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MOVSX

Sign-extends a smaller source operand to a larger destination operand. The most significant bit of the source is replicated to fill the remaining high-order bits of the destination.

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

DO NOT support LOCK

The destination operand MUST be larger than the source operand. In x86-64 mode, the destination MUST be a qword (r64 or m64). The instruction is supported in 64-bit mode and compatibility mode.

To avoid undefined behavior or incorrect results, ensure the source operand is treated as a signed integer (iN), as the instruction performs sign-extension rather than zero-extension. If zero-extension is required, the MOZX instruction SHOULD be used instead.