Intel 80286, 80287 manual Implementing the Ieee P754 Standard

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IMPLEMENTING THE IEEE P754 STANDARD

In designing the EH287.LIB, it was felt that it would be a disadvantage to most users to increase the size of the Normalizing routine by the amount necessary to provide this expanded arithmetic. Because the TEMP_REAL exponent field is so much larger than the LONG_REAL exponent field, it is extremely unlikely that TEMP_REAL underflow will be encountered in most applications.

If meeting the Standard is a more important criterion for your application than the choice between Normalizing and warning modes, then you can select warning mode (Denormal operand exceptions masked), which fully meets the Standard.

If you do wish to implement the Normalization of denormal operands in TEMP_REAL format using extra exponent bits, the list below indicates some useful pointers about handling Denormal operand exceptions:

1.TEMP_REAL numbers are considered Denormal by the NPX whenever the Biased Exponent is o (minimum exponent). This is true even if the explicit integer bit of the significand is 1. Such numbers can occur as the result of Underflow.

2.The 80287 FLD instruction can cause a Denormal Operand error if a number is being loaded from memory. It will not cause this exception if the number is being loaded from elsewhere in the 80287 stack.

3.The 80287 FCOM and FTST instructions will cause a Denormal Operand exception for un normal operands as well as for denormal operands.

4.In cases where both the Denormal Operand and Invalid Operation exceptions occur, you will want to know which is signalled first. When a comparison instruction operates between a nonexistent stack element and a denormal number in 80286 memory, the D and I exceptions are issued simul- taneously In all other situations, a Denormal Operand exception takes precedence over a nonstack Invalid operation exception, while a stack Invalid Operation exception takes precedence over a Denormal Operand exception.

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