movzxmovsx

MOVZX and MOVSX are instructions in the x86 assembly language used for data movement and type conversion between registers and memory. These instructions are particularly useful when dealing with data of different sizes, such as moving a byte into a word or a word into a doubleword, while preserving the sign or zero-extending the value.

MOVZX (Move with Zero-Extend) is used to move data from a smaller source operand to a larger destination operand, filling the upper bits of the destination with zeros. This is useful when the source operand is unsigned. For example, MOVZX AX, BL moves the byte in BL to the lower 8 bits of AX and fills the upper 8 bits of AX with zeros.

MOVSX (Move with Sign-Extend) is similar to MOVZX but is used when the source operand is signed. It moves the data from a smaller source operand to a larger destination operand and fills the upper bits of the destination with the sign bit of the source operand. This preserves the sign of the value. For example, MOVSX AX, BL moves the byte in BL to the lower 8 bits of AX and fills the upper 8 bits of AX with the sign bit of BL.

Both instructions are useful for ensuring that the data is correctly interpreted when moving between different data sizes. They are commonly used in low-level programming, such as in operating system kernels, device drivers, and other performance-critical applications.