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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.
FYL2X Results
| - | 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();
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| FPU flags affected |
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C1 Set to 0 if stack underflow occurred.
Set if result was rounded up; cleared otherwise.
C0, C2, C3 Undefined.
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| 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. |
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| Protected Mode Exceptions |
#NM | EM or TS in CR0 is set. |
|
| Real-Address Mode Exceptions |
#NM | EM or TS in CR0 is set. |
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| Virtual-8086 Mode Exceptions |
#NM | EM or TS in CR0 is set. |
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| Instruction |
Latency |
Throughput |
Execution Unit |
CPUID | 0F3n/0F2n | 0F3n/0F2n | 0F2n |
FYL2X | 100-250/140-190 | 85 | - |
|