;You need to have some bytes defined
mydata db 3,1,4,1,5,9,2,6,5 ;etc.

;But the data is something raw and you don't need the bytes separately
;i.e. finding averages or summing up can be done up to 16 bytes at a time
;You need to cast DWORD or QWORD everytime you need to access [mydata]
mov eax,dword[mydata+4]
movq qword[mydata+0],mm5

;This won't do much harm if you have few, but in longer code, this comes
;efficient:
access_data_dword rd 0 ;This is like putting a label, but with a cast
access_data_word rw 0
mydata db 1,2,3,4,5,6,7,8,9,10,11,12 ;etc.
    

label mydata_dq qword
label mydata_dd dword
mydata db 1,2,3,4,...
...
mov al,[mydata]
mov eax,[mydata_dd]
movq mm0,[mydata_dq]    

mydata_dq qword:
;and...
qword mydata_dq