Exercise 5.15.  Add instruction counting to the register machine 
simulation. That is, have the machine model keep track of the number of 
instructions executed. Extend the machine model's interface to accept a 
new message that prints the value of the instruction count and resets 
the count to zero.

————————————————————————————————————————————————————————————————————————

Code is in 5.15.scm

Using the same factorial machine to test, calculating n! seems to take 
11n - 6 instructions:

1 ]=> (test 2)
(instruction-count = 16)
;Value: 2

1 ]=> (test 3)
(instruction-count = 27)
;Value: 6

1 ]=> (test 4)
(instruction-count = 38)
;Value: 24

1 ]=> (test 5)
(instruction-count = 49)
;Value: 120

1 ]=> (test 6)
(instruction-count = 60)
;Value: 720

1 ]=> (test 7)
(instruction-count = 71)
;Value: 5040

1 ]=> (test 8)
(instruction-count = 82)
;Value: 40320

The changes made to the simulator:

--- ch5-regsim.scm	2010-03-16 02:00:34.000000000 -0600
+++ 5.15.scm	2010-03-27 22:49:41.000000000 -0600
@@ -99,7 +99,8 @@
   (let ((pc (make-register 'pc))
         (flag (make-register 'flag))
         (stack (make-stack))
-        (the-instruction-sequence '()))
+        (the-instruction-sequence '())
+        (instruction-counter 0))
     (let ((the-ops
            (list (list 'initialize-stack
                        (lambda () (stack 'initialize)))
@@ -126,6 +127,7 @@
           (if (null? insts)
               'done
               (begin
+                (set! instruction-counter (+ instruction-counter 1))
                 ((instruction-execution-proc (car insts)))
                 (execute)))))
       (define (dispatch message)
@@ -140,6 +142,9 @@
                (lambda (ops) (set! the-ops (append the-ops ops))))
               ((eq? message 'stack) stack)
               ((eq? message 'operations) the-ops)
+              ((eq? message 'read-instruction-count)
+               (display (list 'instruction-count '= instruction-counter)) (newline)
+               (set! instruction-counter 0))
               (else (error "Unknown request -- MACHINE" message))))
       dispatch)))