sum:
        addiu   $sp, $sp, -12
        sw $ra, 4($sp)
        sw $fp, 0($sp)
        move $fp, $sp

        move    $v0, $a0
        beq     $a0, $zero, exit
        nop
        subi    $a0, $a0, 1
        sw      $a0, 8($sp)
        jal     sum
        nop
        lw      $a0, 8($sp)
        nop
        add     $v0, $v0, $a0

exit:
        lw      $fp, 0($sp)
        lw      $ra, 4($sp)
        addiu   $sp, $sp, 12
        jr      $ra
        nop