fyl2x
Y * log2(X)
FYL2X
Computes ST(1) * log2(ST(0)).
Details
Computes ST(1) * log₂(ST(0)) and replaces both stack entries with the result in ST(0); the stack pointer is decremented. ST(0) must be strictly positive; the result is typically used for computing logarithmic and exponential functions. The x87 status flags are updated; C2 may be set for out-of-range arguments.
Pseudocode Operation
ST(0) ← ST(1) * log2(ST(0)); pop(); TOS ← TOS - 1Example
FYL2X
Encoding
Binary Layout
D9
+0
F1
+1
Operands
Reference (Intel® SDM)
Instruction Forms
| Opcode | Instruction | Op/En | 64/32-bit Mode | CPUID | Description |
|---|---|---|---|---|---|
| D9 F1 | FYL2X | Valid Valid | Replace ST(1) with (ST(1) ∗ log2ST(0)) and pop the register stack. |
Description
Computes (ST(1) ∗ log2 (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.
Table 3-50. 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
Operation
ST(1) := ST(1) ∗ log2ST(0); PopRegisterStack;
Exceptions
Protected Mode Exceptions
#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.
Real-Address Mode Exceptions
Same exceptions as in protected mode.
Virtual-8086 Mode Exceptions
Same exceptions as in protected mode.
Compatibility Mode Exceptions
Same exceptions as in protected mode.
64-Bit Mode Exceptions
Same exceptions as in protected mode.
FYL2X—Compute y * log2x Vol. 2A 3-423