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FSTCW

Stores the FPU Control Word into the specified destination.

The table after the description covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available in 64-bit mode, but it operates on the x87 FPU state. In 64-bit mode, the x87 FPU is supported for compatibility.

The destination MUST be a 16-bit register or memory location. Using a larger destination size (e.g., dword) WILL NOT affect the remaining bytes of the destination, but only the lowest 16 bits are overwritten by the FPU Control Word.