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#+TITLE: 5.1 - Designing Register Machines
#+STARTUP: indent
#+OPTIONS: num:nil
Building a lisp machine in lisp to execute simple instructions modeled
in lisp to evaluate, execute, and eventually compile our lisp written
written in lisp.
* A Language for Describing Register Machines
* Abstraction in Machine Design
* Subroutines
* Using a Stack to Implement Recursion
* Instruction Summary

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#+TITLE: 5.2 - A Register-Machine Simulator
#+STARTUP: indent
#+OPTIONS: num:nil
#+PROPERTY: header-args:scheme :tangle yes
#+name: gcd-machine
#+BEGIN_SRC scheme :tangle no
(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)))
#+END_SRC
* COMMENT Set up source file
#+BEGIN_SRC scheme :tangle yes
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 5.2 - A Register-Machine Simulator
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(define (tagged-list? exp tag)
(if (pair? exp)
(eq? (car exp) tag)
false))
#+END_SRC
* The Machine Model
#+BEGIN_SRC scheme
(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)
((machine 'install-instruction-sequence)
(assemble controller-text machine))
machine))
#+END_SRC
** Registers
#+BEGIN_SRC scheme
(define (make-register name)
(let ((contents '*unassigned*))
(define (dispatch message)
(cond ((eq? message 'get) contents)
((eq? message 'set)
(lambda (value)
(set! contents value)))
(else
(error "Unknown request:
REGISTER"
message))))
dispatch))
(define (get-contents register)
(register 'get))
(define (set-contents! register value)
((register 'set) value))
#+END_SRC
** The stack
#+BEGIN_SRC scheme
(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))
#+END_SRC
** The basic machine
#+BEGIN_SRC scheme
(define (make-new-machine)
(let ((pc (make-register 'pc))
(flag (make-register 'flag))
(stack (make-stack))
(the-instruction-sequence '()))
(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)
(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))
#+END_SRC
* The Assembler
#+BEGIN_SRC scheme
(define (assemble controller-text machine)
(extract-labels controller-text
(lambda (insts labels)
(update-insts! insts labels machine)
insts)))
(define (extract-labels text receive)
(if (null? text)
(receive '() '())
(extract-labels
(cdr text)
(lambda (insts labels)
(let ((next-inst (car text)))
(if (symbol? next-inst)
(receive
insts
(cons
(make-label-entry
next-inst
insts)
labels))
(receive
(cons (make-instruction
next-inst)
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)
(cons text '()))
(define (instruction-text inst) (car inst))
(define (instruction-execution-proc inst)
(cdr inst))
(define (set-instruction-execution-proc!
inst
proc)
(set-cdr! inst proc))
(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))))
#+END_SRC
** Exercise 5.8
The following register-machine code is ambiguous,
because the label `here' is defined more than once:
#+BEGIN_EXAMPLE
start
(goto (label here))
here
(assign a (const 3))
(goto (label there))
here
(assign a (const 4))
(goto (label there))
there
#+END_EXAMPLE
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.
----------------------------------------------------------------------
The contents of register =a= will always be =3=. Labels are looked up
using =assoc=, which will always return the first match in the lookup
table.
#+BEGIN_SRC scheme
(define (extract-labels text receive)
(if (null? text)
(receive '() '())
(extract-labels
(cdr text)
(lambda (insts labels)
(let ((next-inst (car text)))
(if (symbol? next-inst)
(if (assoc next-inst labels)
(error "Duplicate label. AVENGERS, ASSEMBLE!")
(receive
insts
(cons
(make-label-entry
next-inst
insts)
labels)))
(receive
(cons (make-instruction
next-inst)
insts)
labels)))))))
#+END_SRC
* Generating Execution Procedures for Instructions
#+BEGIN_SRC scheme
(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))))
#+END_SRC
** =Assign= instructions
#+BEGIN_SRC scheme
(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))))
#+END_SRC
** =Test=, =branch= and =goto= instructions
#+BEGIN_SRC scheme
(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))
#+END_SRC
** Other instructions
#+BEGIN_SRC scheme
(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))
#+END_SRC
** Execution procedures for subexpressions
#+BEGIN_SRC scheme
(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))))
#+END_SRC
* Monitoring Machine Performance
#+BEGIN_SRC scheme
(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))
#+END_SRC