;
; main routine: establish the stack, then read
; and write.
;
        lcw r1 stack        ; set up stack
        loa r3 r0           ; get variable

        lcw r2 sum          ; call sum
        cal r2 r2           ;

        sto r3 r0           ; write result,
        hlt                 ;    and halt

sum
        psh r2              ; save the return address on the stack
        bgt r3 r0 recurse   ; check base case
        add r3 r0 0         ; if n <= 0, result is 0
        brs done

recurse
        psh r3              ; save n on the stack
        sub r3 r3 1         ; r3 = n-1

        lcw r2 sum          ; make recursive call
        cal r2 r2           ; sum(n-1), answer should be in r3

        pop r2              ; get n into r2
        add r3 r3 r2        ; r3 = n + sum(n-1)
done
        pop r2              ; get the return address
        jmp r2              ; go back to where we were called from

;
; stack area: 50 words
;
        dat 100
stack