A.5.146 MASKMOVQ: Byte Mask Write
MASKMOVQ mm1,mm2 ; 0F F7 /r [KATMAI,MMX]
MASKMOVQ stores data from mm1 to the location specified by ES:(E)DI.
The size of the store depends on the address-size attribute. The most
significant bit in each byte of the mask register mm2 is used to
selectively write the data (0 = no write, 1 = write) on a per-byte
basis.