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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.
Glossary-5