;this code assumes that both DS and ES point to the correct segments.
cld
mov si,offset foo
mov di,offset bar
mov cx,4 ;length of the shorter of the two.

repz cmpsb
; this will continue until the strings are different or CX = 0,
; whichever occurs first. If, after this, the zero flag is set and CX=0,
; the strings were the same
ret

foo byte "test"
bar byte "test"