F2XM1

Calculate 2x - 1

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
D9 F0 F2XM1 None Valid Valid Replace ST(0) with (2ST(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.

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:

x y = 2 (y * log 2 x)

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

Pseudo Code

ST(0) = (pow(2, ST(0)) - 1);

FPU Flags Affected

C1: Set to 0 if stack underflow occurred. Set if result was rounded up; cleared otherwise. C0, C2, C3 are undefined.

Exceptions

Floating-Point Exceptions

Exception Description
#P Value cannot be represented exactly in destination format.
#U Result is too small for destination format.
#D Source is a denormal value.
#IA Source operand is an SNaN value or unsupported format.
#IS Stack underflow occurred.

64-Bit Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1.