1 (define (make-serializer)
2   (let ((mutex (make-mutex)))
3     (lambda (p)
4       (define (serialized-p . args)
5 	(mutex 'acquire)
6 	(let ((val (apply p args)))
7 	  (mutex 'release)
8 	  val))
9       serialized-p)))
10 
11 (define (make-mutex)
12   (let ((cell (list false)))
13     (define (the-mutex m)
14       (cond ((eq? m 'acquire) (if (test-and-set! cell)
15 				  (the-mutex 'acquire)))
16 	    ((eq? m 'release) (clear! cell))))
17     the-mutex))
18 
19 (define (clear! cell)
20   (set-car! cell false))
21 ;; (define (test-and-set! cell)
22 ;;   (if (car cell)
23 ;;       true
24 ;;       (begin (set-car! cell true)
25 ;; 	     false)))
26 (define (test-and-set! cell)
27   (without-interrupts
28    (lambda ()
29      (if (car cell)
30 	 true
31 	 (begin (set-car! cell true)
32 		false)))))
33 
34 ;; Exercise 3.47.  A semaphore (of size n) is a generalization of a mutex. Like a mutex, a semaphore supports acquire and release operations, but it is more general in that up to n processes can acquire it concurrently. Additional processes that attempt to acquire the semaphore must wait for release operations. Give implementations of semaphores
35 
36 ;; a. in terms of mutexes
37 
38 ;; b. in terms of atomic test-and-set! operations. 
39 
40 (define (make-semaphore n)
41   (let ((mutex (make-mutex)))
42     (define (the-semaphore m)
43       (cond ((eq? m 'acquire) 
44 	     (mutex 'acquire)
45 	     (if (> n 0)
46 		 (begin (set! n (- n 1))
47 			(mutex 'release))
48 		 (begin (mutex 'release)
49 			(the-semaphore 'acquire))))
50 	    ((eq? m 'release) 
51 	     (mutex 'acquire)
52 	     (set! n (+ n 1))
53 	     (mutex 'release))))
54     the-semaphore))
55 
56 (define (make-semaphore n)
57   (let ((cell (list false)))
58     (define (clear! cell)
59       (set-car! cell false))
60     (define (the-semaphore m)
61       (cond ((eq? m 'acquire) 
62 	     (if (test-and-set! cell)
63 		 (the-semaphore 'acquire)
64 		 (if (> n 0)
65 		     (begin (set! n (- n 1))
66 			    (clear! cell))
67 		     (begin (clear! cell)
68 			    (the-semaphore 'acquire)))))
69 	    ((eq? m 'release) 
70 	     (if (test-and-set! cell)
71 		 (the-semaphore 'release)
72 		 (begin (set! n (+ n 1))
73 			(clear! cell))))))
74     the-semaphore))