image/svg+xmlFYL2X—Compute y log2xDescriptionComputes (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.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):logbx := (log2b)–1 log2xThis instruction’s operation is the same in non-64-bit modes and 64-bit mode.OperationST(1) := ST(1) log2ST(0);PopRegisterStack;FPU Flags AffectedC1Set to 0 if stack underflow occurred.Set if result was rounded up; cleared otherwise.C0, C2, C3 Undefined.OpcodeInstruction64-Bit ModeCompat/Leg ModeDescriptionD9 F1FYL2XValidValidReplace ST(1) with (ST(1) log2ST(0)) and pop the register stack.Table 3-48. FYL2X ResultsST(0)F±0+0<+F<+1+ 1+ F > + 1+NaN**++* NaNST(1) F****+ F0FNaN 0***+ 000*NaN+ 0***0+ 0+ 0*NaN+ F****F+ 0+ F+NaN+***++NaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNOTES:FMeans finite floating-point value.*Indicates floating-point invalid-operation (#IA) exception.**Indicates floating-point zero-divide (#Z) exception.

image/svg+xmlFloating-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 ExceptionsSame exceptions as in protected mode.Virtual-8086 Mode ExceptionsSame exceptions as in protected mode.Compatibility Mode ExceptionsSame exceptions as in protected mode.64-Bit Mode ExceptionsSame exceptions as in protected mode.

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