Intel 253666-024US - Manual (Page 475)

Vol. 2A 3-429
INSTRUCTION SET REFERENCE, A-M
FXTRACT—Extract Exponent and Significand
FXTRACT—Extract Exponent and Significand
Description
Separates the source value in the ST(0) register into its exponent and significand,
stores the exponent in ST(0), and pushes the significand onto the register stack.
Following this operation, the new top-of-stack register ST(0) contains the value of
the original significand expressed as a floating-point value. The sign and significand
of this value are the same as those found in the source operand, and the exponent is
3FFFH (biased value for a true exponent of zero). The ST(1) register contains the
value of the original operand’s true (unbiased) exponent expressed as a floating-
point value. (The operation performed by this instruction is a superset of the IEEE-
recommended logb(x) function.)
This instruction and the F2XM1 instruction are useful for performing power and range
scaling operations. The FXTRACT instruction is also useful for converting numbers in
double extended-precision floating-point format to decimal representations (e.g., for
printing or displaying).
If the floating-point zero-divide exception (#Z) is masked and the source operand is
zero, an exponent value of –∞ is stored in register ST(1) and 0 with the sign of the
source operand is stored in register ST(0).
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
TEMP ← Significand(ST(0));
ST(0) ← Exponent(ST(0));
TOP← TOP − 1;
ST(0) ← TEMP;
FPU Flags Affected
C1 Set to 0 if stack underflow occurred; set to 1 if stack overflow
occurred.
C0, C2, C3 Undefined.
Floating-Point Exceptions
#IS Stack underflow or overflow occurred.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D9 F4 FXTRACT Valid Valid Separate value in ST(0) into exponent and
significand, store exponent in ST(0), and
push the significand onto the register
stack.