SICP 5.2.1 – 5.2.3 A Register-Machine Simulator
2025-02-21 Fri

Authors present a scheme program which simulates the kind of register machines described in 5.1.

The highest-level abstraction is the make-machine procedure, which can be used to create a machine. It takes the names of the registers, the list of the operations the machine uses, and the controller text.

Set-register-contents! can be used to set the value of a register and get-register-contents to retrieve it. Start is used to make the machine start executing the instructions represented by the controller text.

Authors superbly describe the implementation.

Exercise 5.8

Exercise:

The following register-machine code is ambiguous, because the label here is defined more than once:
start
(goto (label here))
here
(assign a (const 3))
(goto (label there))
here
(assign a (const 4))
(goto (label there))
there
With the simulator as written, what will the contents of register a be when control reaches there? Modify the extract-labels procedure so that the assembler will signal an error if the same label name is used to indicate two different locations.

Answer:

With the simulator as written, what will the contents of register a be when control reaches there?

Labels are stored in a list in the same order they are given by the controller text. Given the implementation of lookup-label — which uses assoc —, when the same labels appears more than once, the first one in the list will be selected. So the answer is 3.

Modify the extract-labels procedure so that the assembler will signal an error if the same label name is used to indicate two different locations.

Here is a modified version of the extract-labels method shown in footnote 4, page 521 (as opposed to the one in the main text).

(define (extract-labels text)
  (if (null? text)
      (cons '() '())
      (let ((result (extract-labels (cdr text))))
        (let ((insts (car result)) (labels (cdr result)))
          (let ((next-inst (car text)))
            (if (symbol? next-inst)
                (if (assoc next-inst labels)
                    (error "Duplicated label:" next-inst)
                    (cons insts
                          (cons (make-label-entry next-inst insts) labels)))
                (cons (cons (make-instruction next-inst) insts)
                      labels)))))))

Exercise 5.9

Exercise:

The treatment of machine operations above permits them to operate on labels as well as on constants and the contents of registers. Modify the expression-processing procedures to enforce the condition that operations can be used only with registers and constants.

Answer:

(define (make-operation-exp exp machine labels operations)
  (let ((op (lookup-prim (operation-exp-op exp) operations))
        (aprocs
         (map (lambda (e)
                (if (label-exp? e)
                    (error "Cannot use labels as operation operands")
                    (make-primitive-exp e machine labels)))
              (operation-exp-operands exp))))
    (lambda ()
      (apply op (map (lambda (p) (p)) aprocs)))))

Exercise 5.10

Exercise:

Design a new syntax for register-machine instructions and modify the simulator to use your new syntax. Can you implement your new syntax without changing any part of the simulator except the syntax procedures in this section?

Answer:

Syntax changes:

(assign (reg <REGISTER-NAME>) ...)

instead of

(assign <REGISTER-NAME> ...)

(op <OPERATION-NAME> <INPUT_1> ... <INPUT_N>)

instead of

((op <OPERATION-NAME>) <INPUT_1> ... <INPUT_N>))

Changes in the code:

;; redefined procedures:

(define (assign-reg-name assign-instruction)
  (cadadr assign-instruction))

(define (operation-exp? exp)
  (and (pair? exp) (tagged-list? exp 'op)))

(define (operation-exp-op operation-exp)
  (cadr operation-exp))

(define (operation-exp-operands operation-exp)
  (cddr operation-exp))

Rephrasing the gcd-machine given the new syntax:

(define gcd-machine
  (make-machine
   '(a b t)
   (list (list 'rem remainder) (list '= =))
   '(test-b
     (test op = (reg b) (const 0))
     (branch (label gcd-done))
     (assign (reg t) op rem (reg a) (reg b))
     (assign (reg a) (reg b))
     (assign (reg b) (reg t))
     (goto (label test-b))
     gcd-done)))

(set-register-contents! gcd-machine 'a 206)

(set-register-contents! gcd-machine 'b 40)

(start gcd-machine)

(get-register-contents gcd-machine 'a)

Exercise 5.11

Exercise:

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
(save y)
(save x)
(restore y)
There are several reasonable possibilities for the meaning of `restore':

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 (see Figure 5.12).

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.

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.

Answer

(define fib-machine
  (make-machine
   '(continue n val)
   (list (list '+ +) (list '< <) (list '- -))
   '(
     (assign continue (label fib-done))
     fib-loop
     (test (op <) (reg n) (const 2))
     (branch (label immediate-answer))
     (save continue)
     (assign continue (label afterfib-n-1))
     (save n)
     (assign n (op -) (reg n) (const 1))
     (goto (label fib-loop))
     afterfib-n-1
     (restore n)
     (restore continue)
     (assign n (op -) (reg n) (const 2))
     (save continue)
     (assign continue (label afterfib-n-2))
     (save val)
     (goto (label fib-loop))
     afterfib-n-2
     ;;(assign n (reg val)) <---------------
     ;;(restore val)        <---------------
     (restore n)        ;;  <---------------
     (restore continue)
     (assign val
             (op +) (reg val) (reg n))
     (goto (reg continue))
     immediate-answer
     (assign val (reg n))
     (goto (reg continue))
     fib-done)))

It works. Here is the modified machine running:

repl.rkt> (define fib-machine
            (make-machine
             '(continue n val)
             (list (list '+ +) (list '< <) (list '- -))
             '(
               (assign continue (label fib-done))
               fib-loop
               (test (op <) (reg n) (const 2))
               (branch (label immediate-answer))
               (save continue)
               (assign continue (label afterfib-n-1))
               (save n)
               (assign n (op -) (reg n) (const 1))
               (goto (label fib-loop))
               afterfib-n-1
               (restore n)
               (restore continue)
               (assign n (op -) (reg n) (const 2))
               (save continue)
               (assign continue (label afterfib-n-2))
               (save val)
               (goto (label fib-loop))
               afterfib-n-2
               ;;(assign n (reg val)) <---------------
               ;;(restore val)        <---------------
               (restore n)        ;;  <---------------
               (restore continue)
               (assign val
                       (op +) (reg val) (reg n))
               (goto (reg continue))
               immediate-answer
               (assign val (reg n))
               (goto (reg continue))
               fib-done)))
repl.rkt> (set-register-contents! fib-machine 'n 4)
'done
repl.rkt> (start fib-machine)
'done
repl.rkt> (get-register-contents fib-machine 'val)
3
repl.rkt> (set-register-contents! fib-machine 'n 5)
'done
repl.rkt> (start fib-machine)
'done
repl.rkt> (get-register-contents fib-machine 'val)
5
repl.rkt> (set-register-contents! fib-machine 'n 6)
'done
repl.rkt> (start fib-machine)
'done
repl.rkt> (get-register-contents fib-machine 'val)
8
repl.rkt>

I have:

modified the definition of push within make-stack:

(define (make-stack)
  ;;...
  (define (push x reg-name)
    (let ((obj (cons x reg-name)))
      (set! s (cons obj s))))
  ;;...
  ))

modified push:

(define (push stack value reg-name)
  ((stack 'push) value reg-name))

created a couple of helper functions (one to extract the value of the object stored in the stack; the other to extract the name of the corresponding register)
```
(define (stack-obj-value stack-obj)
  (car stack-obj))

(define (stack-obj-reg-name stack-obj)
  (cdr stack-obj))
```

modified the make-save and the make-restore procedures:

(define (make-save inst machine stack pc)
  (let ((reg-name (stack-inst-reg-name inst)))
    (let ((reg (get-register machine reg-name)))
      (lambda ()
        (push stack (get-contents reg) reg-name)
        (advance-pc pc)))))

(define (make-restore inst machine stack pc)
  (let ((reg (get-register machine
                           (stack-inst-reg-name inst))))
    (lambda ()
      (let ((stack-obj (pop stack)))
        (let ((stack-val (stack-obj-value stack-obj))
              (stack-reg-name (stack-obj-reg-name stack-obj)))
          (if (not (eq? (stack-inst-reg-name inst)
                        stack-reg-name))
              (error "Cannot restore value in different register. Original register: ...; Attempting to save in ...")
              (begin
                (set-contents! reg stack-val)
                (advance-pc pc))))))))

Notice that the error is raised at simulation time.

We can treat s as a list of stacks; each stacks being associated to a register.

A register stack in the list can be represented as a list whose car is the name of the relevant register and whose cdr is the list of the elements in the stack.

We can establish that if a register foo has no stack in the list, then its stack is empty.

Given so, we can keep the initialize-stack operation as it is. By setting the list of register stacks to an empty list, each register will have an empty stack.

When a value bar for register foo is saved and there is no foo stack in the list of stacks, then a foo stack is created and bar saved into it.

Modified parts of the code:

(define (make-stack);; actually it's a list of stacks (los) now
  (let ((s '()))
    (define (find-reg-stack los name)
      (cond ((null? los)
             '())
            ((eq? (caar los) name) ;; check tag
             (car los))
            (else
             (find-reg-stack (cdr los) name))))
    (define (push-in-los x reg-name)
      (display "push-in-los ")
      (let ((reg-stack (find-reg-stack s reg-name)))
        (if (null? reg-stack) ;; register stack not found
            ;; create reg-stack
            ;; push value
            ;; add reg-stack to los
            (let ((new-reg-stack (cons reg-name
                                       (cons x '()))))
              (set! s (cons new-reg-stack s)))
            ;; else
            ;; push-value in reg-stack
            (set-cdr! reg-stack (cons x (cdr reg-stack))))))
    (define (pop-from-los reg-name)
      (display "pop-from-los ")
      (let ((reg-stack (find-reg-stack s reg-name)))
        (if (null? reg-stack) ;; not found
            (error "Empty stack --- POP")
            (if (null? (cdr reg-stack))
                (error "Empty stack --- POP")
                (let ((top (car (cdr reg-stack))))
                  (set-cdr! reg-stack (cdr (cdr reg-stack)))
                  top)))))
    (define (initialize)
      (set! s '())
      'done)
    (define (dispatch message)
      (cond ((eq? message 'push) push-in-los)
            ((eq? message 'pop) pop-from-los)
            ((eq? message 'initialize) (initialize))
            (else (error "Unknown request -- STACK"
                         message))))
    dispatch))

(define (pop stack reg-name)
  ((stack 'pop) reg-name))

(define (push stack value reg-name)
  ((stack 'push) value reg-name))

(define (make-save inst machine stack pc)
  (let ((reg-name (stack-inst-reg-name inst)))
    (let ((reg (get-register machine
                             reg-name)))
      (lambda ()
        (push stack (get-contents reg) reg-name)
        (advance-pc pc)))))

(define (make-restore inst machine stack pc)
  (let ((reg-name (stack-inst-reg-name inst)))
    (let ((reg (get-register machine reg-name)))
      (lambda ()
        (set-contents! reg (pop stack reg-name))
        (advance-pc pc)))))

Exercise 5.12

Exercise:

The simulator can be used to help determine the data paths required for implementing a machine with a given controller. Extend the assembler to store the following information in the machine model:

a list of all instructions, with duplicates removed, sorted by instruction type (assign, goto, and so on);

a list (without duplicates) of the registers used to hold entry points (these are the registers referenced by goto instructions);

a list (without duplicates) of the registers that are ~save~d or `restore'd;

for each register, a list (without duplicates) of the sources from which it is assigned (for example, the sources for register val in the factorial machine of *Note Figure 5-11:: are (const 1) and ((op *) (reg n) (reg val))).

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.

Answer:

I've decided to modify update-insts!, since it contains a for-each which applies a lambda to each instruction. Such lambda, instead of only updating the sequence of instructions, now it extracts the relevant information from each instruction and stores that information in the reg-sources, entry-point-regs, sorted-instructions, and saved-restored-regs lists. After the the lists are built, we install the lists in the model using message-passing.

Here is the code I've modified or added:

(require sicp)

;; make copy of object
;;
;; I use to it to prevent mutations of the lists stored in the model
(define (make-copy obj)
  (cond ((null? obj)
         '())
        ((pair? obj)
         (cons (make-copy (car obj))
               (make-copy (cdr obj))))
        (else obj)))

(define (make-new-machine)
  (let ((pc (make-register 'pc))
        (flag (make-register 'flag))
        (stack (make-stack))
        (the-instruction-sequence '())
        (sorted-instructions '())
        (entry-point-regs '())
        (saved-restored-regs '())
        (reg-sources '()))
    (let ((the-ops
           (list (list 'initialize-stack
                       (lambda () (stack 'initialize)))))
          (register-table
           (list (list 'pc pc) (list 'flag flag))))
      (define (allocate-register name)
        (if (assoc name register-table)
            (error "Multiply defined register: " name)
            (set! register-table
                  (cons (list name (make-register name))
                        register-table)))
        'register-allocated)
      (define (lookup-register name)
        (let ((val (assoc name register-table)))
          (if val
              (cadr val)
              (error "Unknown register:" name))))
      (define (execute)
        (let ((insts (get-contents pc)))
          (if (null? insts)
              'done
              (begin
                ((instruction-execution-proc (car insts)))
                (execute)))))
      (define (dispatch message)
        (cond ((eq? message 'start)
               (set-contents! pc the-instruction-sequence)
               (execute))
              ((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 'install-operations)
               (lambda (ops) (set! the-ops (append the-ops ops))))
              ((eq? message 'stack) stack)
              ((eq? message 'operations) the-ops)
              ((eq? message 'sorted-instructions) (make-copy sorted-instructions))
              ((eq? message 'install-sorted-instructions)
               (lambda (seq) (set! sorted-instructions seq)))
              ((eq? message 'entry-point-regs) (make-copy entry-point-regs))
              ((eq? message 'install-entry-point-regs)
               (lambda (seq) (set! entry-point-regs seq)))
              ((eq? message 'saved-or-restored-regs) (make-copy saved-restored-regs))
              ((eq? message 'install-saved-or-restored-regs)
               (lambda (seq) (set! saved-restored-regs seq)))
              ((eq? message 'reg-sources) (make-copy reg-sources))
              ((eq? message 'install-reg-sources)
               (lambda (seq) (set! reg-sources seq)))
              (else (error "Unknown request -- MACHINE" message))))
      dispatch)))

;; I'm using this list to insert alphabetically in the
;; sorted-instructions list
(define insts-names-in-order
  '(assign branch goto perform restore save test))

;; compare instructions alphabetically
;; return 0 if i1 = i2
;;        1 if i1 > i2
;;       -1 if i1 < i2
(define (inst-alph-cmp i1 i2)
  (define (iter insts-names-in-order)
    (cond ((eq? i1 i2)
           0)
          ((eq? i1 (car insts-names-in-order))
           -1)
          ((eq? i2 (car insts-names-in-order))
           1)
          (else
           (iter (cdr insts-names-in-order)))))
  (iter insts-names-in-order))

;; return true if instruction type i1 comes alphabetically before
;; instruction type i2
(define (comes-before? i1 i2)
  (= (inst-alph-cmp i1 i2) -1))

;; return true if instruction type i1 comes alphabetically after
;; instruction type i2
(define (comes-after? i1 i2)
  (= (inst-alph-cmp i1 i2) 1))

;; insert inst into current list of sorted insts. In alphabetic
;; order. No duplicates.
(define (insert-inst-to-sorted-insts inst-text items)
  (let ((name (car inst-text)))
    (cond ((null? items)
           (cons inst-text items))
          ((comes-before? name (caar items))
           (cons inst-text items))
          ((equal? inst-text (car items)) ;; same inst
           items)
          ((eq? name (caar items)) ;; same inst type but not same inst
           (cons (car items)
                 (insert-inst-to-sorted-insts inst-text (cdr items))))
          ((comes-after? name (caar items))
           (cons (car items)
                 (insert-inst-to-sorted-insts inst-text (cdr items)))))))

(define (insert-no-duplicates x items)
  (cond ((null? items)
         (cons x items))
        ((eq? x (car items))
         items)
        (else
         (cons (car items)
               (insert-no-duplicates x (cdr items))))))

(define (update-sorted-instructions inst-text current-list)
  ;; inst is an inst obj: (text . <#proc>)
  (insert-inst-to-sorted-insts inst-text current-list))

(define (update-entry-point-regs inst-text current-list)
  (if (and (eq? (car inst-text) 'goto)
           (eq? (caadr inst-text) 'reg))
      (let ((reg-name (cadadr inst-text)))
        (insert-no-duplicates reg-name current-list))
      current-list))

(define (update-saved-or-restored-regs inst-text current-list)
  (if (or (eq? (car inst-text) 'save)
          (eq? (car inst-text) 'restore))
      (let ((reg-name (cadr inst-text)))
        (insert-no-duplicates reg-name current-list))
      current-list))

(define (insert-no-equal-duplicates x items)
  (cond ((null? items)
         (cons x items))
        ((equal? x (car items))
         items)
        (else (cons (car items)
                    (insert-no-equal-duplicates x (cdr items))))))

(define (update-reg-sources inst-text current-list)
  (if (eq? (car inst-text) 'assign)
      (insert-source inst-text current-list)
      current-list))

(define (insert-source inst-text current-list)
  (let ((reg-name (cadr inst-text))
        (source (cddr inst-text)))
    (cond ((null? current-list)
           (list (list reg-name
                       source)))
          ((eq? (caar current-list) reg-name)
           (cons (cons reg-name
                       (insert-no-equal-duplicates
                        source
                        (cdar current-list)))
                 (cdr current-list)))
          (else ;; not the same reg
           (cons (car current-list)
                 (insert-source inst-text (cdr current-list)))))))

(define (update-insts! insts labels machine)
  (let ((pc (get-register machine 'pc))
        (flag (get-register machine 'flag))
        (stack (machine 'stack))
        (ops (machine 'operations))
        (sorted-instructions '())
        (entry-point-regs '())
        (saved-restored-regs '())
        (reg-sources '()))
    (for-each
     (lambda (inst)
       (set-instruction-execution-proc!
        inst
        (make-execution-procedure
         (instruction-text inst) labels machine
         pc flag stack ops))
       (let ((inst-text (instruction-text inst)))
         ;; add info into the local lists
         (set! sorted-instructions
               (update-sorted-instructions inst-text sorted-instructions))
         (set! entry-point-regs
               (update-entry-point-regs inst-text entry-point-regs))
         (set! saved-restored-regs
               (update-saved-or-restored-regs inst-text saved-restored-regs))
         (set! reg-sources
               (update-reg-sources inst-text reg-sources))))
     insts)
    ;; install lists into the model
    ((machine 'install-sorted-instructions) sorted-instructions)
    ((machine 'install-entry-point-regs) entry-point-regs)
    ((machine 'install-saved-or-restored-regs) saved-restored-regs)
    ((machine 'install-reg-sources) reg-sources)))

These are the results using the fib-machine:

(define fib-machine
  (make-machine
   '(continue n val)
   (list (list '+ +) (list '< <) (list '- -))
   '(
     (assign continue (label fib-done))
     fib-loop
     (test (op <) (reg n) (const 2))
     (branch (label immediate-answer))
     (save continue)
     (assign continue (label afterfib-n-1))
     (save n)
     (assign n (op -) (reg n) (const 1))
     (goto (label fib-loop))
     afterfib-n-1
     (restore n)
     (restore continue)
     (assign n (op -) (reg n) (const 2))
     (save continue)
     (assign continue (label afterfib-n-2))
     (save val)
     (goto (label fib-loop))
     afterfib-n-2
     (assign n (reg val))
     (restore val)
     (restore continue)
     (assign val
             (op +) (reg val) (reg n))
     (goto (reg continue))
     immediate-answer
     (assign val (reg n))
     (goto (reg continue))
     fib-done)))

(set-register-contents! fib-machine 'n 4)
(start fib-machine)
(get-register-contents fib-machine 'val)

(display "\nsorted instructions:\n")
(fib-machine 'sorted-instructions)

(display "\nentry-point-regs:\n")
(fib-machine 'entry-point-regs)

(display "\nsaved-or-restored-regs:\n")
(fib-machine 'saved-or-restored-regs)

(display "\nreg-sources:\n")
(fib-machine 'reg-sources)

done
done
3

sorted instructions:
((assign continue (label fib-done))
 (assign continue (label afterfib-n-1))
 (assign n (op -) (reg n) (const 1))
 (assign n (op -) (reg n) (const 2))
 (assign continue (label afterfib-n-2))
 (assign n (reg val))
 (assign val (op +) (reg val) (reg n))
 (assign val (reg n))
 (branch (label immediate-answer))
 (goto (label fib-loop))
 (goto (reg continue))
 (restore n)
 (restore continue)
 (restore val)
 (save continue)
 (save n)
 (save val)
 (test (op <) (reg n) (const 2)))

entry-point-regs:
(continue)

saved-or-restored-regs:
(continue n val)

reg-sources: ;; manually formatted to make structure explicit
((continue

  ((label fib-done))

  ((label afterfib-n-1))

  ((label afterfib-n-2)))


 (n

  ((op -) (reg n) (const 1))

  ((op -) (reg n) (const 2))

  ((reg val)))


 (val

  ((op +) (reg val) (reg n))

  ((reg n))))

Exercise 5.13

Exercise:

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.

Answer:

Make-machine does not take register names anymore:

(define (make-machine ops controller-text)
  (let ((machine (make-new-machine)))
    ((machine 'install-operations) ops)
    ((machine 'install-instruction-sequence)
     (assemble controller-text machine))
    machine))

I've added a is-reg-allocated? method in the machine — retrievable by passing a message.

(define (make-new-machine)
  (let ((pc (make-register 'pc))
        (flag (make-register 'flag))
        (stack (make-stack))
        (the-instruction-sequence '())
        (sorted-instructions '())
        (entry-point-regs '())
        (saved-restored-regs '())
        (reg-sources '()))
    (let ((the-ops
           (list (list 'initialize-stack
                       (lambda () (stack 'initialize)))))
          (register-table
           (list (list 'pc pc) (list 'flag flag))))
      (define (allocate-register name)
        (if (assoc name register-table)
            (error "Multiply defined register: " name)
            (set! register-table
                  (cons (list name (make-register name))
                        register-table)))
        'register-allocated)
      (define (is-reg-allocated? name) ;; <-------------------------------------------
        (assoc name register-table))
      (define (lookup-register name)
        (let ((val (assoc name register-table)))
          (if val
              (cadr val)
              (error "Unknown register:" name))))
      (define (execute)
        (let ((insts (get-contents pc)))
          (if (null? insts)
              'done
              (begin
                ((instruction-execution-proc (car insts)))
                (execute)))))
      (define (dispatch message)
        (cond ((eq? message 'start)
               (set-contents! pc the-instruction-sequence)
               (execute))
              ((eq? message 'install-instruction-sequence)
               (lambda (seq) (set! the-instruction-sequence seq)))
              ((eq? message 'allocate-register) allocate-register)
              ((eq? message 'is-reg-allocated?) is-reg-allocated?) ;; <---------------------
              ((eq? message 'get-register) lookup-register)
              ((eq? message 'install-operations)
               (lambda (ops) (set! the-ops (append the-ops ops))))
              ((eq? message 'stack) stack)
              ((eq? message 'operations) the-ops)
              ((eq? message 'sorted-instructions) (make-copy sorted-instructions))
              ((eq? message 'install-sorted-instructions)
               (lambda (seq) (set! sorted-instructions seq)))
              ((eq? message 'entry-point-regs) (make-copy entry-point-regs))
              ((eq? message 'install-entry-point-regs)
               (lambda (seq) (set! entry-point-regs seq)))
              ((eq? message 'saved-or-restored-regs) (make-copy saved-restored-regs))
              ((eq? message 'install-saved-or-restored-regs)
               (lambda (seq) (set! saved-restored-regs seq)))
              ((eq? message 'reg-sources) (make-copy reg-sources))
              ((eq? message 'install-reg-sources)
               (lambda (seq) (set! reg-sources seq)))
              (else (error "Unknown request -- MACHINE" message))))
      dispatch)))

Make-execution-procedure is now in charge of allocating the registers.

;; Take register name. If a register with that name hasn't been
;; allocated yet, then allocate it.
(define (allocate-reg-perhaps reg-name machine)
  (or ((machine 'is-reg-allocated?) reg-name)
      ((machine 'allocate-register) reg-name)))

;; Take a list of expressions and a machine model. For each of the
;; expressions, if it is a register expression, then, if the register
;; of that expression has not been allocated yet, then allocate it.
(define (allocate-list-regs-perhaps op-inputs machine)
  (cond ((null? op-inputs)
         'done)
        ((register-exp? (car op-inputs))
         (begin ;; do we need this begin?
           (allocate-reg-perhaps (register-exp-reg (car op-inputs)) machine)
           (allocate-list-regs-perhaps (cdr op-inputs) machine)))
        (else
         (allocate-list-regs-perhaps (cdr op-inputs) machine))))

;; Take assign instruction. Allocate those registers in it which
;; haven't been allocated yet.
(define (allocate-assign-regs-perhaps inst machine)
  (allocate-reg-perhaps (assign-reg-name inst) machine)
  (cond ((register-exp? (car (assign-value-exp inst)))
         (allocate-reg-perhaps (assign-reg-name (car (assign-value-exp inst))) machine))
        ((operation-exp? (assign-value-exp inst))
         (allocate-list-regs-perhaps (operation-exp-operands (assign-value-exp inst)) machine))))

;; Take test instruction. Find registers in it, if any. Allocate those
;; which haven't been allocated yet.
(define (allocate-test-regs-perhaps inst machine)
  (allocate-list-regs-perhaps (test-condition inst) machine))

;; Take save instruction. Allocate the register it refers to, if it
;; hasn't been allocated yet.
(define (allocate-save-reg-perhaps inst machine)
  (allocate-reg-perhaps (cadr inst) machine))

;; Take restore instruction. Allocate the register it refers to, if it
;; hasn't been allocated yet.
(define (allocate-restore-reg-perhaps inst machine)
  (allocate-reg-perhaps (cadr inst) machine))

;; Take perform instruction. Find registers in it, if any. Allocate
;; those which haven't been allocated yet.
(define (allocate-perform-regs-perhaps inst machine)
  (allocate-list-regs-perhaps (perform-action inst)))

(define (make-execution-procedure inst labels machine
                                  pc flag stack ops)
  (cond ((eq? (car inst) 'assign)
         (allocate-assign-regs-perhaps inst machine)
         (make-assign inst machine labels ops pc))
        ((eq? (car inst) 'test)
         (allocate-test-regs-perhaps inst machine)
         (make-test inst machine labels ops flag pc))
        ((eq? (car inst) 'branch)
         (make-branch inst machine labels flag pc))
        ((eq? (car inst) 'goto)
         (make-goto inst machine labels pc))
        ((eq? (car inst) 'save)
         (allocate-save-reg-perhaps inst machine)
         (make-save inst machine stack pc))
        ((eq? (car inst) 'restore)
         (allocate-restore-reg-perhaps inst machine)
         (make-restore inst machine stack pc))
        ((eq? (car inst) 'perform)
         (allocate-perform-regs-perhaps inst machine)
         (make-perform inst machine labels ops pc))
        (else (error "Unknown instruction type -- ASSEMBLE"
                     inst))))

Now a machine can be created without passing the register names. For example:

(define fib-machine
  (make-machine
   (list (list '+ +) (list '< <) (list '- -))
   '(
     (assign continue (label fib-done))
     fib-loop
     (test (op <) (reg n) (const 2))
     (branch (label immediate-answer))
     (save continue)
     (assign continue (label afterfib-n-1))
     (save n)
     (assign n (op -) (reg n) (const 1))
     (goto (label fib-loop))
     afterfib-n-1
     (restore n)
     (restore continue)
     (assign n (op -) (reg n) (const 2))
     (save continue)
     (assign continue (label afterfib-n-2))
     (save val)
     (goto (label fib-loop))
     afterfib-n-2
     (assign n (reg val))
     (restore val)
     (restore continue)
     (assign val
             (op +) (reg val) (reg n))
     (goto (reg continue))
     immediate-answer
     (assign val (reg n))
     (goto (reg continue))
     fib-done)))

SICP 5.2.1 – 5.2.3 A Register-Machine Simulator 2025-02-21 Fri

Exercise 5.8

Exercise 5.9

Exercise 5.10

Exercise 5.11

Answer

Exercise 5.12

Exercise 5.13

SICP 5.2.1 – 5.2.3 A Register-Machine Simulator
2025-02-21 Fri