mov   r0,#Source
   mov   r1,#Destination     ;set up pointers
   clr   c                   ;clear carry for the first (LSB) addc 
Loop:
   mov   a,@r0               ;pick a byte from source 
   inc   r0
   addc  a,@r1               ;add the byte from destination
   mov   @r1,a               ;store back to destination
   inc   r1                  ;  and advance pointer
   cjne  r0,#Source+4,Loop   ;loop until end