sicp-ex-5.11

  
  
 (a)  
 (assign n (reg val)) 
 (restore val) 
 we can use (restore n) replace these two instructions. Because val contain Fib(n-2), (restore n) make n containing Fib(n-1), then (assign val (op +) (reg val) (reg n)) works still. 
  
 (b) 
 ;; in make-save, push the register name and contents on the stack. 
 (define (make-save inst machine stack pc) 
   (let ((reg (get-register machine 
                            (stack-inst-reg-name inst)))) 
     (lambda () 
       (push stack (cons (stack-inst-reg-name inst) (get-contents reg))) 
       (advance-pc pc)))) 
  
 ;; when pop stack, check if the register name on the stack is equal to 
 ;;  reg-name in (restore reg-name) 
 (define (make-restore inst machine stack pc) 
   (let* ((reg-name (stack-inst-reg-name inst)) 
                  (reg (get-register machine reg-name))) 
     (lambda () 
          (let ((pop-reg (pop stack)))  
           (if (eq? (car pop-reg) reg-name) 
           (begin 
                    (set-contents! reg (cdr pop-reg))     
            (advance-pc pc)) 
                   (error "the value is not from register:" reg-name)))))) 
  
 (c) 
 ;; first modify make-stack making it including the name of register.  
 ;; all the named-register are in s, we can use assoc to find it. 
 ;; add interface add-reg-stack to add register to stack.  
 (define (make-stack) 
   (let ((s '())) 
     (define (push reg-name value) 
          (let ((reg (assoc reg-name s))) 
           (if reg 
               (set-cdr! reg (cons value (cdr reg))) 
                   (error "the register is not in the stack -- PUSH" reg-name)))) 
     (define (pop reg-name) 
          (let ((reg (assoc reg-name s))) 
           (if reg 
               (if (null? (cdr reg)) 
                       (error "Empty stack for register -- POP " reg-name) 
                           (let ((top (cadr reg))) 
                            (set-cdr! reg (cddr reg)) 
                            top)) 
                   (error "the register is not in the stack -- POP" reg-name)))) 
         (define (add-reg-stack reg-name) 
          (if (assoc reg-name s) 
              (error "this register is already in the stack -- ADD-REG-STACK" reg-name) 
                  (set! s (cons (list reg-name) s)))) 
     (define (initialize) 
          (for-each 
           (lambda (stack) 
            (set-cdr! stack '())) 
           s) 
       'done) 
     (define (dispatch message) 
       (cond ((eq? message 'push) push) 
             ((eq? message 'pop) pop) 
                         ((eq? message 'add-reg-stack) add-reg-stack) 
             ((eq? message 'initialize) (initialize)) 
             (else (error "Unknown request -- STACK" message)))) 
     dispatch)) 
  
 (define (add-reg-stack stack reg-name) 
  ((stack 'add-reg-stack) reg-name)) 
  
 ;; change make-store and make-restore. because their parameters had change. 
 (define (make-save inst machine stack pc) 
   (let ((reg (get-register machine 
                            (stack-inst-reg-name inst)))) 
     (lambda () 
       (push stack (stack-inst-reg-name inst) (get-contents reg)) 
       (advance-pc pc)))) 
  
 (define (make-restore inst machine stack pc) 
   (let* ((reg-name (stack-inst-reg-name inst)) 
                  (reg (get-register machine reg-name))) 
     (lambda () 
          (let ((value (pop stack reg-name)))  
           (set-contents! reg value)     
       (advance-pc pc))))) 
  
 ;; modify allocate-register in make-new-machine, when allocate a register, 
 ;; add it to stack. 
   (define (allocate-register name) 
         (if (assoc name register-table) 
           (error "Multiply defined register: " name) 
                   (begin 
                    (add-reg-stack stack name) 
            (set! register-table  
                  (cons (list name (make-register name)) 
                        register-table)))) 
         'register-allocated)