functions reference Floating-point Support Functions Reference

Exceptions arising from IEEE 754 floating-point arithmetic

3.5.7 Exceptions arising from IEEE 754 floating-point arithmetic

Floating-point arithmetic operations can run into various problems. These are known as exceptions, because they indicate unusual or exceptional situations.

For example, the result computed might be either too big or too small to fit into the format, or there might be no way to calculate the result (as in trying to take the square root of a negative number, or trying to divide zero by zero).
The ARM® floating-point environment can handle an exception by inventing a plausible result for the operation and returning that result, or by trapping the exception.
For example, the square root of a negative number can produce a NaN, and trying to compute a value too big to fit in the format can produce infinity. If an exception occurs and is ignored, a flag is set in the floating-point status word to tell you that something went wrong at some time in the past.
When an exception occurs, a piece of code called a trap handler is run. The system provides a default trap handler that prints an error message and terminates the application. However, you can supply your own trap handlers to clean up the exceptional condition in whatever way you choose. Trap handlers can even supply a result to be returned from the operation.
For example, if you had an algorithm where it was convenient to assume that 0 divided by 0 was 1, you could supply a custom trap handler for the Invalid Operation exception to identify that particular case and substitute the answer you required.
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