fyl2x

Y * log2(X)

FYL2X

Computes ST(1) * log2(ST(0)).

Pseudocode Operation

ST(0) ← ST(1) * log2(ST(0)); pop(); TOS ← TOS - 1

Encoding

Binary Layout
D9
+0
F1
+1
 
Format Legacy
Opcode D9 F1
Extension x87 FPU

Operands

Related

More in x87 FPU

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
D9 F1 FYL2X Valid Valid Replace ST(1) with (ST(1) ∗ log2ST(0)) and pop the register stack.

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. Table 3-50. FYL2X Results ST(0)

- ∞       - F     ±0      +0<+F<+1       + 1     + F > + 1     + ∞       NaN
- ∞           *             *        + ∞         + ∞               *        - ∞       - ∞       NaN
ST(1)      - F            *             *            **          + F          - 0        - F       - ∞       NaN
- 0            *             *             *           + 0          - 0        - 0            *        NaN
+ 0            *             *             *           - 0          + 0        + 0            *        NaN
+ F            *             *            **          - F          + 0        + F       + ∞       NaN
+ ∞           *            *         - ∞         - ∞               *        + ∞       + ∞       NaN

NaN NaN NaN NaN NaN NaN NaN NaN NaN

Operation

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

Exceptions

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. FYL2X-Compute y * log2x Vol. 2A 3-423