image/svg+xml FYL2X—Compute y ∗ log 2 x Description 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. 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. Operation ST(1) := ST(1) ∗ log 2 ST(0); PopRegisterStack; FPU Flags Affected C1Set to 0 if stack underflow occurred. Set if result was rounded up; cleared otherwise. C0, C2, C3 Undefined. OpcodeInstruction64-Bit Mode Compat/ Leg Mode Description D9 F1FYL2XValidValidReplace ST(1) with (ST(1) ∗ log 2 ST(0)) and pop the register stack. Table 3-48. 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 NaNNaNNaNNaNNaNNaNNaNNaNNaN NOTES: FMeans finite floating-point value. *Indicates floating-point invalid-operation (#IA) exception. **Indicates floating-point zero-divide (#Z) exception. image/svg+xml Floating-Point Exceptions #ISStack underflow occurred. #IAEither operand is an SNaN or unsupported format. Source operand in register ST(0) is a negative finite value (not - 0). #ZSource operand in register ST(0) is ± 0. #DSource operand is a denormal value. #UResult is too small for destination format. #OResult is too large for destination format. #PValue cannot be represented exactly in destination format. Protected Mode Exceptions #NMCR0.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. This UNOFFICIAL reference was generated from the official Intel® 64 and IA-32 Architectures Software Developer’s Manual by a dumb script. There is no guarantee that some parts aren't mangled or broken and is distributed WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE .