Vol. 2A 3-431
INSTRUCTION SET REFERENCE, A-M
FYL2X—Compute y * log2x
FYL2X—Compute y ∗ log
2
x
Description
Computes (ST(1) ∗ log
2
(ST(0))), stores the result in resister ST(1), and pops the
FPU register stack. The source operand in ST(0) must be a non-zero positive number.
The following table shows the results obtained when taking the log of various classes
of numbers, assuming that neither overflow nor underflow occurs.
If the divide-by-zero exception is masked and register ST(0) contains ±0, the instruc-
tion returns ∞ with a sign that is the opposite of the sign of the source operand in
register ST(1).
The FYL2X instruction is designed with a built-in multiplication to optimize the calcu-
lation of logarithms with an arbitrary positive base (b):
log
b
x ← (log
2
b)
–1
∗ log
2
x
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D9 F1 FYL2X Valid Valid Replace ST(1) with (ST(1) ∗ log
2
ST(0))
and pop the register stack.
Table 3-53. FYL2X Results
ST(0)
−∞ −F ±0 +0 < +F < +1 +1 +F > +1+∞ NaN
−∞ **+
∞ +∞ * −∞ −∞ NaN
ST(1)
−F**** +F −0 −F −∞ NaN
−0*** +0 −0 −0*NaN
+0*** −0 +0 +0*NaN
+F**** −F +0 +F+
∞ NaN
+
∞ **−∞ −∞ *+∞ +∞ NaN
NaN NaN NaN NaN NaN NaN NaN NaN NaN
NOTES:
F Means finite floating-point value.
* Indicates floating-point invalid-operation (#IA) exception.
** Indicates floating-point zero-divide (#Z) exception.
