MOVSXD

MOVSXD is an x86-64 instruction that moves a sign-extended 32-bit value into a 64-bit destination register. It sign-extends a 32-bit source operand to 64 bits, preserving the sign, and stores the result in a 64-bit destination register. This instruction is commonly used when widening a signed 32-bit value to a 64-bit register for subsequent 64-bit operations.

Syntax and operands: MOVSXD r64, r/m32. The destination must be a 64-bit general-purpose register, and the source is a 32-bit register or memory operand. In other words, it performs sign-extension from a 32-bit source to a 64-bit destination. In many assemblers the form is written as MOVSXD rax, edi, meaning the 32-bit value in EDI is sign-extended into RAX. The opcode encoding is typically 0F BF /r, where the modrm specifies the registers involved.

Usage notes: MOVSXD is available in 64-bit mode as the 64-bit form of the signed-extend move from a doubleword to a quadword. It is distinct from MOVSX, which sign-extends from 8 or 16 bits to 16 or 32 bits, and from MOVZX, which zero-extends instead of sign-extending. The operation preserves the sign bit of the 32-bit source: values in the range 0x00000000 to 0x7FFFFFFF sign-extend to 0x0000000000000000–0x000000007FFFFFFFFF, while values in 0x80000000 to 0xFFFFFFFF sign-extend to 0xFFFFFFFF80000000–0xFFFFFFFFFFFFFFFF.

See also: MOVSX, MOVZX, sign-extension concepts in x86-64 assembly.