diff --git a/parse.y b/parse.y
index 7132063c9..60cbeba07 100644
--- a/parse.y
+++ b/parse.y
@@ -19,7 +19,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: parse.y,v 1.79 1999/12/31 17:39:00 steve Exp $"
+#ident "$Id: parse.y,v 1.80 2000/01/01 23:47:58 steve Exp $"
 #endif
 
 # include  "parse_misc.h"
@@ -127,7 +127,7 @@ extern void lex_end_table();
 %type <expr>  expression expr_primary
 %type <expr>  lavalue lpvalue
 %type <expr>  delay_value
-%type <exprs> delay1 delay3 delay3_opt
+%type <exprs> delay1 delay3 delay3_opt parameter_value_opt
 %type <exprs> expression_list
 %type <exprs> assign assign_list
 
@@ -1088,7 +1088,7 @@ module_item
 	| gatetype delay3_opt gate_instance_list ';'
 		{ pform_makegates($1, $2, $3);
 		}
-	| IDENTIFIER delay3_opt gate_instance_list ';'
+	| IDENTIFIER parameter_value_opt gate_instance_list ';'
 		{ pform_make_modgates($1, $2, $3);
 		  delete $1;
 		}
@@ -1217,6 +1217,20 @@ parameter_assign_list
 	| parameter_assign_list ',' parameter_assign
 	;
 
+
+  /* The parameters of a module instance can be overridden by writing
+     a list of expressions in a syntax much line a delay list. (The
+     difference being the list can have any length.) The pform that
+     attaches the expression list to the module checks that the
+     expressions are constant. */
+
+parameter_value_opt
+	: '#' '(' expression_list ')'
+		{ $$ = $3; }
+	|
+		{ $$ = 0; }
+	;
+
   /* The port (of a module) is a fairle complex item. Each port is
      handled as a Module::port_t object. A simple port reference has a
      name and a PWire object, but more complex constructs are possible
diff --git a/pform.cc b/pform.cc
index 9a8b021b3..086587d04 100644
--- a/pform.cc
+++ b/pform.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: pform.cc,v 1.49 1999/12/30 19:06:14 steve Exp $"
+#ident "$Id: pform.cc,v 1.50 2000/01/01 23:47:58 steve Exp $"
 #endif
 
 # include  "compiler.h"
@@ -355,6 +355,13 @@ void pform_make_modgates(const string&type,
 			 svector<PExpr*>*overrides,
 			 svector<lgate>*gates)
 {
+      for (unsigned idx = 0 ;  idx < overrides->count() ;  idx += 1)
+	    if (! pform_expression_is_constant((*overrides)[idx])) {
+		  VLerror("error: Parameter override expression"
+			  " must be constant.");
+		  return;
+	    }
+
       for (unsigned idx = 0 ;  idx < gates->count() ;  idx += 1) {
 	    lgate cur = (*gates)[idx];
 
@@ -765,6 +772,9 @@ int pform_parse(const char*path, map<string,Module*>&modules,
 
 /*
  * $Log: pform.cc,v $
+ * Revision 1.50  2000/01/01 23:47:58  steve
+ *  Fix module parameter override syntax.
+ *
  * Revision 1.49  1999/12/30 19:06:14  steve
  *  Support reg initial assignment syntax.
  *