Chapter 4. Floating-Point Unit (FPU)
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Operand Data Formats and Types
4.2.3.3 Infinities
Infinities can be positive or negative and represent real values that exceed the overflow
threshold. A result’s exponent greater than or equal to the maximum exponent value
indicates an overflow for a given data format and operation. This overflow description
ignores the effects of rounding and the user-selectable rounding models. For single- and
double-precision infinities, the fraction is a zero. See Figure 4-5.
Figure 4-5. Infinity Format
4.2.3.4 Not-A-Number
When created by the FPU, NANs represent the results of operations having no
mathematical interpretation, such as infinity divided by infinity. Operations using a NAN
operand as an input return a NAN result. User-created NANs can protect against
uninitialized variables and arrays or can represent user-defined data types. See Figure 4-6.
Figure 4-6. Not-a-Number Format
If an input operand to an operation is a NAN, the result is an FPU-created default NAN.
When the FPU creates a NAN, the NAN always contains the same bit pattern in the
mantissa: all mantissa bits are ones and the sign bit is zero. When the user creates a NAN,
any nonzero bit pattern can be stored in the mantissa and the sign bit.
4.2.3.5 Denormalized Numbers
Denormalized numbers represent real values near the underflow threshold. Denormalized
numbers can be positive or negative. For denormalized numbers in single- and
double-precision, the implied integer bit is a zero. See Figure 4-7.
Figure 4-7. Denormalized Number Format
Traditionally, the detection of underflow causes floating-point number systems to perform
a flush-to-zero. The IEEE-754 standard implements gradual underflow: the result mantissa
is shifted right (denormalized) while the result exponent is incremented until reaching the
Exponent = Maximum
Fraction = 0
Sign of Mantissa, 0 or 1
Exponent = Maximum
Fraction = Any nonzero bit pattern
Sign of Mantissa, 0 or 1
Exponent = 0
Fraction = Any nonzero bit pattern
Sign of Mantissa, 0 or 1
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