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PROGRAMMING NUMERIC APPLICATIONS
Table 2-4.
Condition
Code
Interpretation
after
FPREM
Condition Code
Interpretation
after
FPREM
C3 C2
C1
CO
X 1 X
X
Incomplete Reduction;
further iteration is required for complete
reduction.
Complete Reduction;
C1, C3, and
CO
contain the three least-
significant bits
of
quotient:
0 0 0
0
(Quotient) MOD 8 = 0
0 0 0
1 (Quotient) MOD 8 = 4
0 0
1 0
(Quotient) MOD 8 = 1
0 0 1 1 (Quotient) MOD 8 = 5
1
0 0
0
(Quotient) MOD 8 = 2
1
0 0 1
(Quotient)
MOD 8 = 6
1
0 1
0
(Quotient) MOD 8 = 3
1
0 1 1 (Quotient) MOD 8 = 7
FXTRACT
FXTRACT (extract exponent and significand) "decomposes" the number
in
the stack top into
two
numbers that represent the actual value of the operand's exponent and significand fields. The "exponent"
replaces the original operand
on
the stack and the "significand"
is
pushed onto the stack. Following
execution of FXTRACT,
ST
(the
new
stack top) contains the value of the original significand expressed
as a real number: its sign
is
the same as the operand's, its exponent
is
0 true (16,383 or 3FFFH biased),
and its significant
is
identical
to
the original operand's. ST(1) contains the value of the original operand's
true (unbiased) exponent expressed as a real number.
If
the original operand
is
zero, FXTRACT
produces zeros in
ST
and
ST(l)
and both are signed
as
the original operand.
To clarify the operation of FXTRACT, assume
ST
contains a number of
whose
true exponent
is
+4
(i.e., its exponent field contains 4003H). After executing FXTRACT,
ST(l)
will
contain the real number
+4.0;
its sign
will
be positive, its exponent field
will
contain
400lH
(+2
true) and its significand field
will contain
1.l00
...
00B. In other words, the value in ST(I)
will
be
1.0
X
22
=
4.
If
ST contains an
operand whose
true exponent
is
-7
(Le., its exponent field contains 3FF8H), then FXTRACT
will
return an "exponent" of
-7.0;
after the instruction executes, ST(1)'s sign and exponent fields
will
contain
COOIH
(negative sign, true exponent of 2), and its significand
will
be 1.l1100
...
00B.
In other
words, the value
in
ST(1)
will
be
-1.11
X
22
=
-7.0.
In both cases, following FXTRACT, ST's sign
and significand fields
will
be the same as the original operand's, and its exponent field
will
contain
3FFFH
(0
true).
FXTRACT
is
useful in conjunction with FBSTP for converting numbers in 80287 temporary real format
to decimal representations (e.g., for printing or displaying).
It
can also
be
useful for debugging, because
it allows the exponent and significant parts of a real number
to
be
examined separately.
2-9