Than the largest allowable exponent for the selected

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Unformatted text preview: to a QNaN Two QNaNs x87 FPU -- QNaN source operand with the larger significand SSE/SSE2/SSE3 -- First operand SNaN and a floating-point value QNaN and a floating-point value SNaN (for instructions that take only one operand) QNaN (for instructions that take only one operand) SNaN source operand, converted into a QNaN QNaN source operand SNaN source operand, converted into a QNaN QNaN source operand NOTE: 1. For SSE/SSE2/SSE3 instructions, the first operand is generally a source operand that becomes the destination operand. Within the Result column, the x87 FPU notation also applies to the FISTTP instruction in SSE3; the SSE3 notation applies to the SIMD floating-point instructions. Using SNaNs and QNaNs in Applications Except for the rules given at the beginning of Section, "NaNs," for encoding SNaNs and QNaNs, software is free to use the bits in the significand of a NaN for any purpose. Both SNaNs and QNaNs can be encoded to carry and store data, such as diagnostic information. By unmasking the invalid operation exception, the programmer can use signaling NaNs to trap to the exception handler. The generality of this approach and the large number of NaN values that are available provide the sophisticated programmer with a tool that can be applied to a variety of special situations. For example, a compiler can use signaling NaNs as references to uninitialized (real) array elements. The compiler can preinitialize each array element with a signaling NaN whose significand contained the index (relative position) of the element. Then, if an application program attempts to access an element that it had not initialized, it can use the NaN placed there by the compiler. If the invalid operation exception is unmasked, an interrupt will occur, and the exception handler will be invoked. The exception handler can determine which element has been accessed, since the Vol. 1 4-21 DATA TYPES operand address field of the exception pointer will point to the NaN, and the NaN will contain the index number of the array element. Quiet NaNs are often used to speed up debugging. In its early testing phase, a program often contains multiple errors. An exception handler can be written to save diagnostic information in memory whenever it was invoked. After storing the diagnostic data, it can supply a quiet NaN as the result of the erroneous instruction, and that NaN can point to its associated diagnostic area in memory. The program will then continue, creating a different NaN for each error. When the program ends, the NaN results can be used to access the diagnostic data saved at the time the errors occurred. Many errors can thus be diagnosed and corrected in one test run. In embedded applications that use computed results in further computations, an undetected QNaN can invalidate all subsequent results. Such applications should therefore periodically check for QNaNs and provide a recovery mechanism to be used if a QNaN result is detected.
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