모두의 코드
FYL2X (Intel x86/64 assembly instruction)

작성일 : 2020-09-01 이 글은 792 번 읽혔습니다.

FYL2X

Compute y * log2x

참고 사항

아래 표를 해석하는 방법은 x86-64 명령어 레퍼런스 읽는 법 글을 참조하시기 바랍니다.

Opcode

Instruction

64-Bit
Mode

Compat/
Leg Mode

Description

D9 F1

FYL2X

Valid

Valid

Replace ST(1) with (ST(1) * log2ST(0)) and pop the register stack.

Description

Computes (ST(1) \esc{*} log2 (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.

Table 3-48. FYL2X Results

- $\infty$
- $\infty$ *

- F
*

$\pm$0
+ $\infty$

ST(0)
+0<+F<+1
+ $\infty$

+ 1
*

+ F > + 1
- $\infty$

+ $\infty$
- $\infty$

NaN
NaN

ST(1) - F *

*

**

+ F

- 0

- F

- $\infty$

NaN

- 0 *

*

*

+ 0

- 0

- 0

*

NaN

+ 0 *

*

*

- 0

+ 0

+ 0

*

NaN

+ F *

*

**

- F

+ 0

+ F

+ $\infty$

NaN

+ $\infty$ *

*

- $\infty$

- $\infty$

*

+ $\infty$

+ $\infty$

NaN

NaN NaN

NaN

NaN

NaN

NaN

NaN

NaN

NaN

NOTES:

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 $\pm$0, the instruction returns $\infty$ 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):

log x <- (log b)-1b 2 * log2x

This instruction's operation is the same in non-64-bit modes and 64-bit mode.

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 $\pm$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.

Protected Mode Exceptions

#NM

  • CR0.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 Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode.

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