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FSTSW

Stores the current floating-point status word into the specified destination operand.

The table below covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available in 64-bit mode, but it specifically accesses the x87 FPU status word. In x86-64, the x87 FPU is maintained for backward compatibility.

The destination operand MUST be a 16-bit register or memory location. Attempting to use a larger destination size will not affect the remaining bits of the destination register/memory beyond the first 2 bytes. The status word contains flags regarding floating-point exceptions (#I, #Z, #D, #O, #U, #P) and the current rounding control.