f2xm1

Compute 2^x - 1

F2XM1

Computes (2^ST(0)) - 1.

Pseudocode Operation

if (ST(0) < -1 or ST(0) > 1) {
  undefined_behavior()
} else {
  ST(0) ← (2 ^ ST(0)) - 1
}

Encoding

Binary Layout

Format Legacy

Opcode D9 F0

Extension x87 FPU

Operands

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 F0	F2XM1		Valid Valid		Replace ST(0) with (2^ST(0) - 1).

Description

Computes the exponential value of 2 to the power of the source operand minus 1. The source operand is located in register ST(0) and the result is also stored in ST(0). The value of the source operand must lie in the range -1.0 to +1.0. If the source value is outside this range, the result is undefined. The following table shows the results obtained when computing the exponential value of various classes of numbers, assuming that neither overflow nor underflow occurs. Table 3-18. Results Obtained from F2XM1

ST(0) SRC                                                  ST(0) DEST
- 1.0 to -0                                           - 0.5 to - 0
- 0                                                 - 0
+ 0                                                 + 0
+ 0 to +1.0                                            + 0 to 1.0

Values other than 2 can be exponentiated using the following formula: xy := 2(y ∗ log2x)

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

Operation

ST(0) := (2ST(0) - 1);

Exceptions

Protected Mode Exceptions

#NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #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. F2XM1-Compute 2^x - 1 Vol. 2A 3-307

Compatibility Mode Exceptions