diff --git a/Makefile b/Makefile
index b4e8f221..d19d5b86 100644
--- a/Makefile
+++ b/Makefile
@@ -203,6 +203,9 @@ test-sort: chibi-scheme$(EXE)
 test-records: chibi-scheme$(EXE)
 	LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" ./chibi-scheme$(EXE) tests/record-tests.scm
 
+test-weak: chibi-scheme$(EXE)
+	LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" ./chibi-scheme$(EXE) tests/weak-tests.scm
+
 test-libs: chibi-scheme$(EXE)
 	LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" ./chibi-scheme$(EXE) tests/lib-tests.scm
 
diff --git a/eval.c b/eval.c
index 55aba87f..de5612a4 100644
--- a/eval.c
+++ b/eval.c
@@ -1496,7 +1496,7 @@ sexp sexp_make_primitive_env (sexp ctx, sexp version) {
     op = sexp_copy_opcode(ctx, &opcodes[i]);
     if (sexp_opcode_opt_param_p(op) && sexp_opcode_data(op)) {
       sym = sexp_intern(ctx, (char*)sexp_opcode_data(op), -1);
-      sexp_opcode_data(op) = sexp_env_cell_create(ctx, e, sym, SEXP_VOID, NULL);
+      sexp_opcode_data(op) = sexp_env_ref(e, sym, SEXP_FALSE);
     }
     sexp_env_define(ctx, e, sexp_intern(ctx, sexp_opcode_name(op), -1), op);
   }
@@ -1587,18 +1587,31 @@ sexp sexp_add_module_directory_op (sexp ctx sexp_api_params(self, n), sexp dir,
   return SEXP_VOID;
 }
 
-sexp sexp_load_standard_parameters (sexp ctx, sexp e) {
+static void sexp_set_parameter (sexp ctx, sexp env, sexp name, sexp value) {
+  sexp param = sexp_env_ref(env, name, SEXP_FALSE);
+  if (sexp_opcodep(param)) {
+    if (! sexp_pairp(sexp_opcode_data(param)))
+      sexp_opcode_data(param) = sexp_cons(ctx, name, value);
+    else
+      sexp_cdr(sexp_opcode_data(param)) = value;
+  }
+}
+
+sexp sexp_load_standard_parameters (sexp ctx, sexp env) {
   /* add io port and interaction env parameters */
-  sexp p = sexp_make_input_port(ctx, stdin, SEXP_FALSE);
+  sexp_gc_var1(p);
+  sexp_gc_preserve1(ctx, p);
+  p = sexp_make_input_port(ctx, stdin, SEXP_FALSE);
   sexp_port_no_closep(p) = 1;
-  sexp_env_define(ctx, e, sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL), p);
+  sexp_set_parameter(ctx, env, sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL), p);
   p = sexp_make_output_port(ctx, stdout, SEXP_FALSE);
   sexp_port_no_closep(p) = 1;
-  sexp_env_define(ctx, e, sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL), p);
+  sexp_set_parameter(ctx, env, sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL), p);
   p = sexp_make_output_port(ctx, stderr, SEXP_FALSE);
   sexp_port_no_closep(p) = 1;
-  sexp_env_define(ctx, e, sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL), p);
-  sexp_env_define(ctx, e, sexp_global(ctx, SEXP_G_INTERACTION_ENV_SYMBOL), e);
+  sexp_set_parameter(ctx, env, sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL), p);
+  sexp_set_parameter(ctx, env, sexp_global(ctx, SEXP_G_INTERACTION_ENV_SYMBOL), env);
+  sexp_gc_release1(ctx);
   return SEXP_VOID;
 }
 
diff --git a/include/chibi/eval.h b/include/chibi/eval.h
index 290243ae..6c30ab77 100644
--- a/include/chibi/eval.h
+++ b/include/chibi/eval.h
@@ -67,6 +67,7 @@ enum sexp_opcode_names {
   SEXP_OP_DROP,
   SEXP_OP_GLOBAL_REF,
   SEXP_OP_GLOBAL_KNOWN_REF,
+  SEXP_OP_PARAMETER_REF,
   SEXP_OP_STACK_REF,
   SEXP_OP_LOCAL_REF,
   SEXP_OP_LOCAL_SET,
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 49eb8126..d8c4afd0 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -342,8 +342,8 @@ struct sexp_struct {
 #endif
       char tailp, tracep, timeoutp, waitp;
       sexp_uint_t pos, depth, last_fp;
-      sexp bc, lambda, stack, env, fv, parent, child, globals,
-        proc, name, specific, event;
+      sexp bc, lambda, stack, env, fv, parent, child,
+        globals, params, proc, name, specific, event;
     } context;
   } value;
 };
@@ -771,6 +771,7 @@ SEXP_API sexp sexp_make_unsigned_integer(sexp ctx, sexp_luint_t x);
 #define sexp_context_tailp(x)    (sexp_field(x, context, SEXP_CONTEXT, tailp))
 #define sexp_context_tracep(x)   (sexp_field(x, context, SEXP_CONTEXT, tracep))
 #define sexp_context_globals(x)  (sexp_field(x, context, SEXP_CONTEXT, globals))
+#define sexp_context_params(x)   (sexp_field(x, context, SEXP_CONTEXT, params))
 #define sexp_context_last_fp(x)  (sexp_field(x, context, SEXP_CONTEXT, last_fp))
 #define sexp_context_refuel(x)   (sexp_field(x, context, SEXP_CONTEXT, refuel))
 #define sexp_context_ip(x)       (sexp_field(x, context, SEXP_CONTEXT, ip))
diff --git a/main.c b/main.c
index 6128c538..b613c262 100644
--- a/main.c
+++ b/main.c
@@ -22,6 +22,11 @@
 #define exit_failure() exit(70)
 #endif
 
+static sexp sexp_param_ref (sexp ctx, sexp env, sexp name) {
+  sexp res=sexp_env_ref(env, name, SEXP_FALSE);
+  return sexp_opcodep(res) ? sexp_cdr(sexp_opcode_data(res)) : SEXP_VOID;
+}
+
 static void repl (sexp ctx) {
   sexp in, out, err;
   sexp_gc_var4(obj, tmp, res, env);
@@ -31,9 +36,9 @@ static void repl (sexp ctx) {
   sexp_env_define(ctx, sexp_context_env(ctx),
                   sexp_global(ctx, SEXP_G_INTERACTION_ENV_SYMBOL), env);
   sexp_context_tracep(ctx) = 1;
-  in  = sexp_env_ref(env, sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL), SEXP_FALSE);
-  out = sexp_env_ref(env, sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL), SEXP_FALSE);
-  err = sexp_env_ref(env, sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL), SEXP_FALSE);
+  in  = sexp_param_ref(ctx, env, sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL));
+  out = sexp_param_ref(ctx, env, sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL));
+  err = sexp_param_ref(ctx, env, sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL));
   sexp_port_sourcep(in) = 1;
   while (1) {
     sexp_write_string(ctx, "> ", out);
@@ -88,10 +93,16 @@ static sexp sexp_load_standard_repl_env (sexp ctx, sexp env, sexp k) {
   sexp p, res = sexp_load_standard_env(ctx, env, k);
 #if SEXP_USE_GREEN_THREADS
   p  = sexp_env_ref(env, sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL), SEXP_FALSE);
+  if (sexp_opcodep(p)) p = sexp_opcode_data(p);
+  if (sexp_pairp(p)) p = sexp_cdr(p);
   if (sexp_portp(p)) fcntl(sexp_port_fileno(p), F_SETFL, O_NONBLOCK);
   p  = sexp_env_ref(env, sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL), SEXP_FALSE);
+  if (sexp_opcodep(p)) p = sexp_opcode_data(p);
+  if (sexp_pairp(p)) p = sexp_cdr(p);
   if (sexp_portp(p)) fcntl(sexp_port_fileno(p), F_SETFL, O_NONBLOCK);
   p  = sexp_env_ref(env, sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL), SEXP_FALSE);
+  if (sexp_opcodep(p)) p = sexp_opcode_data(p);
+  if (sexp_pairp(p)) p = sexp_cdr(p);
   if (sexp_portp(p)) fcntl(sexp_port_fileno(p), F_SETFL, O_NONBLOCK);
 #endif
   return res;
diff --git a/opcodes.c b/opcodes.c
index f4e66948..8c0ea700 100644
--- a/opcodes.c
+++ b/opcodes.c
@@ -11,9 +11,14 @@
 #define _FN2OPTP(rt, a1, a2, s, d, f) _FN(SEXP_OP_FCALL2, 1, 3, rt, a1, a2, SEXP_FALSE, s, d, f)
 #define _FN3(rt, a1, a2, a3, s, d, f) _FN(SEXP_OP_FCALL3, 3, 0, rt, a1, a2, a3, s, d, f)
 #define _FN4(rt, a1, a2, a3, s, d, f) _FN(SEXP_OP_FCALL4, 4, 0, rt, a1, a2, a3, s, d, f)
-#define _PARAM(n, a, t) _OP(SEXP_OPC_PARAMETER, SEXP_OP_NOOP, 0, 3, t, t, SEXP_FALSE, SEXP_FALSE, 0, n, a, 0)
+#define _PARAM(n, t) _OP(SEXP_OPC_PARAMETER, SEXP_OP_PARAMETER_REF, 0, 1, t, t, SEXP_FALSE, SEXP_FALSE, 0, n, SEXP_FALSE, 0)
 
 static struct sexp_opcode_struct opcodes[] = {
+_PARAM("current-input-port", _I(SEXP_IPORT)),
+_PARAM("current-output-port", _I(SEXP_OPORT)),
+_PARAM("current-error-port", _I(SEXP_OPORT)),
+_PARAM("current-exception-handler", _I(SEXP_PROCEDURE)),
+_PARAM("interaction-environment", _I(SEXP_ENV)),
 _OP(SEXP_OPC_GETTER, SEXP_OP_CAR, 1, 0, _I(SEXP_OBJECT), _I(SEXP_PAIR), SEXP_FALSE, SEXP_FALSE, 0, "car", 0, NULL),
 _OP(SEXP_OPC_SETTER, SEXP_OP_SET_CAR, 2, 0, SEXP_VOID, _I(SEXP_PAIR), _I(SEXP_OBJECT), SEXP_FALSE, 0, "set-car!", 0, NULL),
 _OP(SEXP_OPC_GETTER, SEXP_OP_CDR, 1, 0, _I(SEXP_OBJECT), _I(SEXP_PAIR), SEXP_FALSE, SEXP_FALSE, 0, "cdr", 0, NULL),
@@ -81,14 +86,14 @@ _OP(SEXP_OPC_TYPE_PREDICATE, SEXP_OP_TYPEP,  1, 0, _I(SEXP_BOOLEAN), _I(SEXP_OBJ
 _OP(SEXP_OPC_GENERIC, SEXP_OP_APPLY1, 2, 0, _I(SEXP_OBJECT), _I(SEXP_PROCEDURE), SEXP_NULL, SEXP_FALSE, 0, "apply1", 0, NULL),
 _OP(SEXP_OPC_GENERIC, SEXP_OP_CALLCC, 1, 0, _I(SEXP_OBJECT), _I(SEXP_PROCEDURE), SEXP_FALSE, SEXP_FALSE, 0, "%call/cc", 0, NULL),
 _OP(SEXP_OPC_GENERIC, SEXP_OP_RAISE, 1, 0, _I(SEXP_OBJECT), _I(SEXP_OBJECT), SEXP_FALSE, SEXP_FALSE, 0, "raise", 0, NULL),
-_OP(SEXP_OPC_IO, SEXP_OP_WRITE_CHAR, 1, 3, SEXP_VOID, _I(SEXP_CHAR), _I(SEXP_OPORT), SEXP_FALSE, 0, "write-char", (sexp)"*current-output-port*", NULL),
-_OP(SEXP_OPC_IO, SEXP_OP_NEWLINE, 0, 3, SEXP_VOID, _I(SEXP_OPORT), SEXP_FALSE, SEXP_FALSE, 0, "newline", (sexp)"*current-output-port*", NULL),
-_OP(SEXP_OPC_IO, SEXP_OP_READ_CHAR, 0, 3, _I(SEXP_CHAR), _I(SEXP_IPORT), SEXP_FALSE, SEXP_FALSE, 0, "read-char", (sexp)"*current-input-port*", NULL),
-_OP(SEXP_OPC_IO, SEXP_OP_PEEK_CHAR, 0, 3, _I(SEXP_CHAR), _I(SEXP_IPORT), SEXP_FALSE, SEXP_FALSE, 0, "peek-char", (sexp)"*current-input-port*", NULL),
-_FN1OPTP(_I(SEXP_OBJECT), _I(SEXP_IPORT), "read", (sexp)"*current-input-port*", sexp_read_op),
-_FN2OPTP(SEXP_VOID,_I(SEXP_OBJECT), _I(SEXP_OPORT), "write", (sexp)"*current-output-port*", sexp_write_op),
-_FN2OPTP(SEXP_VOID, _I(SEXP_OBJECT), _I(SEXP_OPORT), "display", (sexp)"*current-output-port*", sexp_display_op),
-_FN1OPTP(SEXP_VOID, _I(SEXP_OPORT), "flush-output", (sexp)"*current-output-port*", sexp_flush_output_op),
+_OP(SEXP_OPC_IO, SEXP_OP_WRITE_CHAR, 1, 3, SEXP_VOID, _I(SEXP_CHAR), _I(SEXP_OPORT), SEXP_FALSE, 0, "write-char", (sexp)"current-output-port", NULL),
+_OP(SEXP_OPC_IO, SEXP_OP_NEWLINE, 0, 3, SEXP_VOID, _I(SEXP_OPORT), SEXP_FALSE, SEXP_FALSE, 0, "newline", (sexp)"current-output-port", NULL),
+_OP(SEXP_OPC_IO, SEXP_OP_READ_CHAR, 0, 3, _I(SEXP_CHAR), _I(SEXP_IPORT), SEXP_FALSE, SEXP_FALSE, 0, "read-char", (sexp)"current-input-port", NULL),
+_OP(SEXP_OPC_IO, SEXP_OP_PEEK_CHAR, 0, 3, _I(SEXP_CHAR), _I(SEXP_IPORT), SEXP_FALSE, SEXP_FALSE, 0, "peek-char", (sexp)"current-input-port", NULL),
+_FN1OPTP(_I(SEXP_OBJECT), _I(SEXP_IPORT), "read", (sexp)"current-input-port", sexp_read_op),
+_FN2OPTP(SEXP_VOID,_I(SEXP_OBJECT), _I(SEXP_OPORT), "write", (sexp)"current-output-port", sexp_write_op),
+_FN2OPTP(SEXP_VOID, _I(SEXP_OBJECT), _I(SEXP_OPORT), "display", (sexp)"current-output-port", sexp_display_op),
+_FN1OPTP(SEXP_VOID, _I(SEXP_OPORT), "flush-output", (sexp)"current-output-port", sexp_flush_output_op),
 _FN2(_I(SEXP_BOOLEAN), _I(SEXP_OBJECT), _I(SEXP_OBJECT), "equal?", 0, sexp_equalp_op),
 _FN1(_I(SEXP_BOOLEAN), _I(SEXP_OBJECT), "list?", 0, sexp_listp_op),
 _FN1(_I(SEXP_BOOLEAN), _I(SEXP_OBJECT), "identifier?", 0, sexp_identifierp_op),
@@ -106,8 +111,8 @@ _FN1(SEXP_VOID, _I(SEXP_OPORT), "close-output-port", 0, sexp_close_port_op),
 _FN0(_I(SEXP_ENV), "make-environment", 0, sexp_make_env_op),
 _FN1(_I(SEXP_ENV), _I(SEXP_FIXNUM), "null-environment", 0, sexp_make_null_env_op),
 _FN1(_I(SEXP_ENV), _I(SEXP_FIXNUM), "scheme-report-environment", 0, sexp_make_standard_env_op),
-_FN2OPTP(_I(SEXP_OBJECT), _I(SEXP_OBJECT), _I(SEXP_ENV), "eval", (sexp)"*interaction-environment*", sexp_eval_op),
-_FN2OPTP(SEXP_VOID, _I(SEXP_STRING), _I(SEXP_ENV), "load", (sexp)"*interaction-environment*", sexp_load_op),
+_FN2OPTP(_I(SEXP_OBJECT), _I(SEXP_OBJECT), _I(SEXP_ENV), "eval", (sexp)"interaction-environment", sexp_eval_op),
+_FN2OPTP(SEXP_VOID, _I(SEXP_STRING), _I(SEXP_ENV), "load", (sexp)"interaction-environment", sexp_load_op),
 _FN4(SEXP_VOID, _I(SEXP_ENV), _I(SEXP_ENV), _I(SEXP_OBJECT), "%env-copy!", 0, sexp_env_copy_op),
 _FN2(SEXP_VOID, _I(SEXP_EXCEPTION), _I(SEXP_OPORT), "print-exception", 0, sexp_print_exception_op),
 _FN1(_I(SEXP_OBJECT), _I(SEXP_EXCEPTION), "exception-type", 0, sexp_exception_type_op),
@@ -122,11 +127,6 @@ _FN2(_I(SEXP_OBJECT), _I(SEXP_OBJECT), SEXP_NULL, "memq", 0, sexp_memq_op),
 _FN2(_I(SEXP_OBJECT), _I(SEXP_OBJECT), SEXP_NULL, "assq", 0, sexp_assq_op),
 _FN3(_I(SEXP_SYNCLO), _I(SEXP_ENV), SEXP_NULL, _I(SEXP_OBJECT), "make-syntactic-closure", 0, sexp_make_synclo_op),
 _FN1(_I(SEXP_OBJECT), _I(SEXP_OBJECT), "strip-syntactic-closures", 0, sexp_strip_synclos),
-_PARAM("current-input-port", (sexp)"*current-input-port*", _I(SEXP_IPORT)),
-_PARAM("current-output-port", (sexp)"*current-output-port*", _I(SEXP_OPORT)),
-_PARAM("current-error-port", (sexp)"*current-error-port*", _I(SEXP_OPORT)),
-_PARAM("current-exception-handler", (sexp)"*current-exception-handler*", _I(SEXP_PROCEDURE)),
-_PARAM("interaction-environment", (sexp)"*interaction-environment*", _I(SEXP_ENV)),
 _FN0(_I(SEXP_OPORT), "open-output-string", 0, sexp_make_output_string_port_op),
 _FN1(_I(SEXP_IPORT), _I(SEXP_STRING), "open-input-string", 0, sexp_make_input_string_port_op),
 _FN1(_I(SEXP_STRING), _I(SEXP_OPORT), "get-output-string", 0, sexp_get_output_string_op),
diff --git a/sexp.c b/sexp.c
index 0a662538..e068a018 100644
--- a/sexp.c
+++ b/sexp.c
@@ -111,7 +111,7 @@ static struct sexp_type_struct _sexp_type_specs[] = {
   {SEXP_SEQ, sexp_offsetof(seq, ls), 2, 2, 0, 0, sexp_sizeof(seq), 0, 0, 0, 0, 0, 0, 0, 0, "sequence", SEXP_FALSE, SEXP_FALSE, NULL},
   {SEXP_LIT, sexp_offsetof(lit, value), 2, 2, 0, 0, sexp_sizeof(lit), 0, 0, 0, 0, 0, 0, 0, 0, "literal", SEXP_FALSE, SEXP_FALSE, NULL},
   {SEXP_STACK, sexp_offsetof(stack, data), 1, 1, sexp_offsetof(stack, top), 1, sexp_sizeof(stack), offsetof(struct sexp_struct, value.stack.length), sizeof(sexp), 0, 0, 0, 0, 0, 0, "stack", SEXP_FALSE, SEXP_FALSE, NULL},
-  {SEXP_CONTEXT, sexp_offsetof(context, bc), 12, 12, 0, 0, sexp_sizeof(context), 0, 0, 0, 0, 0, 0, 0, 0, "context", SEXP_FALSE, SEXP_FALSE, NULL},
+  {SEXP_CONTEXT, sexp_offsetof(context, bc), 13, 13, 0, 0, sexp_sizeof(context), 0, 0, 0, 0, 0, 0, 0, 0, "context", SEXP_FALSE, SEXP_FALSE, NULL},
   {SEXP_CPOINTER, sexp_offsetof(cpointer, parent), 1, 0, 0, 0, sexp_sizeof(cpointer), sexp_offsetof(cpointer, length), 1, 0, 0, 0, 0, 0, 0, "cpointer", SEXP_FALSE, SEXP_FALSE, NULL},
 };
 
@@ -263,10 +263,10 @@ void sexp_init_context_globals (sexp ctx) {
   sexp_global(ctx, SEXP_G_QUASIQUOTE_SYMBOL) = sexp_intern(ctx, "quasiquote", -1);
   sexp_global(ctx, SEXP_G_UNQUOTE_SYMBOL) = sexp_intern(ctx, "unquote", -1);
   sexp_global(ctx, SEXP_G_UNQUOTE_SPLICING_SYMBOL) = sexp_intern(ctx, "unquote-splicing", -1);
-  sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL) = sexp_intern(ctx, "*current-input-port*", -1);
-  sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL) = sexp_intern(ctx, "*current-output-port*", -1);
-  sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL) = sexp_intern(ctx, "*current-error-port*", -1);
-  sexp_global(ctx, SEXP_G_INTERACTION_ENV_SYMBOL) = sexp_intern(ctx, "*interaction-environment*", -1);
+  sexp_global(ctx, SEXP_G_CUR_IN_SYMBOL) = sexp_intern(ctx, "current-input-port", -1);
+  sexp_global(ctx, SEXP_G_CUR_OUT_SYMBOL) = sexp_intern(ctx, "current-output-port", -1);
+  sexp_global(ctx, SEXP_G_CUR_ERR_SYMBOL) = sexp_intern(ctx, "current-error-port", -1);
+  sexp_global(ctx, SEXP_G_INTERACTION_ENV_SYMBOL) = sexp_intern(ctx, "interaction-environment", -1);
   sexp_global(ctx, SEXP_G_EMPTY_VECTOR) = sexp_alloc_type(ctx, vector, SEXP_VECTOR);
   sexp_vector_length(sexp_global(ctx, SEXP_G_EMPTY_VECTOR)) = 0;
 #if ! SEXP_USE_GLOBAL_TYPES
@@ -318,6 +318,7 @@ sexp sexp_make_context (sexp ctx, size_t size) {
   sexp_context_name(res) = sexp_context_specific(res) = SEXP_FALSE;
   sexp_context_fv(res) = SEXP_NULL;
   sexp_context_saves(res) = NULL;
+  sexp_context_params(res) = SEXP_NULL;
   sexp_context_depth(res)=sexp_context_tracep(res)=sexp_context_pos(res)=0;
   sexp_context_tailp(res) = 1;
 #if SEXP_USE_GREEN_THREADS
diff --git a/vm.c b/vm.c
index 3d30dafd..97c162ea 100644
--- a/vm.c
+++ b/vm.c
@@ -206,7 +206,14 @@ static void generate_opcode_app (sexp ctx, sexp app) {
       inv_default = 1;
     } else {
       emit_push(ctx, sexp_opcode_data(op));
-      if (sexp_opcode_opt_param_p(op)) emit(ctx, SEXP_OP_CDR);
+      if (sexp_opcode_opt_param_p(op)) {
+#if SEXP_USE_GREEN_THREADS
+        emit(ctx, SEXP_OP_PARAMETER_REF);
+        emit_word(ctx, (sexp_uint_t)sexp_opcode_data(op));
+        sexp_push(ctx, sexp_bytecode_literals(sexp_context_bc(ctx)), sexp_opcode_data(op));
+#endif
+        emit(ctx, SEXP_OP_CDR);
+      }
       sexp_context_depth(ctx)++;
       num_args++;
     }
@@ -273,6 +280,11 @@ static void generate_opcode_app (sexp ctx, sexp app) {
     break;
   case SEXP_OPC_PARAMETER:
     emit_push(ctx, sexp_opcode_data(op));
+#if SEXP_USE_GREEN_THREADS
+    emit(ctx, SEXP_OP_PARAMETER_REF);
+    emit_word(ctx, (sexp_uint_t)op);
+    sexp_push(ctx, sexp_bytecode_literals(sexp_context_bc(ctx)), op);
+#endif
     emit(ctx, ((num_args == 0) ? SEXP_OP_CDR : SEXP_OP_SET_CDR));
     break;
   default:
@@ -782,7 +794,20 @@ sexp sexp_vm (sexp ctx, sexp proc) {
     _PUSH(sexp_cdr(_WORD0));
     ip += sizeof(sexp);
     break;
-  case SEXP_OP_STACK_REF:            /* `pick' in forth */
+#if SEXP_USE_GREEN_THREADS
+  case SEXP_OP_PARAMETER_REF:
+    _ALIGN_IP();
+    tmp2 = _WORD0;
+    ip += sizeof(sexp);
+    for (tmp1=sexp_context_params(ctx); sexp_pairp(tmp1); tmp1=sexp_cdr(tmp1))
+      if (sexp_car(tmp1) == tmp2) {
+        _PUSH(sexp_car(tmp1));
+        goto loop;
+      }
+    _PUSH(sexp_opcode_data(tmp2));
+    break;
+#endif
+  case SEXP_OP_STACK_REF:
     _ALIGN_IP();
     stack[top] = stack[top - _SWORD0];
     ip += sizeof(sexp);