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;;
;; Here is a slim program that computes the sum of the digits
;; of a non-negative integer n into result
;;

    allocate-registers n result continue sp decision remainder
    allocate-registers zero one ten
    allocate-registers digit-sum base-case
    
    li zero 0
    li one 1
    li ten 10
    
    li sp 0
    
    li digit-sum digit-sum-label
    li base-case base-case-label

    read n
    li continue end-label
    
digit-sum-label:
    sge decision n ten
    jeqz decision base-case

    ;; save registers
    st n sp
    add sp sp one
    st continue sp
    add sp sp one
    
    ;; prepare for recursive call
    quo n n ten
    li continue after-recursive-call-label
    
    ;; make recursive call
    j digit-sum

after-recursive-call-label:
    ;; restore registers
    sub sp sp one
    ld continue sp
    sub sp sp one
    ld n sp
    
    ;; use recursive result to compute sum-of-digits into result
    rem remainder n ten
    add result result remainder
    
    ;; jump where you are supposed to jump
    j continue

base-case-label:
    add result n zero
    j continue

end-label:
    write result
    halt