User's Manual

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.
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