x86 Instruction Set Reference

F2XM1

Compute 2x-1

Opcode	Mnemonic	Description
`D9 F0`	`F2XM1`	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) Source	ST(0) Destination
-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))

Operation
ST(0) = 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 Undefined.

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

Protected Mode Exceptions

#NM EM or TS in CR0 is set.

Real-Address Mode Exceptions

#NM EM or TS in CR0 is set.

Virtual-8086 Mode Exceptions

#NM EM or TS in CR0 is set.

Instruction	Latency	Throughput	Execution Unit
`CPUID`	0F3n/0F2n	0F3n/0F2n	0F2n
`F2XM1`	100-200/90-150	60	-