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 ;; using a list to contain break points, every element in the list  
 ;; is a pair containing the label and line.  the code changed  
 ;; had been marked. 
 (define (make-new-machine) 
  (let ((pc (make-register 'pc)) 
            (flag (make-register 'flag)) 
            (stack (make-stack)) 
            (the-instruction-sequence '()) 
            (instruction-number 0) 
            (trace-on false) 
            (labels '())                                            ;; *** 
            (current-label '*unassigned*)             ;; *** 
            (current-line 0)                                    ;; *** 
            (breakpoint-line 0)                              ;; *** 
            (break-on true))                                   ;; **** 
   (let ((the-ops 
                  (list (list 'initialize-stack 
                                   (lambda () (stack 'initialize))) 
                        (list 'print-stack-statistics  
                                       (lambda () (stack 'print-statistics))))) 
           (list (list 'pc pc) (list 'flag flag)))) 
   (define (print-instruction-number) 
    (display (list "current instruction number is: " instruction-number)) 
    (set! instruction-number 0) 
    (define (allocate-register name) 
         (if (assoc name register-table) 
             (error "Multiply defined register: " name) 
                 (set! register-table 
                       (cons (list name (make-register name)) 
    (define (lookup-register name) 
         (let ((val (assoc name register-table))) 
          (if val 
              (cadr val) 
                   (allocate-register name) 
                   (lookup-register name))))) 
    (define (execute) 
         (let ((insts (get-contents pc))) 
          (if (null? insts) 
                  (begin                                                 ;; *** 
                   (if (and (not (null? (instruction-label (car insts)))) 
                                (assoc (instruction-label (car insts)) labels)) 
                            (set! current-label (instruction-label (car insts))) 
                            (set! breakpoint-line (cdr (assoc current-label labels))) 
                            (set! current-line 0))) 
                   (set! current-line (+ current-line 1)) 
                   (if (and (= current-line breakpoint-line) break-on) 
                            (set! break-on false) 
                        (display (list "breakpoint here" current-label current-line))) 
                            (if trace-on 
                                  (display (instruction-text (car insts))) 
                            (set! break-on true)                                    ;; *** 
                        ((instruction-execution-proc (car insts))) 
                        (set! instruction-number (+ instruction-number 1)) 
         (define (cancel-breakpoint label)                                 ;; *** 
          (define (delete-label acc-labels orig-labels) 
           (cond ((null? orig-labels) 
                          (error "the label is not in the machine -- CANCEL-REAKPOINT" label)) 
                 ((eq? (caar orig-labels) label) (append acc-labels (cdr orig-labels))) 
                     (else (delete-label (cons (car orig-labels) acc-labels) (cdr orig-labels))))) 
          (set! labels (delete-label '() labels))) 
         (define (dispatch message) 
          (cond ((eq? message 'start) 
                         (set-contents! pc the-instruction-sequence) 
                ((eq? message 'install-instruction-sequence) 
                         (lambda (seq) (set! the-instruction-sequence seq))) 
                    ((eq? message 'allocate-register) allocate-register) 
                    ((eq? message 'trace-on) (set! trace-on true)) 
                    ((eq? message 'trace-off) (set! trace-on false)) 
                    ((eq? message 'get-register) lookup-register) 
                    ((eq? message 'install-operations) 
                         (lambda (ops) (set! the-ops (append the-ops ops)))) 
                    ((eq? message 'instruction-number) print-instruction-number) 
                    ((eq? message 'stack) stack) 
                    ((eq? message 'operations) the-ops) 
                    ((eq? message 'set-breakpoint)                           ;; *** 
                         (lambda (label n) (set! labels (cons (cons label n) labels)))) 
                    ((eq? message 'cancel-breakpoint)                     ;; *** 
                         (lambda (label) (cancel-breakpoint label))) 
                    ((eq? message 'cancel-all-breakpoint) (set! labels '())) 
                    ((eq? message 'process-machine) (execute))     ;; *** 
                    (else (error "Unkown request -- MACHINE" message)))) 
 (define (set-breakpoint machine label n) 
  ((machine 'set-breakpoint) label n)) 
 (define (cancel-breakpoint machine label) 
  ((machine 'cancel-breakpoint) label)) 
 (define (cancel-all-breakpoint machine) 
  (machine 'cancel-all-breakpoint)) 
 (define (process-machine machine) 
  (machine 'process-machine))