fyl2xp1
Compute y * log2(x + 1)
Computes ST(1) * log2(ST(0) + 1).
Pseudocode Operation
if (ST(0) <= -1 or ST(0) >= 1) {
undefined_behavior()
} else {
result ← ST(1) * log2(ST(0) + 1)
FPU_stack_pop()
ST(0) ← result
}Encoding
Operands
Related
More in x87 FPU
Reference
Instruction Forms
| Opcode | Instruction | Op/En | 64/32-bit Mode | CPUID | Description |
|---|---|---|---|---|---|
| D9 F9 | FYL2XP1 | Valid Valid | Replace ST(1) with ST(1) ∗ log2(ST(0) + 1.0) and pop the register stack. |
Description
Computes (ST(1) ∗ log2(ST(0) + 1.0)), stores the result in register ST(1), and pops the FPU register stack. The source operand in ST(0) must be in the range:
- ( 1 - 2 / 2 ))to ( 1 - 2 / 2 )
The source operand in ST(1) can range from -∞ to +∞. If the ST(0) operand is outside of its acceptable range, the result is undefined and software should not rely on an exception being generated. Under some circumstances exceptions may be generated when ST(0) is out of range, but this behavior is implementation specific and not guaranteed. The following table shows the results obtained when taking the log epsilon of various classes of numbers, assuming that underflow does not occur. Table 3-51. FYL2XP1 Results ST(0)
-(1 - ( 2 / 2 )) to -0 -0 +0 +0 to +(1 - ( 2 / 2 )) NaN - ∞ +∞ * * - ∞ NaN ST(1) - F +F +0 -0 - F NaN - 0 +0 +0 -0 - 0 NaN +0 - 0 - 0 +0 +0 NaN +F - F - 0 +0 +F NaN +∞ - ∞ * * +∞ NaN NaN NaN NaN NaN NaN NaN
Operation
ST(1) := ST(1) ∗ log2(ST(0) + 1.0); PopRegisterStack;