Cleaner cps interface

reference/runtime.c

@@ -1,135 +0,0 @@
-#include "runtime.h"
-
-box_t cons(box_t car, box_t cdr) {
-  cons_t* kons = alloc();
-  kons->car = car;
-  kons->cdr = cdr;
-  box_t box;
-  box.type = CONS;
-  box.cons = kons;
-  return box;
-}
-
-void chomp_ws() {
-  char c = getchar();
-  for(;;) {
-    if (c == ' ' || c == '\t' || c == '\n' || c == '\r') {
-      c = getchar();
-    } else {
-      ungetchar(c);
-      break;
-    }
-  }
-}
-
-box_t read_integer() {
-  char buf[256];
-  size_t i = 0;
-  char next = getchar();
-  while(isdigit(next)) {
-    buf[i] = next;
-    next = getchar();
-    i++;
-  }
-  ungetchar(next);
-  buf[i] = 0;
-  box_t res;
-  res.type = INTEGER;
-  res.integer = atoi(buf);
-  chomp_ws();
-  return res;
-}
-
-box_t read_symbol() {
-  char buf[256];
-  size_t i = 0;
-  char next = getchar();
-  while (isalpha(next)) { // Totally insufficent but we'll look into it later
-    buf[i] = next;
-    next = getchar();
-    i++;
-  }
-  ungetchar(next);
-  buf[i] = 0;
-  box_t res;
-  char *str = strndup(buf, i);
-  res.type = SYMBOL;
-  res.symbol = str;
-  chomp_ws(); // Clean up whitespace afterwards
-  return res;
-}
-
-box_t read_list() {
-  char c = getchar();
-  box_t val;
-  if (c == ')') {
-    chomp_ws();
-    val.type = CONS;
-    val.cons = the_empty_list;
-    return val;
-  } else {
-    ungetchar(c);
-    box_t car = read();
-    box_t cdr = read_list();
-    return cons(car, cdr);
-  }
-}
-    
-box_t read() {
-  chomp_ws();
-  char next = getchar();
-  if (next == '(') {
-    chomp_ws();
-    return read_list();
-  } else if (isdigit(next)) {
-    ungetchar(next);
-    return read_integer();
-  } else if (isalpha(next) || ispunct(next)) {
-    ungetchar(next);
-    return read_symbol();
-  } else if (next == EOF) {
-    box_t bye;
-    bye.type = BYE;
-    return bye;
-  } else {
-    fprintf(stderr, "Bad input");
-    exit(1);
-  }
-}
-
-void scm_print_cons(box_t exp) {
-  printf("(");
-  while(exp.type = CONS && exp.cons != the_empty_list) {
-    scm_print(exp.cons->car);
-    if (exp.cons->cdr.cons != the_empty_list) {
-      printf(" ");
-    }
-    exp = exp.cons->cdr;
-  }
-  if (exp.cons == the_empty_list) {
-    printf(")");
-  } else {
-    printf(". ");
-    scm_print(exp);
-    printf(")");
-  }
-}
-
-void scm_print(box_t exp) {
-  switch (exp.type) {
-  case INTEGER:
-    printf("%d", exp.integer);
-    break;
-  case SYMBOL:
-    printf("%s", exp.symbol);
-    break;
-  case CONS:
-    scm_print_cons(exp);
-    break;
-  }
-}
-    
-
-box_t eval(box_t exp) {
-  return exp;
-}

reference/runtime.h

@@ -1,17 +0,0 @@
-#ifndef _RUNTIME_H_
-#define _RUNTIME_H_
-
-#include <stdlib.h>
-#include <stdio.h>
-#include <ctype.h>
-#include <string.h>
-#include "common.h"
-#include "gc.h"
-
-#define ungetchar(X) ungetc(X, stdin)
-
-box_t read();
-box_t eval(box_t);
-void scm_print(box_t);
-
-#endif // _RUNTIME_H_

reference/scheme.c

@@ -1,25 +0,0 @@
-
-#include "gc.h"
-#include "runtime.h"
-
-void prompt() {
-  printf("> ");
-}
-
-int repl() {
-  box_t res;
-  do {
-    prompt();
-    box_t in = read();
-    res = eval(in);
-    scm_print(res);
-    printf("\n");
-  } while (res.type != BYE);
-
-  return 0;
-}
-
-int main() {
-  gc_init();
-  return repl();
-}

scmvm.scm

@@ -1,9 +1,7 @@
 (define-module (scmvm)
-  #:use-module (scmvm vm)
-  #:use-module (scmvm assembler)
-  #:use-module (scmvm debugger)
   #:use-module (ice-9 ports)
-  #:export (read-all-instructions instructions-from-file))
+  #:export (read-all-instructions
+	    instructions-from-file))
 
 (define (read-all-instructions)
   (let ([inst (read)])

scmvm/assembler.scm

@@ -1,88 +1,82 @@
 (define-module (scmvm assembler)
-  #:use-module (srfi srfi-1)
   #:use-module (scmvm vm)
+  #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-9)
   #:use-module (rnrs bytevectors)
-  #:use-module (rnrs io ports)
-  #:use-module ((scheme base)
-                #:select (write-u8 write-bytevector))
-  #:export (assemble))
+  #:use-module ((scheme base) #:select (write-bytevector))
+  #:export ((make-assembler* . make-assembler)
+	    assembler?
+	    assembler-pos assembler-pos-set!
+	    assembler-buf
+	    assembler-labels
+	    emit-label
+	    emit-instruction
+	    emit-literal
+	    emit-reference
+	    finalize-references
+	    assembler-dump-program))
 
-(define *aliases*
-  '((if . branch)))
+(define (make-label) (cons #f '()))
 
-(define (or-alias inst)
-  (or (assq-ref *aliases* inst) inst))
+(define-record-type <assembler>
+  (make-assembler pos buf labels)
+  assembler?
+  (pos assembler-pos assembler-pos-set!)
+  (buf assembler-buf assembler-buf-set!)
+  (labels assembler-labels))
 
-(define (lookup-instruction inst)
-  (define inst-obj (assq (or-alias inst) *instruction-set*))
-  (if inst-obj
-      inst-obj
-      (error (format #f "could not find instruction ~a" inst))))
+(define (make-assembler*)
+  (make-assembler 0 (make-bytevector 1024) (make-hash-table)))
 
-(define label? (compose not pair?))
-(define (variable? x)
-  (and (pair? x) (eq? (car x) 'variable)))
-(define (ref? x)
-  (and (pair? x) (eq? (car x) 'ref)))
-(define (set!? x)
-  (and (pair? x) (eq? (car x) 'set!)))
-(define (instruction? x)
-  (and (not (label? x))
-       (not (variable? x))
-       (not (ref? x))
-       (not (set!? x))))
-(define (instruction-size inst)
-  (case (car inst)
-    [(push) 5]
-    [(ref set!) 6]
-    [else 1]))
+(define (assembler-buf-grow! asm)
+  (let ([buf (make-bytevector (ash (bytevector-length (assembler-buf asm)) -1))])
+    (bytevector-copy! (assembler-buf asm) 0 buf 0 (bytevector-length (assembler-buf asm)))
+    (assembler-buf-set! asm buf)))
 
-(define (label-pass instructions address)
-  (cond
-   [(null? instructions) '()]
-   [(label? (car instructions))
-    (acons (car instructions) address (label-pass (cdr instructions) address))]
-   [(variable? (car instructions))
-    (acons (cadar instructions) address (label-pass (cdr instructions) (+ address 4)))]
-   [else
-    (label-pass (cdr instructions) (+ address (instruction-size (car instructions))))]))
+(define* (write-word word asm #:optional (pos (assembler-pos asm)))
+  (when (> (+ pos 4) (bytevector-length (assembler-buf asm)))
+    (assembler-buf-grow! asm))
+  (bytevector-u32-native-set! (assembler-buf asm) pos word))
 
-(define (write-word word)
-  (define bv (make-bytevector 4))
-  (bytevector-u32-native-set! bv 0 word)
-  (write-bytevector bv))
+(define* (write-byte byte asm #:optional (pos (assembler-pos asm)))
+  (when (> (+ pos 1) (bytevector-length (assembler-buf asm)))
+    (assembler-buf-grow! asm))
+  (bytevector-u8-set! (assembler-buf asm) pos byte))
 
-(define (assembly-pass seq labels)
-  (cond
-   [(null? seq) '()]
-   [(label? (car seq)) (assembly-pass (cdr seq) labels)]
-   [(variable? (car seq))
-    (write-word (caddar seq))
-    (assembly-pass (cdr seq) labels)]
-   [(ref? (car seq))
-    (write-u8 (cadr (lookup-instruction 'push)))
-    (write-word (assq-ref labels (cadar seq)))
-    (write-u8 (cadr (lookup-instruction '@)))
-    (assembly-pass (cdr seq) labels)]
-   [(set!? (car seq))
-    (write-u8 (cadr (lookup-instruction 'push)))
-    (write-word (assq-ref labels (cadar seq)))
-    (write-u8 (cadr (lookup-instruction '!)))
-    (assembly-pass (cdr seq) labels)]
-   [else
-    (let* ([inst (car seq)]
-           [inst-obj (lookup-instruction (car inst))])
-      (write-u8 (instruction-code inst-obj))
-      (when (eq? (car inst) 'push)
-        (if (number? (cadr inst))
-            (write-word (cadr inst))
-            (let ([address (assq-ref labels (cadr inst))])
-              (if address
-                  (write-word (assq-ref labels (cadr inst)))
-                  (error (format #f "Could not find label ~a" (cadr inst)))))))
-      (assembly-pass (cdr seq) labels))]))
+(define (assembler-label-add-reference asm name addr)
+  (when (not (hash-ref (assembler-labels asm) name))
+    (hash-set! (assembler-labels asm) name (make-label)))
+  (let ([label (hash-ref (assembler-labels asm) name)])
+    (set-cdr! label (cons addr (cdr label)))))
 
-(define (assemble instructions port)
-  (define labels (label-pass instructions 1))
-  (with-output-to-port port (lambda () (assembly-pass instructions labels)))
-  labels)
+(define (assembler-label-add-value asm name val)
+  (when (not (hash-ref (assembler-labels asm) name))
+    (hash-set! (assembler-labels asm) name (make-label)))
+  ;; 1+ to fudge for null pointers
+  (set-car! (hash-ref (assembler-labels asm) name) (1+ val)))
+
+(define (emit-label asm name)
+  (assembler-label-add-value asm name (assembler-pos asm)))
+
+(define (emit-instruction asm inst)
+  (let ([inst-object (assq inst *instruction-set*)])
+    (write-byte (instruction-code inst-object) asm)
+    (assembler-pos-set! asm (+ (assembler-pos asm) 1))))
+
+(define (emit-literal asm val)
+  (write-word val asm)
+  (assembler-pos-set! asm (+ (assembler-pos asm) 4)))
+
+(define (emit-reference asm name)
+  (assembler-label-add-reference asm name (assembler-pos asm))
+  (assembler-pos-set! asm (+ (assembler-pos asm) 4)))
+
+(define (finalize-references asm)
+  (define (install-location _name label)
+    (for-each
+     (cute write-word (car label) asm <>)
+     (cdr label)))
+  (hash-for-each install-location (assembler-labels asm)))
+
+(define (assembler-dump-program asm port)
+  (write-bytevector (assembler-buf asm) port 0 (assembler-pos asm)))

scmvm/debugger.scm

@@ -15,20 +15,20 @@
             debugger-step debugger-continue))
 
 (define-record-type <debugger>
-  (make-debugger vm source breakpoints continuation stepping)
+  (make-debugger vm asm breakpoints continuation stepping)
   debugger?
   (vm debugger-vm)
-  (source debugger-source)
+  (asm debugger-asm)
   (breakpoints debugger-breakpoints)
   (continuation debugger-continuation debugger-continuation-set!)
   (stepping debugger-stepping? debugger-stepping-set!))
 
-(define (make-breakpoints labels)
+(define (make-breakpoints asm)
   (define the-breakpoints '())
   (define (->index index/label)
     (if (number? index/label)
         index/label
-        (assq-ref labels index/label)))
+        (car (hash-ref (assembler-labels asm) index/label))))
   (define-syntax-rule (ilambda (i) e ...)
     (lambda (v) (let ([i (->index v)]) e ...)))
   (match-lambda
@@ -43,12 +43,12 @@
     ['ref
      (ilambda (i) (assq-ref the-breakpoints i))]))
 
-(define (make-debugger* source)
-  (define-values (prgm symbols)
+(define (make-debugger* asm)
+  (define prgm
     (call-with-values open-bytevector-output-port
       (lambda (port get-bv)
-        (define symbols (assemble source port))
-        (values (get-bv) symbols))))
+	(assembler-dump-program asm port)
+	(get-bv))))
   (define the-debugger #f)
   (define (debug)
     (shift k
@@ -58,7 +58,7 @@
          (k))))
   (define vm (make-vm #:debugger debug))
   (vm-load-program! vm prgm)
-  (set! the-debugger (make-debugger vm source (make-breakpoints symbols) #f #f))
+  (set! the-debugger (make-debugger vm asm (make-breakpoints asm) #f #f))
   (debugger-breakpoint-add! the-debugger 1)
   (reset (run-vm vm))
   the-debugger)

scmvm/language/assembly.scm

@@ -0,0 +1,59 @@
+(define-module (scmvm language assembly)
+  #:use-module (scmvm vm)
+  #:use-module (scmvm assembler)
+  #:use-module (srfi srfi-1)
+  #:use-module ((scheme base) #:select (write-bytevector))
+  #:export (assemble assemble-instructions))
+
+(define *aliases*
+  '((if . branch)))
+
+(define (or-alias inst)
+  (or (assq-ref *aliases* inst) inst))
+
+(define (lookup-instruction inst)
+  (define inst-obj (assq (or-alias inst) *instruction-set*))
+  (or inst-obj
+      (error (format #f "could not find instruction ~a" inst))))
+
+(define label? (negate pair?))
+(define (variable? x)
+  (and (pair? x) (eq? (car x) 'variable)))
+(define (ref? x)
+  (and (pair? x) (eq? (car x) 'ref)))
+(define (set!? x)
+  (and (pair? x) (eq? (car x) 'set!)))
+(define (push? x)
+  (and (pair? x) (eq? (car x) 'push)))
+
+(define (emit-push asm v)
+  (emit-instruction asm 'push)
+  (if (number? v)
+      (emit-literal asm v)
+      (emit-reference asm v)))
+
+(define (assemble-instructions asm inst-seq)
+  (when (pair? inst-seq)
+    (cond
+     [(label? (car inst-seq))
+      (emit-label asm (car inst-seq))]
+     [(variable? (car inst-seq))
+      (emit-label asm (second (car inst-seq)))
+      (emit-literal asm (third (car inst-seq)))]
+     [(ref? (car inst-seq))
+      (emit-push asm (second (car inst-seq)))
+      (emit-instruction asm '@)]
+     [(set!? (car inst-seq))
+      (emit-push asm (second (car inst-seq)))
+      (emit-instruction asm '!)]
+     [(push? (car inst-seq))
+      (emit-push asm (second (car inst-seq)))]
+     [else
+      (emit-instruction asm (instruction-name (lookup-instruction (first (car inst-seq)))))])
+    (assemble-instructions asm (cdr inst-seq))))
+
+(define (assemble instructions port)
+  (define asm (make-assembler))
+  (assemble-instructions asm instructions)
+  (finalize-references asm)
+  (assembler-dump-program asm port))

scmvm/language/scheme.scm

@@ -0,0 +1,194 @@
+(define-module (scmvm language scheme)
+  #:use-module (scmvm assembler)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:use-module (ice-9 match)
+  #:export (compile decompile ir-convert))
+
+;; Scheme compiler
+;; Scheme subset we're targeting
+;; <prgm> ::= <top> ...
+;; <top>  ::= <def> | <exp>
+;; <def>  ::= (define <var> <exp>)
+;;          | (define (<var> <var> ...) <exp> ...)
+;; <exp>  ::= (lambda (<var> ...) <exp> ...)
+;;          | (if <exp> <exp> <exp>)
+;;          | (<exp> <exp> ...)
+;;          | (let ((<var> <exp>) ...) <exp> ...)
+;;          | (begin <exp> ...)
+;;          | <num> | <sym> | <var> | #t | #f
+
+(define (atomic? x)
+  (or (number? x)
+      (symbol? x)
+      (boolean? x)))
+
+(define (primitive? x)
+  (memq x '(+ - * / = < > <= >=)))
+
+(define-syntax-rule (define-cps-loop name unit)
+  (define (name v* k)
+    (if (null? v*)
+	(k '())
+	(unit (car v*)
+	      (lambda (t)
+		(name (cdr v*)
+		      (lambda (t*)
+			(k (cons t t*)))))))))
+
+;; Desugaring
+;; Transforms to simplify the language
+;; - lambdas and lets can only have 1 expression in body position
+;; - define is always simple binds, function defs bind a lambda
+;; <prgm> ::= <top> ...
+;; <top>  ::= <def> | <exp>
+;; <def>  ::= (define <var> <exp>)
+;; <exp>  ::= (lambda (<var> ...) <exp>)
+;;          | (if <exp> <exp> <exp>)
+;;          | (<exp> <exp> ...)
+;;          | (let ((<var> <exp>) ...) <exp>)
+;;          | (begin <exp> ...)
+;;          | <num> | <sym> | <var> | #t | #f
+
+(define (desugar-prgm prgm)
+  (map (lambda (top)
+	 (if (and (pair? top) (eq? (car top) 'define))
+	     (desugar-define top)
+	     (desugar-exp top)))
+       prgm))
+
+(define (desugar-define def)
+  (match def
+    [`(define ,(name params ...) . ,e*)
+     `(define ,name ,(desugar-exp `(lambda ,params ,@e*)))]
+    [`(define ,name ,exp)
+     `(define ,name ,(desugar-exp exp))]))
+
+(define (desugar-exp exp)
+  (match exp
+    [`(lambda ,params . ,body)
+     `(lambda ,params ,(desugar-body body))]
+    [`(if ,exp1 ,exp2 ,exp3)
+     `(if ,(desugar-exp exp1) ,(desugar-exp exp2) ,(desugar-exp exp3))]
+    [`(,f . ,args)
+     `(,(desugar-exp f) ,@(map desugar-exp args))]
+    [`(let ,((v* e*) ...) . ,body)
+     `(let (,(map (lambda (v e) `(,v ,(desugar-exp e))) v* e*))
+	,(desugar-body body))]
+    [`(begin . ,body) (desugar-body body)]
+    [(? atomic?) exp]))
+
+(define (desugar-body body)
+  (match body
+    ['() '()]
+    [(e) (desugar-exp e)]
+    [(e* ...) `(begin ,@(map desugar-exp e*))]))
+
+
+;; CPS conversion
+;; Re-structure the program into "Continuation Passing Style", where non-atomic
+;; expressions must pass their continuations explicitly, changing to a very
+;; "lambda-like" format
+;; - begin expressions are decomposed
+;; - let expressions are transformed into closed function applications
+;; <prgm> ::= <top> ...
+;; <top>  ::= <def> | <exp>
+;; <def>  ::= (define <var> <exp>)
+;; <exp>  ::= <aexp>
+;;          | <cexp>
+;; <cexp> ::= (<aexp> <aexp> ...)
+;;          | (if <aexp> <cexp> <cexp>)
+;;          | (set-then! <var> <aexp> <cexp>)
+;;          | (define-then! <var> <aexp> <cexp>)
+;; <aexp> ::= (lambda (<var> ...) exp)
+;;          | <num> | <var> | #t | #f
+;; 
+;; We choose a hybrid transformation based on https://matt.might.net/articles/cps-conversion/
+
+(define undefined-value (make-symbol "undefined"))
+
+(define (M expr)
+  ;; M dispatches to the appropriate transformer
+  (match expr
+    [('lambda (var ...) e)
+     (let ([$k (gensym "$k")])
+       `(lambda (,@var ,$k) ,(T-c e $k)))]
+    [(? atomic?) expr]))
+
+(define (T-k expr k)
+  ;; T-k takes an explicit continuation and calls it when done
+  ;; As an invariant, T-k cannot nest a T-c call directly
+  (match expr
+    [`(lambda . ,_) (k (M expr))]
+    [ (? atomic?)   (k (M expr))]
+    [ ('define v e) (T-k `(define-then! ,v ,e) k)]
+    [ ('begin e) (T-k e k)]
+    [ ('begin e e* ...)
+      (T-k e (lambda _ (T-k `(begin ,@e*) k)))]
+    [ ('let ([v* e*] ...) body)
+      (T-k `((lambda (,@v*) ,body) ,@e*) k)]
+    [ ('if exp1 exp2 exp3)
+      (T-k exp1 (lambda ($exp1)
+                  `(if ,$exp1
+                       ,(T-k exp2 k)
+                       ,(T-k exp3 k))))]
+    [ ('set! var expr)
+      (T-k expr (lambda ($expr)
+                  `(set-then! ,var ,$expr ,(k undefined-value))))]
+    [((? primitive? f) e* ...)
+     (let* ([$rv (gensym "$rv")]
+            [cont `(lambda (,$rv) ,(k $rv))])
+       (T*-k e* (lambda ($e*)
+                  `((cps ,f) ,@$e* ,cont))))]
+    [(f e* ...)
+     (let* ([$rv (gensym "$rv")]
+	    [cont `(lambda (,$rv) ,(k $rv))])
+       (T-k f (lambda ($f)
+		(T*-k e* (lambda ($e*)
+			   `(,$f ,@$e* ,cont))))))]))
+
+(define (T-c expr c)
+  ;; T-c takes a symbolic continuation, and uses it to construct CPS
+  (match expr
+    [`(lambda . ,_) `(,c ,(M expr))]
+    [ (? atomic?)   `(,c ,(M expr))]
+    [ ('define v e) (T-c `(define-then! ,v ,e) c)]
+    [ ('begin e) (T-c e c)]
+    [ ('begin e e* ...)
+      (T-k e (lambda _ (T-c `(begin ,@e*) c)))]
+    [ ('let ([v* e*] ...) body)
+      (T-c `((lambda (,@v*) ,body) ,@e*) c)]
+    [ ('if exp1 exp2 exp3)
+      (let ([$k (gensym "$k")]) ;; Bind cont to avoid blow up
+        `((lambda (,$k)
+            ,(T-k exp1 (lambda (aexp)
+                         `(if ,aexp
+                              ,(T-c exp2 $k)
+                              ,(T-c exp3 $k)))))
+          ,c))]
+    [ ('set! var expr)
+      (T-k expr (lambda ($expr)
+                  `(set-then! ,var ,$expr (,c ,undefined-value))))]
+    [ ((? primitive? f) e* ...)
+      (T*-k e* (lambda ($e*)
+                 `((cps ,f) ,@$e* ,c)))]
+    [ (f e* ...)
+      (T-k f (lambda ($f)
+	       (T*-k e* (lambda ($e*)
+			  `(,$f ,@$e* ,c)))))]))
+
+(define (cps-convert-prgm prgm)
+  (T-c `(begin ,@prgm) 'ktail))
+
+(define-cps-loop T*-k T-k)
+
+(define (ir-convert prgm)
+  (cps-convert-prgm (desugar-prgm prgm)))
+
+;; Useful for testing
+;; (define (cps prim)
+;;   (lambda vars
+;;     (let* ([rev (reverse vars)]
+;;            [k (car rev)]
+;;            [args (reverse (cdr rev))])
+;;       (k (apply prim args)))))

scmvm/vm.scm

@@ -13,7 +13,7 @@
 		       vm-data-stack vm-ret-stack
 		       vm-debugger vm-debugger-set!
 		       vm-pc vm-pc-set!
-		       *instruction-set* instruction-type instruction-code))
+		       *instruction-set* instruction-name instruction-code))
 
 ;;; Data Structures
 (define *stack-size* 512)
@@ -58,6 +58,9 @@
       [(->list)
        (lambda ()
 	 (reverse-vector->list the-stack 0 top))]
+      [(set!)
+       (lambda (k obj)
+         (vector-set! the-stack k obj))]
       [else (error "stack dispatch unknown value")])))
 
 (define (push stack v)
@@ -81,6 +84,9 @@
 (define (stack->list stack)
   ((stack '->list)))
 
+(define (stack-set! stack k obj)
+  ((stack 'set!) k obj))
+
 
 ;;; IO
 (define (read-word)
@@ -116,8 +122,10 @@
     (rot    #x18)
     (over   #x19)
     (not    #x1a)
+    (set!    #x1b)
     (bye    #xff)))
 
+(define instruction-name car)
 (define instruction-code cadr)
 
 (define (op-lookup code)
@@ -242,6 +250,11 @@
        (push data-stack b)
        (push data-stack a)
        (push data-stack b))]
+    [(set!)
+     ;; use let* to induce an order of evaluation
+     (let* ([idx (pop data-stack)]
+            [obj (pop data-stack)])
+       (stack-set! data-stack idx obj))]
     [(bye) (set! exit? #t)])
   (when (not exit?)
     (run-vm vm)))

scratch.scm

@@ -1,8 +0,0 @@
-(use-modules (scmvm)
-	     (scmvm vm)
-	     (scmvm assembler)
-	     (scmvm debugger))
-
-(define my-instructions (instructions-from-file "./asm/runtime.scm"))
-(define my-debugger (make-debugger my-instructions))
-(define my-vm (debugger-vm my-debugger))

tests.scm

@@ -1,14 +1,16 @@
 (use-modules (d- test)
-             (scmvm assembler)
+	     (scmvm assembler)
              (scmvm vm)
 	     (scmvm debugger)
+	     (scmvm language assembly)
+	     (scmvm language scheme)
              (rnrs bytevectors)
              (rnrs io ports)
              ((scheme base)
               #:select (open-output-bytevector get-output-bytevector)))
 
 ;;; Data
-(define adder-program-asm
+(define adder-program-assembly
   '((variable result 0)
     (push 1)
     (push 2)
@@ -17,7 +19,7 @@
     (!)
     (bye)))
 
-(define fib-program-asm
+(define fib-program-assembly
   '(  (variable result 0)
       (ref result)
       (push fib)
@@ -103,17 +105,16 @@
        #x02          ; Store fib(n)
        #xff          ; Exit program
        ))
-
 
 ;;; Tests
-(define-test-suite "assembler"
+(define-test-suite "assembly"
   (define-test "adder"
     (define out (open-output-bytevector))
-    (assemble adder-program-asm out)
+    (assemble adder-program-assembly out)
     (assert-equal adder-program-bytecode (get-output-bytevector out)))
   (define-test "fib"
     (define out (open-output-bytevector))
-    (assemble fib-program-asm out)
+    (assemble fib-program-assembly out)
     (assert-equal fib-program-bytecode (get-output-bytevector out))))
 
 (define-test-suite "vm"
@@ -133,6 +134,9 @@
 
 (define-test-suite "debugger"
   (define-test "modify-running-program"
+    (define fib-program-asm (make-assembler))
+    (assemble-instructions fib-program-asm fib-program-assembly)
+    (finalize-references fib-program-asm)
     (define my-debugger (make-debugger fib-program-asm))
     (define my-vm (debugger-vm my-debugger))
     (define my-data (vm-data-stack my-vm))
@@ -146,6 +150,9 @@
     (debugger-continue my-debugger)
     (assert-equal 1 (vm-memory-ref my-vm 1)))
   (define-test "stepping"
+    (define fib-program-asm (make-assembler))
+    (assemble-instructions fib-program-asm fib-program-assembly)
+    (finalize-references fib-program-asm)
     (define my-debugger (make-debugger fib-program-asm))
     (define my-vm (debugger-vm my-debugger))
     (vm-memory-set! my-vm 1 10)