NASM 2.05 based x86 Instruction Reference
[ch086]
A.5.66 F2XM1: Calculate 2**X-1 F2XM1 ; D9 F0 [8086,FPU] F2XM1 raises 2 to the power of ST0, subtracts one, and stores the result back into ST0. The initial contents of ST0 must be a number in the range -1.0 to +1.0.