Chapter 5, Computing with Register Machines
Exercise 5.19
Here is my approach:
- The main idea is to store the
linenoin instructions data-structure. - When installing a breakpoint, compute the lineno of the intruction where breakpoint needs to be installed and then store this lineno in a breakpoint table.
- While executing, stop the execution if the current lineno being executed is inside the breakpoint table.
- For the case when we proceed from a breakpoint, maintain a flag
resume-from-breakpoint?. If this value is set then don’t stop at the breakpoint as well as unset it so that we can stop again at the breakpoint. - To compute lineno from the offset
ngiven by user, we need lables table. Thus, insidemake-machine, now i also installed labels in the machine.
Well, thats it!
I shall first present the main changes and then at the bottom(after example output), will put the complete code.
So, here are the main changes:
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;;NEW PROCEDURES
(define (cancel-all-breakpoints machine)
(machine 'remove-all-breakpoints))
(define (proceed-machine machine)
(machine 'resume-from-breakpoint))
(define (cancel-breakpoint machine label-name offset)
(let ((labels (machine 'labels)))
(let ((label-insts (lookup-label labels label-name)))
(if (null? label-insts)
(error "Invalid request as a breakpoint can't be at the end of instructions!")
((machine 'remove-breakpoint)
(+ (instruction-lineno (car label-insts))
(- offset 1))
(cons label-name
offset))))))
(define (set-breakpoint machine label-name offset)
(let ((labels (machine 'labels)))
(let ((label-insts (lookup-label labels label-name)))
(if (null? label-insts)
(error "Can not define breakpoint at the end of instructions!")
((machine 'install-breakpoint)
(+ (instruction-lineno (car label-insts))
(- offset 1))
(cons label-name
offset))))))
;;;Changes are marked with ;;;
(define (make-new-machine)
(let ((pc (make-register 'pc))
(flag (make-register 'flag))
(stack (make-stack))
(the-instruction-sequence '())
(instruction-count 0)
(trace-on false)
(labels '()) ;;;
(breakpoint-lineno-table '()) ;;;
;;total available instructions
;;not to be confused with instruction-count
;;which contains the number of instructions executed
(total-instructions 0) ;;;
(resume-from-breakpoint? false)) ;;;
(define (reset-instruction-count)
(newline)
(display (list "Total instructions executed: " instruction-count))
(set! instruction-count 0)
'done)
(define (install-breakpoint lineno breakpoint) ;;;
(if (> lineno total-instructions) ;;;
(begin ;;;
(newline) ;;;
(display "Ignoring breakpoint: ") ;;;
(display breakpoint) ;;;
(display " as this offset lies after all the instructions!") ;;;
'ignored) ;;;
(let ((breakpoint-at-lineno (assoc lineno breakpoint-lineno-table))) ;;;
(if breakpoint-at-lineno ;;;
(begin ;;;
(newline) ;;;
(display "Ignoring breakpoint: ") ;;;
(display breakpoint) ;;;
(display " as this point is already set by an existing breakpoint: ") ;;;
(display (cdr breakpoint-at-lineno))) ;;;
(begin
(set! breakpoint-lineno-table (cons (cons lineno breakpoint) ;;;
breakpoint-lineno-table));;;
'installed-breakpoint)))))
(define (remove-breakpoint lineno breakpoint) ;;;
(let ((breakpoint-at-lineno (assoc lineno breakpoint-lineno-table))) ;;;
(if breakpoint-at-lineno
(begin ;;;
(set! breakpoint-lineno-table ;;;
(delete! breakpoint-at-lineno breakpoint-lineno-table)) ;;;
'removed-breakpoint) ;;;
(begin ;;;
(newline) ;;;
(display "Ignoring remove breakpoint since : ") ;;;
(display breakpoint) ;;;
(display " is not present!") ;;;
'not-present)))) ;;;
(let ((the-ops
(list (list 'initialize-stack
(lambda () (stack 'initialize)))
;;**next for monitored stack (as in section 5.2.4)
;; -- comment out if not wanted
(list 'print-stack-statistics
(lambda () (stack 'print-statistics)))
(list 'reset-instruction-count reset-instruction-count)))
(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
(let ((breakpoint-at-lineno (assoc (instruction-lineno (car insts)) ;;;
breakpoint-lineno-table))) ;;;
(if (and (not resume-from-breakpoint?) ;;;
breakpoint-at-lineno) ;;;
(begin (newline) ;;;
(display "Stopped at break-point " ) ;;;
(display (cdr breakpoint-at-lineno)) ;;;
'breakpoint) ;;;
(begin
(if resume-from-breakpoint? ;;;
(set! resume-from-breakpoint? false)) ;;;
(if trace-on
(begin
(for-each (lambda(label)
(newline)
(display "Moving over label: ")
(display label))
(instruction-labels (car insts)))
(newline)
(display "Executing instruction: ")
(display (instruction-text (car insts)))))
((instruction-execution-proc (car insts)))
(set! instruction-count (+ instruction-count 1))
(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)
(set! total-instructions (length 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 'reset-instruction-count) (reset-instruction-count))
((eq? message 'trace-on) (set! trace-on true))
((eq? message 'trace-off) (set! trace-on false))
((eq? message 'registers-trace-on)
(lambda (first-reg . reg-names)
(for-each (lambda(reg-name)
((lookup-register reg-name) 'trace-on))
(cons first-reg reg-names))))
((eq? message 'registers-trace-off)
(lambda (first-reg . reg-names)
(for-each (lambda(reg-name)
((lookup-register reg-name) 'trace-off))
(cons first-reg reg-names))))
((eq? message 'install-labels) ;;;
(lambda (lbls) (set! labels lbls))) ;;;
((eq? message 'labels) labels) ;;;
((eq? message 'install-breakpoint) install-breakpoint) ;;;
((eq? message 'remove-breakpoint) remove-breakpoint) ;;;
((eq? message 'remove-all-breakpoints) ;;;
(set! breakpoint-lineno-table '()) ;;;
'done) ;;;
((eq? message 'resume-from-breakpoint) ;;;
(set! resume-from-breakpoint? true) ;;;
(execute)) ;;;
(else (error "Unknown request -- MACHINE" message))))
dispatch)))
;;There is a small change is extract-labels for linenos
(define (extract-labels lineno text receive)
(if (null? text)
(receive '() '())
(let ((next-inst (car text)))
(if (not (symbol? next-inst))
(set! lineno (+ lineno 1)))
(extract-labels lineno (cdr text)
(lambda (insts labels)
(if (symbol? next-inst)
(begin
(if (not (null? insts))
(add-a-instruction-label! (car insts)
next-inst))
(receive insts
(cons (make-label-entry next-inst
insts)
labels)))
(receive (cons (make-instruction next-inst lineno)
insts)
labels)))))))
;;assemble now also returns labels
(define (assemble controller-text machine)
(extract-labels 0 controller-text
(lambda (insts labels)
(update-insts! insts labels machine)
;;now return the pair of insts and labels
;;instead of just insts.
(cons insts labels)))) ;;;
(define (make-machine register-names ops controller-text)
(let ((machine (make-new-machine)))
(for-each (lambda (register-name)
((machine 'allocate-register) register-name))
register-names)
((machine 'install-operations) ops)
(let ((insts-and-labels (assemble controller-text machine))) ;;;
((machine 'install-instruction-sequence) ;;;
(car insts-and-labels)) ;;;
((machine 'install-labels) ;;;
(cdr insts-and-labels))) ;;;
machine)) ;;;
Test/Output:
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1 ]=>
(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 t (op rem) (reg a) (reg b))
(assign a (reg b))
(assign b (reg t))
(goto (label test-b))
gcd-done)))
(set-breakpoint gcd-machine 'test-b 4)
(set-breakpoint gcd-machine 'test-b 5)
(gcd-machine 'trace-on)
(set-register-contents! gcd-machine 'a 50)
(set-register-contents! gcd-machine 'b 375)
(start gcd-machine)
;Value 53: (register simulator loaded)
1 ]=>
;Value: gcd-machine
1 ]=>
;Value: installed-breakpoint
1 ]=>
;Value: installed-breakpoint
1 ]=>
;Value: #f
1 ]=>
;Value: done
1 ]=>
;Value: done
1 ]=>
Moving over label: test-b
Executing instruction: (test (op =) (reg b) (const 0))
Executing instruction: (branch (label gcd-done))
Executing instruction: (assign t (op rem) (reg a) (reg b))
Stopped at break-point (test-b . 4)
;Value: breakpoint
1 ]=> (proceed-machine gcd-machine)
Executing instruction: (assign a (reg b))
Stopped at break-point (test-b . 5)
;Value: breakpoint
1 ]=> (get-register-contents gcd-machine 'a)
;Value: 375
1 ]=> (get-register-contents gcd-machine 'b)
;Value: 375
1 ]=> (get-register-contents gcd-machine 't)
;Value: 50
1 ]=> (cancel-breakpoint gcd-machine 'test-b 4)
;Value: removed-breakpoint
1 ]=> (proceed-machine gcd-machine)
Executing instruction: (assign b (reg t))
Executing instruction: (goto (label test-b))
Moving over label: test-b
Executing instruction: (test (op =) (reg b) (const 0))
Executing instruction: (branch (label gcd-done))
Executing instruction: (assign t (op rem) (reg a) (reg b))
Executing instruction: (assign a (reg b))
Stopped at break-point (test-b . 5)
;Value: breakpoint
1 ]=> (get-register-contents gcd-machine 't)
;Value: 25
1 ]=> (cancel-all-breakpoints gcd-machine)
;Value: done
1 ]=> (proceed-machine gcd-machine)
Executing instruction: (assign b (reg t))
Executing instruction: (goto (label test-b))
Moving over label: test-b
Executing instruction: (test (op =) (reg b) (const 0))
Executing instruction: (branch (label gcd-done))
Executing instruction: (assign t (op rem) (reg a) (reg b))
Executing instruction: (assign a (reg b))
Executing instruction: (assign b (reg t))
Executing instruction: (goto (label test-b))
Moving over label: test-b
Executing instruction: (test (op =) (reg b) (const 0))
Executing instruction: (branch (label gcd-done))
;Value: done
1 ]=>
(set-breakpoint gcd-machine 'test-b 4)
;Value: installed-breakpoint
1 ]=>
(set-breakpoint gcd-machine 'test-b 4)
Ignoring breakpoint: (test-b . 4) as this point is already set by an existing breakpoint: (test-b . 4)
;Unspecified return value
1 ]=>
(set-breakpoint gcd-machine 'test-b 40)
Ignoring breakpoint: (test-b . 40) as this offset lies after all the instructions!
;Value: ignored
1 ]=>
(set-breakpoint gcd-machine 'gcd-done 4)
;Can not define breakpoint at the end of instructions!
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.
2 error>
Complete code:
This code include changes for exercises 5.14 to 5.19:
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;;;;REGISTER-MACHINE SIMULATOR FROM SECTION 5.2 OF
;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS
;;;;Matches code in ch5.scm
;;;;This file can be loaded into Scheme as a whole.
;;;;Then you can define and simulate machines as shown in section 5.2
;;;**NB** there are two versions of make-stack below.
;;; Choose the monitored or unmonitored one by reordering them to put the
;;; one you want last, or by commenting one of them out.
;;; Also, comment in/out the print-stack-statistics op in make-new-machine
;;; To find this stack code below, look for comments with **
(define (make-machine register-names ops controller-text)
(let ((machine (make-new-machine)))
(for-each (lambda (register-name)
((machine 'allocate-register) register-name))
register-names)
((machine 'install-operations) ops)
(let ((insts-and-labels (assemble controller-text machine)))
((machine 'install-instruction-sequence)
(car insts-and-labels))
((machine 'install-labels)
(cdr insts-and-labels)))
machine))
(define (make-register name)
(let ((contents '*unassigned*)
(trace-on false))
(define (dispatch message)
(cond ((eq? message 'get) contents)
((eq? message 'set)
(lambda (value)
(if trace-on
(begin
(newline)
(display "Changing reg-")
(display name)
(display " old-val: ")
(display contents)
(display " new-val: ")
(display value)))
(set! contents value)))
((eq? message 'trace-on) (set! trace-on true))
((eq? message 'trace-off) (set! trace-on false))
(else
(error "Unknown request -- REGISTER" message))))
dispatch))
(define (get-contents register)
(register 'get))
(define (set-contents! register value)
((register 'set) value))
;;**original (unmonitored) version from section 5.2.1
(define (make-stack)
(let ((s '()))
(define (push x)
(set! s (cons x s)))
(define (pop)
(if (null? s)
(error "Empty stack -- POP")
(let ((top (car s)))
(set! s (cdr s))
top)))
(define (initialize)
(set! s '())
'done)
(define (dispatch message)
(cond ((eq? message 'push) push)
((eq? message 'pop) (pop))
((eq? message 'initialize) (initialize))
(else (error "Unknown request -- STACK"
message))))
dispatch))
(define (pop stack)
(stack 'pop))
(define (push stack value)
((stack 'push) value))
;;**monitored version from section 5.2.4
(define (make-stack)
(let ((s '())
(number-pushes 0)
(max-depth 0)
(current-depth 0))
(define (push x)
(set! s (cons x s))
(set! number-pushes (+ 1 number-pushes))
(set! current-depth (+ 1 current-depth))
(set! max-depth (max current-depth max-depth)))
(define (pop)
(if (null? s)
(error "Empty stack -- POP")
(let ((top (car s)))
(set! s (cdr s))
(set! current-depth (- current-depth 1))
top)))
(define (initialize)
(set! s '())
(set! number-pushes 0)
(set! max-depth 0)
(set! current-depth 0)
'done)
(define (print-statistics)
(newline)
(display (list 'total-pushes '= number-pushes
'maximum-depth '= max-depth)))
(define (dispatch message)
(cond ((eq? message 'push) push)
((eq? message 'pop) (pop))
((eq? message 'initialize) (initialize))
((eq? message 'print-statistics)
(print-statistics))
(else
(error "Unknown request -- STACK" message))))
dispatch))
(define (cancel-all-breakpoints machine)
(machine 'remove-all-breakpoints))
(define (proceed-machine machine)
(machine 'resume-from-breakpoint))
(define (cancel-breakpoint machine label-name offset)
(let ((labels (machine 'labels)))
(let ((label-insts (lookup-label labels label-name)))
(if (null? label-insts)
(error "Invalid request as a breakpoint can't be at the end of instructions!")
((machine 'remove-breakpoint)
(+ (instruction-lineno (car label-insts))
(- offset 1))
(cons label-name
offset))))))
(define (set-breakpoint machine label-name offset)
(let ((labels (machine 'labels)))
(let ((label-insts (lookup-label labels label-name)))
(if (null? label-insts)
(error "Can not define breakpoint at the end of instructions!")
((machine 'install-breakpoint)
(+ (instruction-lineno (car label-insts))
(- offset 1))
(cons label-name
offset))))))
(define (make-new-machine)
(let ((pc (make-register 'pc))
(flag (make-register 'flag))
(stack (make-stack))
(the-instruction-sequence '())
(instruction-count 0)
(trace-on false)
(labels '())
(breakpoint-lineno-table '())
;;total available instructions
;;not to be confused with instruction-count
;;which contains the number of instructions executed
(total-instructions 0)
(resume-from-breakpoint? false))
(define (reset-instruction-count)
(newline)
(display (list "Total instructions executed: " instruction-count))
(set! instruction-count 0)
'done)
(define (install-breakpoint lineno breakpoint)
(if (> lineno total-instructions)
(begin
(newline)
(display "Ignoring breakpoint: ")
(display breakpoint)
(display " as this offset lies after all the instructions!")
'ignored)
(let ((breakpoint-at-lineno (assoc lineno breakpoint-lineno-table)))
(if breakpoint-at-lineno
(begin
(newline)
(display "Ignoring breakpoint: ")
(display breakpoint)
(display " as this point is already set by an existing breakpoint: ")
(display (cdr breakpoint-at-lineno)))
(begin
(set! breakpoint-lineno-table (cons (cons lineno breakpoint)
breakpoint-lineno-table))
'installed-breakpoint)))))
(define (remove-breakpoint lineno breakpoint)
(let ((breakpoint-at-lineno (assoc lineno breakpoint-lineno-table)))
(if breakpoint-at-lineno
(begin
(set! breakpoint-lineno-table
(delete! breakpoint-at-lineno breakpoint-lineno-table))
'removed-breakpoint)
(begin
(newline)
(display "Ignoring remove breakpoint since : ")
(display breakpoint)
(display " is not present!")
'not-present))))
(let ((the-ops
(list (list 'initialize-stack
(lambda () (stack 'initialize)))
;;**next for monitored stack (as in section 5.2.4)
;; -- comment out if not wanted
(list 'print-stack-statistics
(lambda () (stack 'print-statistics)))
(list 'reset-instruction-count reset-instruction-count)))
(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
(let ((breakpoint-at-lineno (assoc (instruction-lineno (car insts))
breakpoint-lineno-table)))
(if (and (not resume-from-breakpoint?)
breakpoint-at-lineno)
(begin (newline)
(display "Stopped at break-point " )
(display (cdr breakpoint-at-lineno))
'breakpoint)
(begin
(if resume-from-breakpoint?
(set! resume-from-breakpoint? false))
(if trace-on
(begin
(for-each (lambda(label)
(newline)
(display "Moving over label: ")
(display label))
(instruction-labels (car insts)))
(newline)
(display "Executing instruction: ")
(display (instruction-text (car insts)))))
((instruction-execution-proc (car insts)))
(set! instruction-count (+ instruction-count 1))
(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)
(set! total-instructions (length 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 'reset-instruction-count) (reset-instruction-count))
((eq? message 'trace-on) (set! trace-on true))
((eq? message 'trace-off) (set! trace-on false))
((eq? message 'registers-trace-on)
(lambda (first-reg . reg-names)
(for-each (lambda(reg-name)
((lookup-register reg-name) 'trace-on))
(cons first-reg reg-names))))
((eq? message 'registers-trace-off)
(lambda (first-reg . reg-names)
(for-each (lambda(reg-name)
((lookup-register reg-name) 'trace-off))
(cons first-reg reg-names))))
((eq? message 'install-labels)
(lambda (lbls) (set! labels lbls)))
((eq? message 'labels) labels)
((eq? message 'install-breakpoint) install-breakpoint)
((eq? message 'remove-breakpoint) remove-breakpoint)
((eq? message 'remove-all-breakpoints)
(set! breakpoint-lineno-table '())
'done)
((eq? message 'resume-from-breakpoint)
(set! resume-from-breakpoint? true)
(execute))
(else (error "Unknown request -- MACHINE" message))))
dispatch)))
(define (start machine)
(machine 'start))
(define (get-register-contents machine register-name)
(get-contents (get-register machine register-name)))
(define (set-register-contents! machine register-name value)
(set-contents! (get-register machine register-name) value)
'done)
(define (get-register machine reg-name)
((machine 'get-register) reg-name))
(define (assemble controller-text machine)
(extract-labels 0 controller-text
(lambda (insts labels)
(update-insts! insts labels machine)
;;now return the pair of insts and labels
;;instead of just insts.
(cons insts labels))))
(define (extract-labels lineno text receive)
(if (null? text)
(receive '() '())
(let ((next-inst (car text)))
(if (not (symbol? next-inst))
(set! lineno (+ lineno 1)))
(extract-labels lineno (cdr text)
(lambda (insts labels)
(if (symbol? next-inst)
(begin
(if (not (null? insts))
(add-a-instruction-label! (car insts)
next-inst))
(receive insts
(cons (make-label-entry next-inst
insts)
labels)))
(receive (cons (make-instruction next-inst lineno)
insts)
labels)))))))
(define (update-insts! insts labels machine)
(let ((pc (get-register machine 'pc))
(flag (get-register machine 'flag))
(stack (machine 'stack))
(ops (machine 'operations)))
(for-each
(lambda (inst)
(set-instruction-execution-proc!
inst
(make-execution-procedure
(instruction-text inst) labels machine
pc flag stack ops)))
insts)))
(define (make-instruction text lineno)
(cons (cons lineno text) (cons '() '())))
(define (instruction-text inst)
(cdar inst))
(define (instruction-lineno inst)
(caar inst))
(define (instruction-execution-proc inst)
(cadr inst))
(define (instruction-labels inst)
(cddr inst))
(define (set-instruction-execution-proc! inst proc)
(set-car! (cdr inst) proc))
(define (set-instruction-labels! inst labels)
(set-cdr! (cdr inst) labels))
(define (add-a-instruction-label! inst label)
(set-instruction-labels! inst (cons label (instruction-labels inst))))
(define (make-label-entry label-name insts)
(cons label-name insts))
(define (lookup-label labels label-name)
(let ((val (assoc label-name labels)))
(if val
(cdr val)
(error "Undefined label -- ASSEMBLE" label-name))))
(define (make-execution-procedure inst labels machine
pc flag stack ops)
(cond ((eq? (car inst) 'assign)
(make-assign inst machine labels ops pc))
((eq? (car inst) 'test)
(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)
(make-save inst machine stack pc))
((eq? (car inst) 'restore)
(make-restore inst machine stack pc))
((eq? (car inst) 'perform)
(make-perform inst machine labels ops pc))
(else (error "Unknown instruction type -- ASSEMBLE"
inst))))
(define (make-assign inst machine labels operations pc)
(let ((target
(get-register machine (assign-reg-name inst)))
(value-exp (assign-value-exp inst)))
(let ((value-proc
(if (operation-exp? value-exp)
(make-operation-exp
value-exp machine labels operations)
(make-primitive-exp
(car value-exp) machine labels))))
(lambda () ; execution procedure for assign
(set-contents! target (value-proc))
(advance-pc pc)))))
(define (assign-reg-name assign-instruction)
(cadr assign-instruction))
(define (assign-value-exp assign-instruction)
(cddr assign-instruction))
(define (advance-pc pc)
(set-contents! pc (cdr (get-contents pc))))
(define (make-test inst machine labels operations flag pc)
(let ((condition (test-condition inst)))
(if (operation-exp? condition)
(let ((condition-proc
(make-operation-exp
condition machine labels operations)))
(lambda ()
(set-contents! flag (condition-proc))
(advance-pc pc)))
(error "Bad TEST instruction -- ASSEMBLE" inst))))
(define (test-condition test-instruction)
(cdr test-instruction))
(define (make-branch inst machine labels flag pc)
(let ((dest (branch-dest inst)))
(if (label-exp? dest)
(let ((insts
(lookup-label labels (label-exp-label dest))))
(lambda ()
(if (get-contents flag)
(set-contents! pc insts)
(advance-pc pc))))
(error "Bad BRANCH instruction -- ASSEMBLE" inst))))
(define (branch-dest branch-instruction)
(cadr branch-instruction))
(define (make-goto inst machine labels pc)
(let ((dest (goto-dest inst)))
(cond ((label-exp? dest)
(let ((insts
(lookup-label labels
(label-exp-label dest))))
(lambda () (set-contents! pc insts))))
((register-exp? dest)
(let ((reg
(get-register machine
(register-exp-reg dest))))
(lambda ()
(set-contents! pc (get-contents reg)))))
(else (error "Bad GOTO instruction -- ASSEMBLE"
inst)))))
(define (goto-dest goto-instruction)
(cadr goto-instruction))
(define (make-save inst machine stack pc)
(let ((reg (get-register machine
(stack-inst-reg-name inst))))
(lambda ()
(push stack (get-contents reg))
(advance-pc pc))))
(define (make-restore inst machine stack pc)
(let ((reg (get-register machine
(stack-inst-reg-name inst))))
(lambda ()
(set-contents! reg (pop stack))
(advance-pc pc))))
(define (stack-inst-reg-name stack-instruction)
(cadr stack-instruction))
(define (make-perform inst machine labels operations pc)
(let ((action (perform-action inst)))
(if (operation-exp? action)
(let ((action-proc
(make-operation-exp
action machine labels operations)))
(lambda ()
(action-proc)
(advance-pc pc)))
(error "Bad PERFORM instruction -- ASSEMBLE" inst))))
(define (perform-action inst) (cdr inst))
(define (make-primitive-exp exp machine labels)
(cond ((constant-exp? exp)
(let ((c (constant-exp-value exp)))
(lambda () c)))
((label-exp? exp)
(let ((insts
(lookup-label labels
(label-exp-label exp))))
(lambda () insts)))
((register-exp? exp)
(let ((r (get-register machine
(register-exp-reg exp))))
(lambda () (get-contents r))))
(else
(error "Unknown expression type -- ASSEMBLE" exp))))
(define (register-exp? exp) (tagged-list? exp 'reg))
(define (register-exp-reg exp) (cadr exp))
(define (constant-exp? exp) (tagged-list? exp 'const))
(define (constant-exp-value exp) (cadr exp))
(define (label-exp? exp) (tagged-list? exp 'label))
(define (label-exp-label exp) (cadr exp))
(define (make-operation-exp exp machine labels operations)
(let ((op (lookup-prim (operation-exp-op exp) operations))
(aprocs
(map (lambda (e)
(make-primitive-exp e machine labels))
(operation-exp-operands exp))))
(lambda ()
(apply op (map (lambda (p) (p)) aprocs)))))
(define (operation-exp? exp)
(and (pair? exp) (tagged-list? (car exp) 'op)))
(define (operation-exp-op operation-exp)
(cadr (car operation-exp)))
(define (operation-exp-operands operation-exp)
(cdr operation-exp))
(define (lookup-prim symbol operations)
(let ((val (assoc symbol operations)))
(if val
(cadr val)
(error "Unknown operation -- ASSEMBLE" symbol))))
;; from 4.1
(define (tagged-list? exp tag)
(if (pair? exp)
(eq? (car exp) tag)
false))
'(REGISTER SIMULATOR LOADED)