Exercise 5.13.  Modify the simulator so that it uses the controller 
sequence to determine what registers the machine has rather than 
requiring a list of registers as an argument to make-machine. Instead of 
pre-allocating the registers in make-machine, you can allocate them one 
at a time when they are first seen during assembly of the instructions.

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

Code is in 5.13.scm, tested with the example machines from Exercise 5.7, 
with the explicit register definitions removed from the make-machine 
calls.

Test output:

1 ]=> (test-expt-recur)

;Value: 343

1 ]=> (test-expt-iter)

;Value: 343

The following changes were made:

--- ch5-regsim.scm	2010-03-16 02:00:34.000000000 -0600
+++ 5.13.scm	2010-03-16 01:55:12.000000000 -0600
@@ -1,12 +1,9 @@
 ;;;;REGISTER-MACHINE SIMULATOR FROM SECTION 5.2 OF
 ;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS
 
-(define (make-machine register-names ops controller-text)
+(define (make-machine ops controller-text)
   (let ((machine (make-new-machine)))
-    (for-each (lambda (register-name)
-                ((machine 'allocate-register) register-name))
-              register-names)
-    ((machine 'install-operations) ops)    
+    ((machine 'install-operations) ops)
     ((machine 'install-instruction-sequence)
      (assemble controller-text machine))
     machine))
@@ -116,11 +113,12 @@
                   (cons (list name (make-register name))
                         register-table)))
         'register-allocated)
-      (define (lookup-register name)
+      (define (lookup-or-assign-register name)
         (let ((val (assoc name register-table)))
           (if val
               (cadr val)
-              (error "Unknown register:" name))))
+              (begin (allocate-register name)
+                     (lookup-or-assign-register name)))))
       (define (execute)
         (let ((insts (get-contents pc)))
           (if (null? insts)
@@ -135,7 +133,7 @@
               ((eq? message 'install-instruction-sequence)
                (lambda (seq) (set! the-instruction-sequence seq)))
               ((eq? message 'allocate-register) allocate-register)
-              ((eq? message 'get-register) lookup-register)
+              ((eq? message 'get-register) lookup-or-assign-register)
               ((eq? message 'install-operations)
                (lambda (ops) (set! the-ops (append the-ops ops))))
               ((eq? message 'stack) stack)