diff --git a/CMakeLists.txt b/CMakeLists.txt
index 91eb4d86..a0088607 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -700,12 +700,14 @@ set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${STA_HOME}/app)
 # Note executable and lib name cannot be the same because
 # on osx something is case insensitive. Using STA for the
 # lib name results in "No rule to make target ../depend.
-add_executable(sta app/Main.cc)
+add_executable(sta app/Main.cc ${STA_WRAP})
+
 target_link_libraries(sta
   OpenSTA
   ${TCL_LIB}
   ${CUDD_LIB}
   )
+
 if (ZLIB_FOUND)
   target_link_libraries(sta ${ZLIB_LIBRARIES})
 endif()
diff --git a/app/Main.cc b/app/Main.cc
index c884275b..f7f927b9 100644
--- a/app/Main.cc
+++ b/app/Main.cc
@@ -15,32 +15,58 @@
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 #include <stdio.h>
+#include <tcl.h>
 #include "Machine.hh"
 #include "StaConfig.hh"  // STA_VERSION
 #include "StringUtil.hh"
 #include "Sta.hh"
 #include "StaMain.hh"
 
+
+namespace sta {
+extern const char *tcl_inits[];
+}
+
 using sta::stringEq;
+using sta::findCmdLineFlag;
 using sta::Sta;
-using sta::staMain;
-using sta::showUsage;
+using sta::initSta;
+using sta::evalTclInit;
+using sta::stringPrintTmp;
+using sta::sourceTclFile;
+using sta::parseThreadsArg;
+using sta::tcl_inits;
 
 // Swig uses C linkage for init functions.
 extern "C" {
 extern int Sta_Init(Tcl_Interp *interp);
 }
 
-namespace sta {
-extern const char *tcl_inits[];
-}
+static int cmd_argc;
+static char **cmd_argv;
+static const char *init_filename = ".sta";
+
+static void
+showUsage(const char *prog,
+	  const char *init_filename);
+static int
+tclAppInit(Tcl_Interp *interp);
+static int
+staTclAppInit(int argc,
+	      char *argv[],
+	      const char *init_filename,
+	      Tcl_Interp *interp);
+static void
+initStaApp(int argc,
+	   char *argv[],
+	   Tcl_Interp *interp);
 
 int
 main(int argc,
      char *argv[])
 {
   if (argc == 2 && stringEq(argv[1], "-help")) {
-    showUsage(argv[0]);
+    showUsage(argv[0], init_filename);
     return 0;
   }
   else if (argc == 2 && stringEq(argv[1], "-version")) {
@@ -48,8 +74,102 @@ main(int argc,
     return 0;
   }
   else {
-    Sta *sta = new Sta;
-    staMain(sta, argc, argv, ".sta", Sta_Init, sta::tcl_inits);
+    // Set argc to 1 so Tcl_Main doesn't source any files.
+    // Tcl_Main never returns.
+#if 0
+    // It should be possible to pass argc/argv to staTclAppInit with
+    // a closure but I couldn't get the signature to match Tcl_AppInitProc.
+    Tcl_Main(1, argv, [=](Tcl_Interp *interp)
+		      { sta::staTclAppInit(argc, argv, interp);
+			return 1;
+		      });
+#else
+    // Workaround.
+    cmd_argc = argc;
+    cmd_argv = argv;
+    Tcl_Main(1, argv, tclAppInit);
+#endif
     return 0;
   }
 }
+
+static int
+tclAppInit(Tcl_Interp *interp)
+{
+  return staTclAppInit(cmd_argc, cmd_argv, init_filename, interp);
+}
+
+// Tcl init executed inside Tcl_Main.
+static int
+staTclAppInit(int argc,
+	      char *argv[],
+	      const char *init_filename,
+	      Tcl_Interp *interp)
+{
+  // source init.tcl
+  Tcl_Init(interp);
+
+  initStaApp(argc, argv, interp);
+
+  if (!findCmdLineFlag(argc, argv, "-no_splash"))
+    Tcl_Eval(interp, "sta::show_splash");
+
+  if (!findCmdLineFlag(argc, argv, "-no_init")) {
+    char *init_path = stringPrintTmp("[file join $env(HOME) %s]",
+				     init_filename);
+    sourceTclFile(init_path, true, true, interp);
+  }
+
+  bool exit_after_cmd_file = findCmdLineFlag(argc, argv, "-exit");
+
+  if (argc > 2 ||
+      (argc > 1 && argv[1][0] == '-'))
+    showUsage(argv[0], init_filename);
+  else {
+    if (argc == 2) {
+      char *cmd_file = argv[1];
+      if (cmd_file) {
+	sourceTclFile(cmd_file, false, false, interp);
+	if (exit_after_cmd_file)
+	  exit(EXIT_SUCCESS);
+      }
+    }
+  }
+  return TCL_OK;
+}
+
+static void
+initStaApp(int argc,
+	   char *argv[],
+	   Tcl_Interp *interp)
+{
+  initSta();
+  Sta *sta = new Sta;
+  Sta::setSta(sta);
+  sta->makeComponents();
+  sta->setTclInterp(interp);
+  int thread_count = parseThreadsArg(argc, argv);
+  sta->setThreadCount(thread_count);
+
+  // Define swig TCL commands.
+  Sta_Init(interp);
+  // Eval encoded sta TCL sources.
+  evalTclInit(interp, tcl_inits);
+  // Import exported commands from sta namespace to global namespace.
+  Tcl_Eval(interp, "sta::define_sta_cmds");
+  Tcl_Eval(interp, "namespace import sta::*");
+}
+
+static void
+showUsage(const char *prog,
+	  const char *init_filename)
+{
+  printf("Usage: %s [-help] [-version] [-no_init] [-exit] cmd_file\n", prog);
+  printf("  -help              show help and exit\n");
+  printf("  -version           show version and exit\n");
+  printf("  -no_init           do not read %s init file\n", init_filename);
+  printf("  -threads count|max use count threads\n");
+  printf("  -no_splash         do not show the license splash at startup\n");
+  printf("  -exit              exit after reading cmd_file\n");
+  printf("  cmd_file           source cmd_file\n");
+}
diff --git a/app/StaMain.cc b/app/StaMain.cc
index 2d96ae24..5b49f134 100644
--- a/app/StaMain.cc
+++ b/app/StaMain.cc
@@ -24,37 +24,6 @@
 
 namespace sta {
 
-typedef sta::Vector<SwigInitFunc> SwigInitFuncSeq;
-
-// "Arguments" passed to staTclAppInit.
-static int sta_argc;
-static char **sta_argv;
-static const char *sta_init_filename;
-static const char **sta_tcl_inits;
-static SwigInitFunc sta_swig_init;
-
-void
-staMain(Sta *sta,
-	int argc,
-	char *argv[],
-	const char *init_filename,
-	SwigInitFunc swig_init,
-	const char *tcl_inits[])
-{
-  initSta();
-
-  Sta::setSta(sta);
-  sta->makeComponents();
-
-  int thread_count = parseThreadsArg(argc, argv);
-  sta->setThreadCount(thread_count);
-
-  staSetupAppInit(argc, argv, init_filename, swig_init, tcl_inits);
-  // Set argc to 1 so Tcl_Main doesn't source any files.
-  // Tcl_Main never returns.
-  Tcl_Main(1, argv, staTclAppInit);
-}
-
 int
 parseThreadsArg(int &argc,
 		char *argv[])
@@ -71,71 +40,6 @@ parseThreadsArg(int &argc,
   return 1;
 }
 
-// Set globals to pass to staTclAppInit.
-void
-staSetupAppInit(int argc,
-		char *argv[],
-		const char *init_filename,
-		SwigInitFunc swig_init,
-		const char *tcl_inits[])
-{
-  sta_argc = argc;
-  sta_argv = argv;
-  sta_init_filename = init_filename;
-  sta_tcl_inits = tcl_inits;
-  sta_swig_init = swig_init;
-}
-
-// Tcl init executed inside Tcl_Main.
-int
-staTclAppInit(Tcl_Interp *interp)
-{
-  int argc = sta_argc;
-  char **argv = sta_argv;
-
-  // source init.tcl
-  Tcl_Init(interp);
-
-  // Define swig commands.
-  sta_swig_init(interp);
-
-  Sta *sta = Sta::sta();
-  sta->setTclInterp(interp);
-
-  // Eval encoded sta TCL sources.
-  evalTclInit(interp, sta_tcl_inits);
-
-  if (!findCmdLineFlag(argc, argv, "-no_splash"))
-    Tcl_Eval(interp, "sta::show_splash");
-
-  // Import exported commands from sta namespace to global namespace.
-  Tcl_Eval(interp, "sta::define_sta_cmds");
-  Tcl_Eval(interp, "namespace import sta::*");
-
-  if (!findCmdLineFlag(argc, argv, "-no_init")) {
-    char *init_path = stringPrintTmp("[file join $env(HOME) %s]",
-				     sta_init_filename);
-    sourceTclFile(init_path, true, true, interp);
-  }
-
-  bool exit_after_cmd_file = findCmdLineFlag(argc, argv, "-exit");
-
-  if (argc > 2 ||
-      (argc > 1 && argv[1][0] == '-'))
-    showUsage(argv[0]);
-  else {
-    if (argc == 2) {
-      char *cmd_file = argv[1];
-      if (cmd_file) {
-	sourceTclFile(cmd_file, false, false, interp);
-	if (exit_after_cmd_file)
-	  exit(EXIT_SUCCESS);
-      }
-    }
-  }
-  return TCL_OK;
-}
-
 bool
 findCmdLineFlag(int &argc,
 		char *argv[],
@@ -220,17 +124,4 @@ evalTclInit(Tcl_Interp *interp,
   delete [] unencoded;
 }
 
-void
-showUsage(const char * prog) 
-{
-  printf("Usage: %s [-help] [-version] [-no_init] [-exit] cmd_file\n", prog);
-  printf("  -help              show help and exit\n");
-  printf("  -version           show version and exit\n");
-  printf("  -no_init           do not read .sta init file\n");
-  printf("  -threads count|max use count threads\n");
-  printf("  -no_splash         do not show the license splash at startup\n");
-  printf("  -exit              exit after reading cmd_file\n");
-  printf("  cmd_file           source cmd_file\n");
-}
-
 } // namespace
diff --git a/app/StaMain.hh b/app/StaMain.hh
index a9fa2d1c..2b188e89 100644
--- a/app/StaMain.hh
+++ b/app/StaMain.hh
@@ -22,28 +22,20 @@ struct Tcl_Interp;
 namespace sta {
 
 class Sta;
-typedef int (*SwigInitFunc)(Tcl_Interp *);
-
-// The swig_init function is called to define the swig interface
-// functions to the tcl interpreter.
-void
-staMain(Sta *sta,
-	int argc,
-	char *argv[],
-	const char *init_filename,
-	SwigInitFunc swig_init,
-	const char *tcl_inits[]);
-
-// Set arguments passed to staTclAppInit inside the tcl interpreter.
-void
-staSetupAppInit(int argc,
-		char *argv[],
-		const char *init_filename,
-		SwigInitFunc swig_init,
-		const char *tcl_inits[]);
 
+// Parse command line argument
 int
-staTclAppInit(Tcl_Interp *interp);
+staTclAppInit(int argc,
+	      char *argv[],
+	      const char *init_filename,
+	      Tcl_Interp *interp);
+
+// Sta initialization.
+// Makes the Sta object and registers TCL commands.
+void
+initSta(int argc,
+	char *argv[],
+	Tcl_Interp *interp);
 
 // TCL init files are encoded into the string init using the three
 // digit decimal equivalent for each ascii character.  This function
@@ -63,8 +55,6 @@ findCmdLineKey(int &argc,
 	       char *argv[],
 	       const char *key);
 
-void
-showUsage(const char *prog);
 int
 parseThreadsArg(int &argc,
 		char *argv[]);
diff --git a/network/ConcreteLibrary.cc b/network/ConcreteLibrary.cc
index 059e104f..eea5ec4c 100644
--- a/network/ConcreteLibrary.cc
+++ b/network/ConcreteLibrary.cc
@@ -117,6 +117,7 @@ ConcreteCell::ConcreteCell(ConcreteLibrary *library,
   name_(stringCopy(name)),
   filename_(stringCopy(filename)),
   liberty_cell_(nullptr),
+  ext_cell_(nullptr),
   port_bit_count_(0),
   is_leaf_(is_leaf)
 {
@@ -145,6 +146,12 @@ ConcreteCell::setLibertyCell(LibertyCell *cell)
   liberty_cell_ = cell;
 }
 
+void
+ConcreteCell::setExtCell(void *ext_cell)
+{
+  ext_cell_ = ext_cell;
+}
+
 ConcretePort *
 ConcreteCell::makePort(const char *name)
 {
@@ -437,6 +444,7 @@ ConcretePort::ConcretePort(ConcreteCell *cell,
   cell_(cell),
   direction_(PortDirection::unknown()),
   liberty_port_(nullptr),
+  ext_port_(nullptr),
   pin_index_(-1),
   is_bundle_(is_bundle),
   is_bus_(is_bus),
@@ -468,6 +476,12 @@ ConcretePort::setLibertyPort(LibertyPort *port)
   liberty_port_ = port;
 }
 
+void
+ConcretePort::setExtPort(void *port)
+{
+  ext_port_ = port;
+}
+
 const char *
 ConcretePort::busName() const
 {
diff --git a/network/ConcreteLibrary.hh b/network/ConcreteLibrary.hh
index 906b135c..18ecd8db 100644
--- a/network/ConcreteLibrary.hh
+++ b/network/ConcreteLibrary.hh
@@ -96,6 +96,8 @@ public:
   const char *filename() const { return filename_; }
   LibertyCell *libertyCell() const { return liberty_cell_; }
   void setLibertyCell(LibertyCell *cell);
+  void *extCell() const { return ext_cell_; }
+  void setExtCell(void *ext_cell);
   int portBitCount() const { return port_bit_count_; }
   ConcretePort *findPort(const char *name) const;
   void findPortsMatching(const PatternMatch *pattern,
@@ -147,6 +149,8 @@ protected:
   // Filename is optional.
   const char *filename_;
   LibertyCell *liberty_cell_;
+  // External application cell.
+  void *ext_cell_;
   // Non-bus and bus ports (but no expanded bus bit ports).
   ConcretePortSeq ports_;
   ConcretePortMap port_map_;
@@ -172,6 +176,9 @@ public:
   PortDirection *direction() const { return direction_; }
   LibertyPort *libertyPort() const { return liberty_port_; }
   void setLibertyPort(LibertyPort *port);
+  // External application port.
+  void *extPort() const { return ext_port_; }
+  void setExtPort(void *port);
   void setDirection(PortDirection *dir);
   // Bundles are groups of related ports that do not use
   // bus notation.
@@ -220,6 +227,8 @@ protected:
   ConcreteCell *cell_;
   PortDirection *direction_;
   LibertyPort *liberty_port_;
+  // External application port.
+  void *ext_port_;
   int pin_index_;
   bool is_bundle_;
   bool is_bus_;