Working on 5.11

ex-5_01-06.scm

@@ -210,10 +210,10 @@
         (goto (label fib-loop))            ; perform recursive call
       afterfib-n-1                         ; upon return, val contains Fib(n - 1)
         (restore n)
-        ;(restore continue)
+        ;(restore continue)                ; uneeded restore
         ;; set up to compute Fib(n - 2)
         (assign n (op -) (reg n) (const 2))
-        ;(save continue)
+        ;(save continue)                   ; uneeded continue
         (assign continue (label afterfib-n-2))
         (save val)                         ; save Fib(n - 1)
         (goto (label fib-loop))

ex-5_07-xx.scm

@@ -103,38 +103,84 @@
 (start triple-inc-machine)
 (assert (get-register-contents triple-inc-machine 'a) 11)
 
-(display "\nex-5.11\n")
+(display "\nex-5.11 - stack-behavior\n")
 
-;Exercise 5.11.  When we introduced save and restore in section 5.1.4, we
-;didn't specify what would happen if you tried to restore a register that was
-;not the last one saved, as in the sequence
+; a
+(define fib-machine
+  (make-machine
+    '(n val continue)
+    (list (list '< <) (list '- -) (list '= +) (list '+ +))
+    '(controller
+        (assign continue (label fib-done))
+      fib-loop
+        (test (op <) (reg n) (const 2))
+        (branch (label immediate-answer))
+        ;; set up to compute Fib(n - 1)
+        (save continue)
+        (assign continue (label afterfib-n-1))
+        (save n)                           ; save old value of n
+        (assign n (op -) (reg n) (const 1)); clobber n to n - 1
+        (goto (label fib-loop))            ; perform recursive call
+      afterfib-n-1                         ; upon return, val contains Fib(n - 1)
+        (restore n)
+        ;; set up to compute Fib(n - 2)
+        (assign n (op -) (reg n) (const 2))
+        (assign continue (label afterfib-n-2))
+        (save val)                         ; save Fib(n - 1)
+        (goto (label fib-loop))
+      afterfib-n-2                         ; upon return, val contains Fib(n - 2)
+        ; (assign n (reg val))             ; n now contains Fib(n - 2)
+        ; (restore val)                    ; val now contains Fib(n - 1)
+        (restore n)                        ; ex-5.11 - save one instruction
+        (assign val                        ;  Fib(n - 1) +  Fib(n - 2)
+                (op +) (reg val) (reg n))
+        (restore continue)
+        (goto (reg continue))              ; return to caller, answer is in val
+      immediate-answer
+        (assign val (reg n))               ; base case:  Fib(n) = n
+        (goto (reg continue))
+      fib-done)))
 
-;(save y)
-;(save x)
-;(restore y)
+; We can replace ((assign n (reg val)) (restore val)) with (restore n) because
+; we add up n and val anyway. It does not matter which is in which register.
 
-;There are several reasonable possibilities for the meaning of restore:
+(set-register-contents! fib-machine 'n 8)
+(start fib-machine)
+(display "a. ")
+(assert (get-register-contents fib-machine 'val) 21)
 
-;a.  (restore y) puts into y the last value saved on the stack, regardless of
-;what register that value came from. This is the way our simulator behaves.
-;Show how to take advantage of this behavior to eliminate one instruction from
-;the Fibonacci machine of section 5.1.4 (figure 5.12).
+; b.
+(define stack-content car)
+(define stack-register cadr)
 
-;b.  (restore y) puts into y the last value saved on the stack, but only if
-;that value was saved from y; otherwise, it signals an error. Modify the
-;simulator to behave this way. You will have to change save to put the register
-;name on the stack along with the value.
+(define (make-save inst machine stack pc)
+  (let ((reg (get-register machine
+                           (stack-inst-reg-name inst))))
+    (lambda ()
+      (push stack (list (get-contents reg) reg))
+      (advance-pc pc))))
 
+(define (make-restore inst machine stack pc)
+  (let ((reg (get-register machine
+                           (stack-inst-reg-name inst))))
+    (lambda ()
+      (let ((stack-element (pop stack)))
+        (if (not (eq? reg (stack-register stack-element)))
+          (error "restore from different reg -- MAKE-RESTORE"
+                 (reg 'name?)
+                 ((stack-register stack-element) 'name?)))
+      (set-contents! reg (stack-content stack-element))
+      (advance-pc pc)))))
+
+(display "b. [implemented]\n")
+
+(display "c. ACTIVE!!!\n")
 ;c.  (restore y) puts into y the last value saved from y regardless of what
 ;other registers were saved after y and not restored. Modify the simulator to
 ;behave this way. You will have to associate a separate stack with each
 ;register. You should make the initialize-stack operation initialize all the
 ;register stacks.
 
-; Extend the message-passing interface to the machine to provide access to
-; this new information. To test your analyzer, define the Fibonacci machine
-; from figure 5.12 and examine the lists you constructed.
-
 (display "\nex-5.12\n")
 
 ; (display "\nex-5.13\n")

misc/sicp-regsim.scm

@@ -29,6 +29,7 @@
       (cond ((eq? message 'get) contents)
             ((eq? message 'set)
              (lambda (value) (set! contents value)))
+            ((eq? message 'name?) name)
             (else
              (error "Unknown request -- REGISTER" message))))
     dispatch))