snoopybbt

    org $400

arr:     dc.w     1, 2, 12, 4, 21
n:     dc.w     5
sum:     dc.w     0


    org $1800

START:
    clr.l     d0        ; i = 0, we use the .l since i takes part in .l calculation
    clr.l     d3        ; sum = 0 (temporarily sum lives in d3)
    move.w    n, d1     ; d1 = n

loop:   cmp.w     d1, d0
    bge       endloop   ; if (i >= n) goto endloop

    movea.l   #arr, a0  ; a0 = &arr[0]
    adda.l    d0, a0
    adda.l    d0, a0    ; a0 = &arr[0] + 2 x i
    add.w     (a0), d3  ; d3 = d3 + arr[i]

    addq.w    #1, d0    ; i = i + 1
    bra       loop      ; repeat loop

endloop:    move.w    d3, sum   ; write the sum into memory

    END START