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Source code

FYL2X

Compute y log2x

stableVMJITAOTinstruction

Encodings

OpcodeInstructionOp/En64-bitCompat/LegacyDescription
D9 F1FYL2XValidValidReplace ST(1) with (ST(1) log2ST(0)) and pop the register stack.

Measured cost

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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.

FYL2X Results

ST(1)-F****+F-0-F-NaN
-0***+0-0-0*NaN
+0***-0+0+0*NaN
+F****-F+0+F+NaN
+**--*++NaN

Operation

ST(1) := ST(1)  log2ST(0);
PopRegisterStack;

FPU Flags Affected

C1                        Set to 0 if stack underflow occurred.

                          Set if result was rounded up; cleared otherwise.

C0, C2, C3                Undefined.

Floating-Point Exceptions

#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.

Exceptions

Protected mode
#NMCR0.EM[bit 2] or CR0.TS[bit 3] = 1.
#MFIf there is a pending x87 FPU exception.
#UDIf the LOCK prefix is used.
Real address mode
Same exceptions as in protected mode.
Virtual-8086 mode
Same exceptions as in protected mode.
Compatibility mode
Same exceptions as in protected mode.
64-bit mode
Same exceptions as in protected mode.

Sources