NASM 2.05 based x86 Instruction Reference[ch094]

A.5.74 FCOS: Cosine FCOS ; D9 FF [386,FPU] FCOS computes the cosine of ST0 (in radians), and stores the result in ST0. The absolute value of ST0 must be less than 2**63. See also FSINCOS (section A.5.100).