This content has not been validated; it was AI-generated following the x86-64 ISA Documentation generation process to maintain high quality. However, as no human has verified this data, errors may exist. If the accuracy of this data is critical, please consult the Intel SDM. The model used to generate this is gemma4:31b

FNSTCW

Stores the current x87 floating-point control word into the specified destination.

The following table describes the supported source and destination.

DO NOT support LOCK

This instruction is available in 64-bit mode, but it operates on the x87 FPU state. In x86-64, the x87 state is preserved across context switches, but the instruction is primarily used for compatibility with 32-bit x87 code.

The destination must be a 16-bit register or memory location. Attempting to store the control word into a destination larger than word size may result in undefined behavior or incorrect data alignment if not handled by the assembler. Since this instruction modifies memory or a general-purpose register, it does not trigger any floating-point exceptions (#I, #Z, #D, #O, #U, #P).