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The latest version of this topic can be found at MMWORD.

Used for 64-bit multimedia operands with MMX and SSE (XMM) instructions.


MMWORD is a type. Prior to MMWORD being added to MASM, equivalent functionality could have been achieved with:

mov mm0, qword ptr [ebx]  

While both instructions work on 64-bit operands, QWORD is the type for 64-bit unsigned integers and MMWORD is the type for a 64-bit multimedia value.

MMWORD is intended to represent the same type as __m64.

movq     mm0, mmword ptr [ebx]