fsin

Fsin, commonly written FSIN, is an x87 floating-point unit (FPU) instruction that computes the sine of the value currently on the top of the x87 register stack, ST(0). The operation uses ST(0) as its input and replaces it with sin(ST(0)). The input is interpreted in radians, and the result is left in ST(0). If the input is NaN, the result is NaN and a floating-point exception may be raised depending on the processor state.

FSIN is part of the original 8087 instruction set and remains available on modern x86 processors that implement the x87 FPU. In contemporary software, FSIN is most commonly encountered in legacy code, hand-tuned assembly, or environments that explicitly rely on the x87 stack model. For typical high-level programming, sine calculations are performed by math libraries or by SIMD-based instructions (such as SSE/AVX), but FSIN can still be used when precise control of the FPU stack and instruction timing is required.

In practice, FSIN does not require explicit operand addressing beyond manipulating the FPU stack; programmers load a value onto ST(0) (for example, via an fld instruction) and then issue FSIN to obtain the sine of that value. The instruction’s behavior is well-defined for finite real inputs and aligns with the mathematical sine function within the precision limits of the x87 FPU.

See also: SIN, x87 FPU, fld, SSE/AVX-based trig routines.