sicp-ex-5.21

  
  
 (a) 
 > (define count-leaves-machine 
   (make-machine 
    (list (list '+ +) (list 'null? null?) 
          (list 'pair? pair?) (list 'car car) (list 'cdr cdr)) 
   '( 
    (assign continue (label count-leaves-done)) 
    (assign val (const 0)) 
   tree-loop 
    (test (op null?) (reg tree)) 
    (branch (label null-tree)) 
    (test (op pair?) (reg tree)) 
    (branch (label left-tree)) 
    (assign val (const 1)) 
    (goto (reg continue)) 
   left-tree 
    (save tree) 
    (save continue) 
    (assign continue (label right-tree)) 
    (assign tree (op car) (reg tree)) 
    (goto (label tree-loop)) 
   right-tree 
    (restore continue) 
    (restore tree) 
    (save continue) 
    (save val) 
    (assign continue (label after-tree)) 
    (assign tree (op cdr) (reg tree)) 
    (goto (label tree-loop)) 
   after-tree 
    (assign var (reg val)) 
    (restore val) 
    (restore continue) 
    (assign val (op +) (reg var) (reg val)) 
    (goto (reg continue)) 
   null-tree 
    (assign val (const 0)) 
    (goto (reg continue)) 
    count-leaves-done))) 
  
 (set-register-contents! count-leaves-machine 'tree '(a (b c (d)) (e f) g)) 
 (start count-leaves-machine) 
 (get-register-contents count-leaves-machine 'val) 
 'done 
 'done 
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