User's Manual

GLOSSARY OF 80287 AND FLOATING-POINT TERMINOLOGY
Trapping NaN: a NaN that causes an I error whenever it enters into a calculation or comparison, even
a nonordered comparison.
Two's Complement: a method of representing Integers.
If the uppermost bit is 0, the number is consid-
ered positive, with the value given by the rest of the bits. If the uppermost bit is 1, the number is
negative, with the value obtained by subtracting (2
b
;t count) from all the given bits. For example, the
8-bit number
11111100 is -4, obtained by subtracting 2' from 252.
Unbiased Exponent: the true value that tells
how far and in which direction to move the Binary Point
oLthe Significand of a Floating-Point Number. For example, if a Short Real Exponent is 131, we
subtract the Bias 127 to obtain the Unbiased Exponent +4. Thus, the Real number being represented
is the Significand with the Binary Point shifted 4 bits to the right.
Underflow: an error condition in which the correct answer
is nonzero, but has a magnitude too small
to be represented
as a Normal number in the destination Floating-Point format. The Standard specifies
that an attempt be made to represent the number as a Denormal.
Unmasked: a term
that applies to each of the six 80287 Exceptions: I,D,Z,O,U,P. An exception is
Unmasked if a corresponding bit in the 80287 Control Word is set to O. If an exception is Unmasked,
the
80287 will generate an interrupt when the error condition occurs. You can provide an interrupt
routine
that customizes your error recovery.
Un normal: a Temporary Real representation in which the explicit Integer bit of the Significand is
zero, and the exponent is nonzero. We consider Unnormal numbers distinct from Denormal numbers.
Word Integer: an Integer format supported by both the
80286 and the 80287 that consists of a 16-bit
Two's Complement quantity.
Zero divide: an error condition in which the inputs are finite, but the correct answer, even with an
unlimited exponent, has infinite magnitude.
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