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How can I change VEX instructions between 16, 32, and 64-bit modes?

Posted by Marvee Amasi
4:24 pm
04/02/2025

Does anyone understand how to work with `MASKMOVDQU`, cus I’m stuck by it’s instruction from the page 902 of the Intel x64 manual . I am confused on how to adjust the register sizes for this instruction. Here the documentation mentions that the default memory location is `DS:DI/EDI/RDI`, but the opcode includes `0x66`

How should I handle this `0x66`? Should I replace it with `0x67` or use the `REX.W` prefix for `EDI/RDI` registers to adjust for the different sizes?

So there is a VEX version of this instruction listed as: