1 (define (make-machine register-names ops controller-text)
2   (let ((machine (make-new-machine)))
3     (for-each (lambda (register-name)
4                 ((machine 'allocate-register) register-name))
5               register-names)
6     ((machine 'install-operations) ops)    
7     ((machine 'install-instruction-sequence)
8      (assemble controller-text machine))
9     machine))
10 (define (make-register name)
11   (let ((contents '*unassigned*))
12     (define (dispatch message)
13       (cond ((eq? message 'get) contents)
14             ((eq? message 'set)
15              (lambda (value) (set! contents value)))
16             (else
17              (error "Unknown request -- REGISTER" message))))
18     dispatch))
19 (define (get-contents register)
20   (register 'get))
21 (define (set-contents! register value)
22   ((register 'set) value))
23 (define (make-stack)
24   (let ((s '())
25         (number-pushes 0)
26         (max-depth 0)
27         (current-depth 0))
28     (define (push x)
29       (set! s (cons x s))
30       (set! number-pushes (+ 1 number-pushes))
31       (set! current-depth (+ 1 current-depth))
32       (set! max-depth (max current-depth max-depth)))
33     (define (pop)
34       (if (null? s)
35           (error "Empty stack -- POP")
36           (let ((top (car s)))
37             (set! s (cdr s))
38             (set! current-depth (- current-depth 1))
39             top)))    
40     (define (initialize)
41       (set! s '())
42       (set! number-pushes 0)
43       (set! max-depth 0)
44       (set! current-depth 0)
45       'done)
46     (define (print-statistics)
47       (newline)
48       (display (list 'total-pushes  '= number-pushes
49                      'maximum-depth '= max-depth)))
50     (define (dispatch message)
51       (cond ((eq? message 'push) push)
52             ((eq? message 'pop) (pop))
53             ((eq? message 'initialize) (initialize))
54             ((eq? message 'print-statistics)
55              (print-statistics))
56             (else
57              (error "Unknown request -- STACK" message))))
58     dispatch))
59 (define (pop stack)
60   (stack 'pop))
61 (define (push stack value)
62   ((stack 'push) value))
63 (define (make-new-machine)
64   (let ((pc (make-register 'pc))
65         (flag (make-register 'flag))
66         (stack (make-stack))
67         (the-instruction-sequence '()))
68     (let ((the-ops
69 	   (list (list 'initialize-stack
70 		       (lambda () (stack 'initialize)))
71 		 (list 'print-stack-statistics
72 		       (lambda () (stack 'print-statistics)))))
73           (register-table
74            (list (list 'pc pc) (list 'flag flag))))
75       (define (allocate-register name)
76         (if (assoc name register-table)
77             (error "Multiply defined register: " name)
78             (set! register-table
79                   (cons (list name (make-register name))
80                         register-table)))
81         'register-allocated)
82       (define (lookup-register name)
83         (let ((val (assoc name register-table)))
84           (if val
85               (cadr val)
86               (error "Unknown register:" name))))
87       (define (execute)
88         (let ((insts (get-contents pc)))
89           (if (null? insts)
90               'done
91               (begin
92                 ((instruction-execution-proc (car insts)))
93                 (execute)))))
94       (define (dispatch message)
95         (cond ((eq? message 'start)
96                (set-contents! pc the-instruction-sequence)
97                (execute))
98               ((eq? message 'install-instruction-sequence)
99                (lambda (seq) (set! the-instruction-sequence seq)))
100               ((eq? message 'allocate-register) allocate-register)
101               ((eq? message 'get-register) lookup-register)
102               ((eq? message 'install-operations)
103                (lambda (ops) (set! the-ops (append the-ops ops))))
104               ((eq? message 'stack) stack)
105               ((eq? message 'operations) the-ops)
106               (else (error "Unknown request -- MACHINE" message))))
107       dispatch)))
108 (define (start machine)
109   (machine 'start))
110 (define (get-register-contents machine register-name)
111   (get-contents (get-register machine register-name)))
112 (define (set-register-contents! machine register-name value)
113   (set-contents! (get-register machine register-name) value)
114   'done)
115 (define (get-register machine reg-name)
116   ((machine 'get-register) reg-name))
117 (define (assemble controller-text machine)
118   (extract-labels controller-text
119     (lambda (insts labels)
120       (update-insts! insts labels machine)
121       insts)))
122 (define (extract-labels text receive)
123   (if (null? text)
124       (receive '() '())
125       (extract-labels (cdr text)
126        (lambda (insts labels)
127          (let ((next-inst (car text)))
128            (if (symbol? next-inst)
129 	       (if (label-defined? labels next-inst)
130 		   (error "Duplicate label -- ASSEMBLE"
131 			  next-inst)
132 		   (receive 
133 		       insts
134 		       (cons (make-label-entry next-inst
135 					       insts)
136 			     labels)))
137                (receive
138 		   (cons (make-instruction next-inst)
139 			 insts)
140 		   labels)))))))
141 (define (update-insts! insts labels machine)
142   (let ((pc (get-register machine 'pc))
143         (flag (get-register machine 'flag))
144         (stack (machine 'stack))
145         (ops (machine 'operations)))
146     (for-each
147      (lambda (inst)
148        (set-instruction-execution-proc! 
149         inst
150         (make-execution-procedure
151          (instruction-text inst) labels machine
152          pc flag stack ops)))
153      insts)))
154 (define (make-instruction text)
155   (cons text '()))
156 (define (instruction-text inst)
157   (car inst))
158 (define (instruction-execution-proc inst)
159   (cdr inst))
160 (define (set-instruction-execution-proc! inst proc)
161   (set-cdr! inst proc))
162 (define (make-label-entry label-name insts)
163   (cons label-name insts))
164 (define (label-defined? labels label-name)
165   (not (false? (assoc label-name labels))))
166 (define (lookup-label labels label-name)
167   (let ((val (assoc label-name labels)))
168     (if val
169         (cdr val)
170         (error "Undefined label -- ASSEMBLE" label-name))))
171 (define (make-execution-procedure inst labels machine
172                                   pc flag stack ops)
173   (cond ((eq? (car inst) 'assign)
174          (make-assign inst machine labels ops pc))
175         ((eq? (car inst) 'test)
176          (make-test inst machine labels ops flag pc))
177         ((eq? (car inst) 'branch)
178          (make-branch inst machine labels flag pc))
179         ((eq? (car inst) 'goto)
180          (make-goto inst machine labels pc))
181         ((eq? (car inst) 'save)
182          (make-save inst machine stack pc))
183         ((eq? (car inst) 'restore)
184          (make-restore inst machine stack pc))
185         ((eq? (car inst) 'perform)
186          (make-perform inst machine labels ops pc))
187         (else (error "Unknown instruction type -- ASSEMBLE"
188                      inst))))
189 (define (make-assign inst machine labels operations pc)
190   (let ((target
191          (get-register machine (assign-reg-name inst)))
192         (value-exp (assign-value-exp inst)))
193     (let ((value-proc
194            (if (operation-exp? value-exp)
195                (make-operation-exp
196                 value-exp machine labels operations)
197                (make-primitive-exp
198                 (car value-exp) machine labels))))
199       (lambda ()                ; execution procedure for assign
200         (set-contents! target (value-proc))
201         (advance-pc pc)))))
202 (define (assign-reg-name assign-instruction)
203   (cadr assign-instruction))
204 (define (assign-value-exp assign-instruction)
205   (cddr assign-instruction))
206 (define (advance-pc pc)
207   (set-contents! pc (cdr (get-contents pc))))
208 (define (make-test inst machine labels operations flag pc)
209   (let ((condition (test-condition inst)))
210     (if (operation-exp? condition)
211         (let ((condition-proc
212                (make-operation-exp
213                 condition machine labels operations)))
214           (lambda ()
215             (set-contents! flag (condition-proc))
216             (advance-pc pc)))
217         (error "Bad TEST instruction -- ASSEMBLE" inst))))
218 (define (test-condition test-instruction)
219   (cdr test-instruction))
220 (define (make-branch inst machine labels flag pc)
221   (let ((dest (branch-dest inst)))
222     (if (label-exp? dest)
223         (let ((insts
224                (lookup-label labels (label-exp-label dest))))
225           (lambda ()
226             (if (get-contents flag)
227                 (set-contents! pc insts)
228                 (advance-pc pc))))
229         (error "Bad BRANCH instruction -- ASSEMBLE" inst))))
230 (define (branch-dest branch-instruction)
231   (cadr branch-instruction))
232 (define (make-goto inst machine labels pc)
233   (let ((dest (goto-dest inst)))
234     (cond ((label-exp? dest)
235            (let ((insts
236                   (lookup-label labels
237                                 (label-exp-label dest))))
238              (lambda () (set-contents! pc insts))))
239           ((register-exp? dest)
240            (let ((reg
241                   (get-register machine
242                                 (register-exp-reg dest))))
243              (lambda ()
244                (set-contents! pc (get-contents reg)))))
245           (else (error "Bad GOTO instruction -- ASSEMBLE"
246                        inst)))))
247 (define (goto-dest goto-instruction)
248   (cadr goto-instruction))
249 (define (make-save inst machine stack pc)
250   (let ((reg (get-register machine
251                            (stack-inst-reg-name inst))))
252     (lambda ()
253       (push stack (get-contents reg))
254       (advance-pc pc))))
255 (define (make-restore inst machine stack pc)
256   (let ((reg (get-register machine
257                            (stack-inst-reg-name inst))))
258     (lambda ()
259       (set-contents! reg (pop stack))    
260       (advance-pc pc))))
261 (define (stack-inst-reg-name stack-instruction)
262   (cadr stack-instruction))
263 (define (make-perform inst machine labels operations pc)
264   (let ((action (perform-action inst)))
265     (if (operation-exp? action)
266         (let ((action-proc
267                (make-operation-exp
268                 action machine labels operations)))
269           (lambda ()
270             (action-proc)
271             (advance-pc pc)))
272         (error "Bad PERFORM instruction -- ASSEMBLE" inst))))
273 (define (perform-action inst) (cdr inst))
274 (define (make-primitive-exp exp machine labels)
275   (cond ((constant-exp? exp)
276          (let ((c (constant-exp-value exp)))
277            (lambda () c)))
278         ((label-exp? exp)
279          (let ((insts
280                 (lookup-label labels
281                               (label-exp-label exp))))
282            (lambda () insts)))
283         ((register-exp? exp)
284          (let ((r (get-register machine
285                                 (register-exp-reg exp))))
286            (lambda () (get-contents r))))
287         (else
288          (error "Unknown expression type -- ASSEMBLE" exp))))
289 (define (tagged-list? exp tag)
290   (and (pair? exp) (eq? (car exp) tag)))
291 (define (register-exp? exp) (tagged-list? exp 'reg))
292 (define (register-exp-reg exp) (cadr exp))
293 (define (constant-exp? exp) (tagged-list? exp 'const))
294 (define (constant-exp-value exp) (cadr exp))
295 (define (label-exp? exp) (tagged-list? exp 'label))
296 (define (label-exp-label exp) (cadr exp))
297 (define (make-operation-exp exp machine labels operations)
298   (let ((op (lookup-prim (operation-exp-op exp) operations)) 
299         (aprocs
300          (map (lambda (e)
301 		;; (if (label-exp? e)
302 		;;     (error "Operation exp cannot operate on labels -- ASSEMBLE"
303 		;; 	   exp)
304 		(make-primitive-exp e machine labels))
305               (operation-exp-operands exp))))
306     (lambda ()
307       (apply op (map (lambda (p) (p)) aprocs)))))
308 (define (operation-exp? exp)
309   (and (pair? exp) (tagged-list? (car exp) 'op)))
310 (define (operation-exp-op operation-exp)
311   (cadr (car operation-exp)))
312 (define (operation-exp-operands operation-exp)
313   (cdr operation-exp))
314 (define (lookup-prim symbol operations)
315   (let ((val (assoc symbol operations)))
316     (if val
317         (cadr val)
318         (error "Unknown operation -- ASSEMBLE" symbol))))
319 
320 ;; test suite
321 
322 (define (test-case actual expected)
323   (newline)
324   (display "Actual:   ")
325   (display actual)
326   (newline)
327   (display "Expected: ")
328   (display expected)
329   (newline))
330 
331 (define gcd-machine
332   (make-machine
333    '(a b t)
334    (list (list 'rem remainder) (list '= =))
335    '(test-b
336        (test (op =) (reg b) (const 0))
337        (branch (label gcd-done))
338        (assign t (op rem) (reg a) (reg b))
339        (assign a (reg b))
340        (assign b (reg t))
341        (goto (label test-b))
342      gcd-done)))
343 (set-register-contents! gcd-machine 'a 206)
344 (set-register-contents! gcd-machine 'b 40)
345 (start gcd-machine)
346 (test-case (get-register-contents gcd-machine 'a) 2)
347 
348 (define fib-machine
349   (make-machine 
350    '(n val continue)
351    `((< ,<) (- ,-) (+ ,+))
352    '(controller
353      (assign continue (label fib-done))
354      fib-loop
355      (test (op <) (reg n) (const 2))
356      (branch (label immediate-answer))
357      (save continue)
358      (assign continue (label afterfib-n-1))
359      (save n)
360      (assign n (op -) (reg n) (const 1))
361      (goto (label fib-loop))
362      afterfib-n-1           
363      (restore n)
364      (restore continue)
365      (assign n (op -) (reg n) (const 2))
366      (save continue)
367      (assign continue (label afterfib-n-2))
368      (save val)
369      (goto (label fib-loop))
370      afterfib-n-2
371      (assign n (reg val))
372      (restore val)       
373      (restore continue)
374      (assign val         
375 	     (op +) (reg val) (reg n)) 
376      (goto (reg continue))
377      immediate-answer
378      (assign val (reg n)) 
379      (goto (reg continue))
380      fib-done)))
381 (set-register-contents! fib-machine 'val 0)
382 (set-register-contents! fib-machine 'n 15)
383 (start fib-machine)
384 (test-case (get-register-contents fib-machine 'val) 610)
385 
386 (define fact-iter
387   (make-machine
388    '(product counter n)
389    `((> ,>) (* ,*) (+ ,+))
390    '((assign product (const 1))
391      (assign counter (const 1))
392      fact-loop
393      (test (op >) (reg counter) (reg n))
394      (branch (label fact-end))
395      (assign product (op *) (reg counter) (reg product))
396      (assign counter (op +) (reg counter) (const 1))
397      (goto (label fact-loop))
398      fact-end)))
399 (set-register-contents! fact-iter 'n 10)
400 (start fact-iter)
401 (test-case (get-register-contents fact-iter 'product) 3628800)
402 
403 (define (sqrt x)
404   (define (good-enough? guess)
405     (< (abs (- (square guess) x)) 0.001))
406   (define (improve guess)
407     (average guess (/ x guess)))
408   (define (sqrt-iter guess)
409     (if (good-enough? guess)
410         guess
411         (sqrt-iter (improve guess))))
412   (sqrt-iter 1.0))
413 
414 (define (good-enough? guess x)
415   (< (abs (- (square guess) x)) 0.001))
416 (define (improve guess x)
417   (average guess (/ x guess)))
418 (define (average x y)
419   (/ (+ x y) 2))
420 (define sqrt-iter-ops
421   (make-machine
422    '(guess x)
423    `((good-enough? ,good-enough?)
424      (improve ,improve)
425      (abs ,abs)
426      (square ,square) 
427      (average ,average)
428      (< ,<)
429      (- ,-)
430      (/ ,/))
431    '((assign guess (const 1.0))
432      sqrt-iter
433      (test (op good-enough?) (reg guess) (reg x))
434      (branch (label sqrt-done))
435      (assign guess (op improve) (reg guess) (reg x))
436      (goto (label sqrt-iter))
437      sqrt-done)))
438 
439 (set-register-contents! sqrt-iter-ops 'x 27)
440 (start sqrt-iter-ops)
441 (test-case (get-register-contents sqrt-iter-ops 'guess)
442 	   5.19615242)
443 
444 (define (good-enough? guess x)
445   (< (abs (- (square guess) x)) 0.001))
446 (define (improve guess x)
447   (average guess (/ x guess)))
448 (define (average x y)
449   (/ (+ x y) 2))
450 (define sqrt-iter
451   (make-machine
452    '(guess x temp)
453    `((abs ,abs)
454      (square ,square) 
455      (average ,average)
456      (< ,<)
457      (- ,-)
458      (/ ,/))
459    '((assign guess (const 1.0))
460      sqrt-iter
461      ;; (test (op good-enough?) (reg guess) (reg x))
462      (assign temp (op square) (reg guess))
463      (assign temp (op -) (reg temp) (reg x))
464      (assign temp (op abs) (reg temp))
465      (test (op <) (reg temp) (const 0.001))
466      (branch (label sqrt-done))
467      ;; (assign guess (op improve) (reg guess) (reg x))
468      (assign temp (op /) (reg x) (reg guess))
469      (assign guess (op average) (reg guess) (reg temp))
470      (goto (label sqrt-iter))
471      sqrt-done)))
472 (set-register-contents! sqrt-iter-ops 'x 91)
473 (start sqrt-iter-ops)
474 (test-case (get-register-contents sqrt-iter-ops 'guess)
475 	   9.53939201)
476 
477 (define (expt b n)
478   (if (= n 0)
479       1
480       (* b (expt b (- n 1)))))
481 
482 (define expt-rec
483   (make-machine
484    '(b n product continue)
485    `((= ,=)
486      (* ,*)
487      (- ,-))
488    '((assign continue (label expt-done))
489      expt-rec
490      (test (op =) (reg n) (const 0))
491      (branch (label base-case))
492      (assign n (op -) (reg n) (const 1))
493      (save continue)
494      (assign continue (label after-b-n-1))
495      (goto (label expt-rec))
496      after-b-n-1
497      (restore continue)
498      (assign product (op *) (reg b) (reg product))
499      (goto (reg continue))
500      base-case
501      (assign product (const 1))
502      (goto (reg continue))
503      expt-done)))
504 
505 (set-register-contents! expt-rec 'b 3.2)
506 (set-register-contents! expt-rec 'n 6)
507 (start expt-rec)
508 (test-case (get-register-contents expt-rec 'product)
509 	   1073.74182)
510 
511 (define (expt b n)
512   (define (expt-iter counter product)
513     (if (= counter 0)
514         product
515         (expt-iter (- counter 1) (* b product))))
516   (expt-iter n 1))
517 
518 (define expt-iter
519   (make-machine
520    '(b n counter product)
521    `((= ,=)
522      (* ,*)
523      (- ,-))
524    '((assign counter (reg n))
525      (assign product (const 1))
526      expt-iter
527      (test (op =) (reg counter) (const 0))
528      (branch (label expt-iter-done))
529      (assign counter (op -) (reg counter) (const 1))
530      (assign product (op *) (reg b) (reg product))
531      (goto (label expt-iter))
532      expt-iter-done)))
533 (set-register-contents! expt-iter 'b 1.6)
534 (set-register-contents! expt-iter 'n 17)
535 (start expt-iter)
536 (test-case (get-register-contents expt-iter 'product) 
537 	   2951.47905)
538 
539 ;; (define amb-machine
540 ;;   (make-machine
541 ;;    '(a)
542 ;;    '()
543 ;;    '(start
544 ;;      (goto (label here))
545 ;;      here
546 ;;      (assign a (const 3))
547 ;;      (goto (label there))
548 ;;      here
549 ;;      (assign a (const 4))
550 ;;      (goto (label there))
551 ;;      there)))
552 
553 ;; (start amb-machine)
554 ;; (test-case (get-register-contents amb-machine 'a)
555 ;; 	   3)
556 ;; this now raises an error
557 
558 ;; 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
559 
560 ;; (save y)
561 ;; (save x)
562 ;; (restore y)
563 
564 ;; There are several reasonable possibilities for the meaning of restore:
565 
566 ;; 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).
567 
568 ;; (assign n (reg val))
569 ;; (restore val)
570 
571 ;; can now be shortened to
572 
573 ;; (restore n)
574 
575 ;; 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.
576 
577 
578 
579 ;; 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.