x86 Instruction Set Reference

# FYL2X

# Compute y * log_2(x)

Opcode Mnemonic Description
D9 F1 FYL2X Replace ST(1) with (ST(1) * log_2(ST(0))) and pop the register stack.

# Description

Computes (ST(1) * log_2 (ST(0))), stores the result in resister 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): -inf ST(0): -F ST(0): +-0 ST(0): +0 < +F < +1 +1 ST(0): +F > +1 +inf NaN
ST(1): -inf * * +inf +inf * -inf -inf NaN
ST(1): -F * * ** +F -0 -F -inf NaN
ST(1): -0 * * * +0 -0 -0 * NaN
ST(1): +0 * * * -0 +0 +0 * NaN
ST(1): +F * * ** -F +0 +F +inf NaN
ST(1): +inf * * -inf -inf * +inf +inf NaN
ST(1): NaN NaN NaN NaN NaN NaN NaN NaN NaN
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 infinite 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):
[code]
log_b(x) = log_2(x) / log_2(b)
[/code]

# Operation

ST(1) = ST(1) * log_2(ST(0));
PopRegisterStack();
1
2
3

# 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.
#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.

# Protected Mode Exceptions
#NM EM or TS in CR0 is set.

# Real-Address Mode Exceptions
#NM EM or TS in CR0 is set.

# Virtual-8086 Mode Exceptions
#NM EM or TS in CR0 is set.
Instruction Latency Throughput Execution Unit
CPUID 0F3n/0F2n 0F3n/0F2n 0F2n
FYL2X 100-250/140-190 85 -

← FYL2XP1 →