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. |
Computes (ST(1) ∗ log_{2} (ST(0))), stores the result in register 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.
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.
If the divide-by-zero exception is masked and register ST(0) contains ±0, the instruction 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 calculation 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.
ST(1) := ST(1) ∗ log_{2}ST(0); PopRegisterStack;
C1 |
Set to 0 if stack underflow occurred. Set if result was rounded up; cleared otherwise. |
C0, C2, C3 | Undefined. |
#IS | Stack underflow occurred. |
#IA |
Either operand is an SNaN or unsupported format. Source operand in register ST(0) is a negative finite value (not -0). |
#Z
Source operand in register ST(0) is ±0.
#D
Source operand is a denormal value.
#U
Result is too small for destination format.
#O
Result is too large for destination format.
#P
Value cannot be represented exactly in destination format.
#NM | CR0.EM[bit 2] or CR0.TS[bit 3] = 1. |
#MF | If there is a pending x87 FPU exception. |
#UD | If the LOCK prefix is used. |
Same exceptions as in protected mode.
Same exceptions as in protected mode.
Same exceptions as in protected mode.
Same exceptions as in protected mode.